Glossary of cellular and molecular biology (M–Z) (this page) lists terms beginning with the letters M through Z.
This glossary is intended as introductory material for novices (for more specific and technical detail, see the article corresponding to each term). It has been designed as a companion to
Glossary of genetics and evolutionary biology, which contains many overlapping and related terms; other related glossaries include
Glossary of virology and
Glossary of chemistry.
Any very large
molecule composed of dozens, hundreds, or thousands of
covalently bonded atoms, especially one with biological significance. Many important biomolecules, such as
nucleic acids and
proteins, are
polymers consisting of a repeated series of smaller
monomers; others such as
lipids and
carbohydrates may not be polymeric but are nevertheless large and complex molecules.
Any of a class of relatively long-lived
phagocytic cells of the mammalian
immune system which are activated in response to the presence of foreign materials in certain tissues and subsequently play important roles in
antigen presentation, stimulating other types of immune cells, and killing or
engulfing parasitic microorganisms, diseased cells, or tumor cells.[3]
The branch of medicine and medical science that involves the study, diagnosis, and management of
hereditary disorders, and more broadly the application of knowledge about human
genetics to medical care.
A specialized type of
cell division that occurs exclusively in
sexually reproducingeukaryotes, during which
DNA replication is followed by two consecutive rounds of division to ultimately produce four genetically unique
haploid daughter cells, each with half the number of
chromosomes as the original
diploid parent cell. Meiosis only occurs in cells of the sex organs, and serves the purpose of generating haploid
gametes such as
sperm,
eggs, or
spores, which are later fused during
fertilization. The two meiotic divisions, known as Meiosis I and Meiosis II, may also include various
genetic recombination events between
homologous chromosomes.
A supramolecular aggregate of
amphipathiclipid molecules which when suspended in a polar solvent tend to arrange themselves into structures which minimize the exposure of their
hydrophobic tails by sheltering them within a ball created by their own
hydrophilic heads (i.e. a
micelle). Certain types of lipids, specifically
phospholipids and other
membrane lipids, commonly occur as
double-layered sheets of molecules when immersed in an aqueous environment, which can themselves assume approximately spherical shapes, acting as semipermeable barriers surrounding a water-filled interior space. This is the basic layout used for the biological membranes enclosing all
cells,
vesicles, and
membrane-bound organelles.
The complete set of chemical reactions which sustain and account for the basic processes of life in all living cells,[2] especially those involving: 1) the conversion of energy from food into energy available for cellular activities; 2) the breakdown of food into simpler compounds which can then be used as
substrates to build complex
biomolecules such as
proteins,
lipids, and
nucleic acids; and 3) the degradation and excretion of toxins, byproducts, and other unusable compounds known as
metabolic wastes. In a broader sense the term may include all chemical reactions occurring in living organisms, even those which are not strictly necessary for life but instead serve accessory functions. Many specific cellular activities are accomplished by
metabolic pathways in which one chemical is ultimately transformed through a stepwise series of reactions into another chemical, with each reaction
catalyzed by a specific
enzyme. Most metabolic reactions can be subclassified as
catabolic or
anabolic.
An intermediate or end product of
metabolism, especially degradative metabolism (catabolism);[2] or any substance produced by or taking part in a metabolic reaction. Metabolites include a huge variety of small molecules generated by cells from various
pathways and having various functions, including as inputs to other pathways and reactions, as
signaling molecules, and as stimulators, inhibitors, and
cofactors of
enzymes. Metabolites may result from the degradation and elimination of naturally occurring compounds as well as of pharmaceutical compounds.
(of a linear
chromosome or chromosome fragment) Having a
centromere positioned in the middle of the chromosome, resulting in
chromatid arms of approximately equal length.[5]
The covalent attachment of a
methyl group (–CH 3) to a chemical compound, protein, or other biomolecule, either spontaneously or by
enzymatic catalysis. Methylation is one of the most widespread natural mechanisms by which
nucleic acids and
proteins are
labelled. The
methylation of nucleobases in a DNA molecule inhibits recognition of the methylated sequence by
DNA-binding proteins, which can effectively
silence the expression of genes. Specific
amino acids in
histones are also commonly methylated, which can change
nucleosome positioning and similarly activate or repress nearby loci. Contrast demethylation.
Any of a class of
transferaseenzymes which catalyze the covalent bonding of a
methyl group (–CH 3) to another compound, protein, or biomolecule, a process known as
methylation.
A group that "aims to provide a standard for the representation of
DNA microarraygene expression data that would facilitate the exchange of microarray information between different data systems".[6]
Any of a diverse class of small
membrane-bound organelles or
vesicles found in the cells of many eukaryotes, especially plants and animals, usually having some specific metabolic function and occurring in great numbers in certain specialized cell types.
Peroxisomes,
glyoxysomes,
glycosomes, and
hydrogenosomes are often considered microbodies.
A
chromosomaldeletion that is too short to cause any apparent change in morphology under a light microscope, though it may still be detectable with other methods such as
sequencing.
A long, thin, flexible, rod-like structure composed of polymeric strands of proteins, usually
actins, that occurs in abundance in the
cytoplasm of eukaryotic cells, forming part of the
cytoskeleton. Microfilaments comprise the cell's structural framework. They are modified by and interact with numerous other cytoplasmic proteins, playing important roles in cell stability, motility, contractility, and facilitating changes in cell shape, as well as in
cytokinesis.
The smaller of the two types of
nuclei that occur in pairs in the cells of some ciliated
protozoa. Whereas the larger
macronucleus is
polyploid, the micronucleus is
diploid and generally transcriptionally inactive except for the purpose of sexual reproduction, where it has important functions during
conjugation.[2]
Also short tandem repeat (STR) or simple sequence repeat (SSR).
A type of
satellite DNA consisting of a relatively short
sequence of
tandem repeats, in which certain
motifs (ranging in length from one to six or more
bases) are repeated, typically 5–50 times. Microsatellites are widespread throughout most organisms' genomes and tend to have higher mutation rates than other regions. They are classified as
variable number tandem repeat (VNTR) DNA, along with longer
minisatellites.
The centrally constricted region that forms across the central axis of a cell during
cell division, constricted by the closing of the
contractile ring until the
daughter cells are finally separated.[2]
In plant cells, the outermost layer of the
cell wall; a continuous, unified layer of extracellular
pectins which is the first layer deposited by the cell during
cytokinesis and which serves to cement together the primary cell walls of adjacent cells.[4]
An incorrect
pairing of
nucleobases on
complementarystrands of
DNA or
RNA; i.e. the presence in one strand of a duplex molecule of a base that is not complementary (by Watson–Crick pairing rules) to the base occupying the corresponding position in the other strand, which prevents normal
hydrogen bonding between the bases. For example, a
guanine paired with a
thymine would be a mismatch, as guanine normally pairs with
cytosine.[10]
The insertion of an incorrect
amino acid in a growing
peptide chain during
translation, i.e. the inclusion of any amino acid that is not the one specified by a particular
codon in an
mRNA transcript. Mistranslation may originate from a
mischargedtransfer RNA or from a malfunctioning
ribosome.[10]
The selective degradation of
mitochondria by means of
autophagy; i.e. the mitochondrion initiates its own degradation. Mitophagy is a regular process in healthy populations of cells by which defective or damaged mitochondria are recycled, preventing their accumulation. It may also occur in response to the changing
metabolic needs of the cell, e.g. during certain developmental stages.
In
eukaryotic cells, the part of the
cell cycle during which the
division of the
nucleus takes place and replicated
chromosomes are separated into two distinct nuclei. Mitosis is generally preceded by the
S phase of
interphase, when the cell's
DNA is replicated, and either occurs simultaneously with or is followed by
cytokinesis, when the
cytoplasm and
plasma membrane are divided into two new
daughter cells. Colloquially, the term "mitosis" is often used to refer to the entire process of cell division, not just the division of the nucleus.
The proportion of cells within a sample which are undergoing
mitosis at the time of observation, typically expressed as a percentage or as a value between 0 and 1. The number of cells dividing by mitosis at any given time can vary widely depending on organism,
tissue, developmental stage, and
culture media, among other factors.[2]
The abnormal
exchange of genetic material between
homologous chromosomes during
mitosis (as opposed to
meiosis, where it occurs normally).
Homologous recombination during mitosis is relatively uncommon; in the laboratory, it can be induced by exposing dividing cells to high-energy electromagnetic radiation such as X rays. As in meiosis, it can separate
heterozygous alleles and thereby propagate potentially significant changes in zygosity to
daughter cells, though unless it occurs very early in development this often has little or no phenotypic effect, since any phenotypic variance shown by mutant lineages arising in terminally differentiated cells is generally masked or compensated for by neighboring
wild-type cells.[2]
The presence of more than one different
ploidy level, i.e. more than one number of sets of
chromosomes, in different cells of the same cellular population.[10]
The branch of biology that studies biological activity at the
molecular level, in particular the various mechanisms underlying the biological processes that occur in and between
cells, including the structures, properties, synthesis, and modification of
biomolecules such as
proteins and
nucleic acids, their interactions with the chemical environment and with other biomolecules, and how these interactions explain the observations of classical biology (which in contrast studies biological systems at much larger scales).[11] Molecular biology relies largely on laboratory techniques of physics and chemistry to manipulate and measure microscopic phenomena. It is closely related to and overlaps with the fields of
cell biology,
biochemistry, and
molecular genetics.
Any of various
molecular biology methods designed to
replicate a particular molecule, usually a
DNAsequence, many times inside the cells of a natural host. Commonly, a
recombinant DNA fragment containing a
gene of interest is
ligated into a
plasmidvector, which
competent bacterial cells are then induced to uptake in a process known as
transformation. The bacteria, carrying the recombinant plasmid, are then allowed to proliferate naturally in
cell culture, so that each time the bacterial cells divide, the plasmids are replicated along with the rest of the bacterial genome. Any functioning gene of interest will be
expressed by the bacterial cells, and thereby its
gene products will also be cloned. The plasmids or gene products, which now exist in many copies, may then be extracted from the bacteria and purified. Molecular cloning is a fundamental tool of
genetic engineering which is employed for a wide variety of purposes, often to study
gene expression, to amplify a specific gene product, or to generate a
selectable phenotype.
Describing cells, proteins, or molecules descended or derived from a single
clone (i.e. from the same genome or genetic lineage) or made in response to a single unique compound. Monoclonal
antibodies are raised against only one
antigen or can only recognize one unique
epitope on the same antigen. Similarly, the cells of some
tissues and
neoplasms may be described as monoclonal if they are all the asexual progeny of one original
parent cell.[2] Contrast polyclonal.
A
molecule or
compound which can exist individually or serve as a building block or
subunit of a larger
macromolecular aggregate known as a
polymer.[4] Polymers form when multiple monomers of the same or similar molecular species are connected to each other by
chemical bonds, either in a linear chain or a non-linear conglomeration. Examples include the individual
nucleotides which form
nucleic acid polymers; the individual
amino acids which form
polypeptides; and the individual proteins which form
protein complexes.
A synthetic
nucleic acid analogue connecting a short sequence of
nucleobases into an artificial
antisenseoligomer, used in
genetic engineering to
knockdowngene expression by
pairing with
complementary sequences in naturally occurring RNA or DNA molecules, especially
mRNA transcripts, thereby inhibiting interactions with other biomolecules such as proteins and
ribosomes. Morpholino oligomers are not themselves
translated, and neither they nor their hybrid duplexes with RNA are attacked by
nucleases; also, unlike the negatively charged
phosphates of normal nucleic acids, the synthetic backbones of Morpholinos are electrically neutral, making them less likely to interact non-selectively with a host cell's charged proteins. These properties make them useful and reliable tools for artificially generating
mutant phenotypes in living cells.[10]
The presence of two or more populations of cells with different
genotypes in an individual organism which has developed from a single fertilized
egg. A mosaic organism can result from many kinds of genetic phenomena, including
nondisjunction of chromosomes,
endoreduplication, or mutations in individual
stem cell lineages during the early development of the embryo. Mosaicism is similar to but distinct from
chimerism.
Any
protein which converts chemical energy derived from the hydrolysis of
nucleoside triphosphates such as
ATP and
GTP into mechanical work in order to effect its own locomotion, by propelling itself along a filament or through the
cytoplasm.[4]
Composed of more than one cell. The term is especially used to describe organisms or tissues consisting of many cells descendant from the same original parent cell which work together in an organized way, but may also describe colonies of nominally single-celled organisms such as protists and bacteria which live symbiotically with each other in large groups. Contrast unicellular.
Any physical or chemical agent that
changes the genetic material (usually
DNA) of an organism and thereby increases the frequency of
mutations above natural background levels.
1. The process by which the genetic information of an organism is changed, resulting in a
mutation. Mutagenesis may occur spontaneously or as a result of exposure to a
mutagen.
2. In
molecular biology, any laboratory technique by which one or more genetic mutations are deliberately
engineered in order to produce a
mutant gene, regulatory element, gene product, or
genetically modified organism so that the functions of a genetic locus, process, or product can be studied in detail.
Any permanent change in the
nucleotide sequence of a strand of
DNA or
RNA, or in the
amino acid sequence of a
peptide. Mutations play a role in both normal and abnormal biological processes; their natural occurrence is integral to the process of
evolution. They can result from errors in
replication, chemical damage, exposure to high-energy radiation, or manipulations by
mobile genetic elements.
Repair mechanisms have evolved in many organisms to correct them. By understanding the effect that a mutation has on
phenotype, it is possible to establish the function of the
gene or sequence in which it occurs.
One of two possible orientations by which a linear DNA fragment can be inserted into a
vector, specifically the one in which the
gene maps of both fragment and vector have the same orientation.[10] Contrast u orientation.
In the process of being synthesized; incomplete; not yet fully processed or mature. The term is commonly used to describe
strands of
DNA or
RNA which are actively undergoing synthesis during
replication or
transcription, respectively, or sometimes a complete, fully transcribed RNA molecule before any
alterations have been made (e.g.
polyadenylation or
RNA editing), or a
peptide chain actively undergoing
translation by a
ribosome.[10]
Any
amino acid, natural or artificial, that is not one of the 20 or 21
proteinogenic amino acids encoded by the
standard genetic code. There are hundreds of such amino acids, many of which have biological functions and are specified by alternative codes or incorporated into proteins accidentally by
errors in translation. Many of the best known naturally occurring ncAAs occur as intermediates in the metabolic pathways leading to the standard amino acids, while others have been made synthetically in the laboratory.[13]
Any segment of
DNA that does not
encode a sequence that may ultimately be
transcribed and
translated into a
protein. In most organisms, only a small fraction of the genome consists of protein-coding DNA, though the proportion varies greatly between species. Some non-coding DNA may still be transcribed into functional
non-coding RNA (as with
transfer RNAs) or may serve important developmental or
regulatory purposes; other regions (as with so-called "
junk DNA") appear to have no known biological function.
Any molecule of
RNA that is not ultimately
translated into a
protein. The
DNA sequence from which a functional non-coding RNA is
transcribed is often referred to as an "RNA gene". Numerous types of non-coding RNAs essential to normal genome function are produced constitutively, including
transfer RNA (tRNA),
ribosomal RNA (rRNA),
microRNA (miRNA), and
small interfering RNA (siRNA); other non-coding RNAs (sometimes described as "junk RNA") have no known function and are likely the product of spurious transcription.
A type of
point mutation which results in a premature
stop codon in the
transcribedmRNA sequence, thereby causing the premature termination of
translation, which results in a truncated, incomplete, and often non-functional
protein.
Also nonsynonymous substitution or replacement mutation.
A type of
mutation in which the
substitution of one
nucleotide base for another results, after
transcription and
translation, in an amino acid sequence that is different from that produced by the original unmutated gene. Because nonsynonymous mutations always result in a biological change in the organism, they are often subject to strong
selection pressure. Contrast synonymous mutation.
The end of a linear chain of
amino acids (i.e. a
peptide) that is terminated by the free
amine group (–NH 2) of the first amino acid added to the chain during
translation. This amino acid is said to be N-terminal. By convention, sequences, domains, active sites, or any other structure positioned nearer to the N-terminus of the
polypeptide or the folded
protein it forms relative to others are described as
upstream. Contrast C-terminus.
Any
DNA molecule contained within the
nucleus of a eukaryotic cell, most prominently the DNA in
chromosomes. It is sometimes used interchangeably with
genomic DNA.
A sub-cellular barrier consisting of two concentric
lipid bilayermembranes that surrounds the
nucleus in
eukaryotic cells. The nuclear envelope is sometimes simply called the "nuclear membrane", though the structure is actually composed of two distinct membranes, an
inner membrane and an
outer membrane.
The principle that the nuclei of essentially all
differentiated cells of a mature multicellular organism are genetically identical to each other and to the nucleus of the
zygote from which they descended; i.e. they all contain the same genetic information on the same chromosomes, having been replicated from the original zygotic set with extremely high fidelity. Even though all adult
somatic cells have the same set of genes, cells can nonetheless differentiate into distinct
cell types by expressing different subsets of these genes. Though this principle generally holds true, the reality is slightly more complex, as mutations such as
insertions,
deletions,
duplications, and
translocations as well as
chimerism,
mosaicism, and various types of
genetic recombination can all cause different somatic lineages within the same organism to be genetically non-identical.
A fibrous network of proteins lining the inner,
nucleoplasmic surface of the
nuclear envelope, composed of protein filaments similar to those that make up the
cytoskeleton. It may function as a scaffold for the various contents of the nucleus such as
nuclear proteins and
chromosomes.[3]
An
amino acid sequence within a
protein which serves as a molecular signal marking the protein for
transport into the
nucleus, typically consisting of one or more short motifs containing positively charged amino acids exposed on the mature protein's surface (especially
lysines and
arginines). Though all proteins are
translated in the cytoplasm, many whose primary biological activities occur inside the nucleus, e.g.
transcription factors, require nuclear localization signals identifiable by
molecular chaperones in order to cross the
nuclear envelope. Contrast nuclear export signal.
A complex of
membrane proteins that creates an opening in the
nuclear envelope through which certain molecules and ions are
permitted to pass in order to cross the envelope and thus enter or exit the
nucleus (analogous to the
channel proteins in the
cell membrane). The nuclear envelope typically has thousands of pores, which selectively regulate the exchange of specific materials between the
nucleoplasm and the
cytoplasm, including nuclear proteins, which are synthesized in the cytoplasm but function in the nucleus, as well as
messenger RNAs, which are transcribed in the nucleus but must be translated in the cytoplasm.[4][3]
Any
RNA molecule located within a cell's
nucleus, whether associated with chromosomes or existing freely in the nucleoplasm, including
small nuclear RNA (snRNA),
enhancer RNA (eRNA), and all newly transcribed
immature RNAs,
coding or
non-coding, prior to their export to the cytosol (hnRNA).
The mechanisms by which molecules cross the
nuclear envelope surrounding a cell's nucleus. Though small molecules and ions can cross the membrane freely, the entry and exit of larger molecules is tightly regulated by
nuclear pores, so that most
macromolecules such as RNAs and proteins require association with transport factors in order to be
chaperoned across.
Any of a class of
enzymes capable of cleaving
phosphodiester bonds connecting adjacent
nucleotides in a nucleic acid molecule (the opposite of a
ligase). Nucleases may
nick one or
cut bothstrands of a duplex molecule, and may cleave randomly or at specific recognition sequences. They are ubiquitous and imperative for normal cellular function, and are also widely employed in laboratory techniques.
A long,
polymericmacromolecule made up of smaller
monomers called
nucleotides which are chemically linked to one another in a chain. Two specific types of nucleic acid,
DNA and
RNA, are common to all living organisms, serving to encode the genetic information governing the construction, development, and ordinary processes of all biological systems. This information, contained within the order or
sequence of the nucleotides, is
translated into
proteins, which direct all of the chemical reactions necessary for life.
The precise order of consecutively linked
nucleotides in a
nucleic acid molecule such as
DNA or
RNA. Long sequences of nucleotides are the principal means by which biological systems store genetic information, and therefore the accurate
replication,
transcription, and
translation of such sequences is of the utmost importance, lest the information be lost or corrupted. Nucleic acid sequences may be equivalently referred to as sequences of nucleotides,
nitrogenous bases,
nucleobases, or, in
duplex molecules,
base pairs, and they correspond directly to sequences of
codons and
amino acids.
Sometimes used interchangeably with nitrogenous base or simply base.
Any of the five primary or canonical
nitrogenous bases –
adenine (A),
guanine (G),
cytosine (C),
thymine (T), and
uracil (U) – that form
nucleosides and
nucleotides, the latter of which are the fundamental building blocks of
nucleic acids. The ability of these bases to form
base pairs via
hydrogen bonding, as well as their flat, compact three-dimensional profiles, allows them to "stack" one upon another and leads directly to the long-chain structures of
DNA and
RNA. When writing sequences in shorthand notation, the letter N is often used to represent a nucleotide containing a generic or unidentified nucleobase.
An irregularly shaped region within a prokaryotic cell which contains most or all of the cell's genetic material, but is not enclosed by a
nuclear membrane as in eukaryotes.
The basic structural subunit of
chromatin used in
packaging nuclear DNA such as chromosomes, consisting of a
core particle of eighthistone proteins around which
double-stranded DNA is wrapped in a manner akin to thread wound around a spool. The technical definition of a nucleosome includes a segment of DNA about 146 base pairs in length which makes 1.67 left-handed turns as it coils around the histone core, as well as a stretch of
linker DNA (generally 38–80 bp) connecting it to an adjacent core particle, though the term is often used to refer to the core particle alone. Long series of nucleosomes are further condensed by association with
histone H1 into higher-order structures such as
30-nm fibers and ultimately
supercoiledchromatids. Because the histone–DNA interaction limits access to the DNA molecule by other proteins and RNAs, the precise positioning of nucleosomes along the DNA sequence plays a fundamental role in controlling whether or not genes are
transcribed and
expressed, and hence mechanisms for moving and ejecting nucleosomes have evolved as a means of
regulating the expression of particular loci.
nucleosome-depleted region (NDR)
A region of a genome or chromosome in which long segments of DNA are bound by few or no
nucleosomes, and thus exposed to manipulation by other proteins and molecules, especially implying that the region is
transcriptionally active.
A large spherical or lobular
organelle surrounded by a
dedicated membrane which functions as the main storage compartment for the genetic material of eukaryotic cells, including the DNA comprising
chromosomes, as well as the site of RNA synthesis during
transcription. The vast majority of eukaryotic cells have a single nucleus, though some cells may have
more than one nucleus, either temporarily or permanently, and in some organisms there exist certain cell types (e.g. mammalian
erythrocytes) which lose their nuclei upon reaching maturity, effectively becoming
anucleate. The nucleus is one of the defining features of eukaryotes; the cells of prokaryotes such as bacteria lack nuclei entirely.[2]
A
gene that has the potential to cause
cancer. In
tumor cells, such genes are often
mutated and/or
expressed at abnormally high levels.
one gene–one polypeptide
Also one gene–one protein or one gene–one enzyme.
The hypothesis that there exists a large class of
genes in which each particular gene directs the synthesis of one particular
polypeptide or
protein.[10] Historically it was thought that all genes and proteins might follow this rule by definition, but it is now known that many or most proteins are composites of different polypeptides and therefore the product of multiple genes, and also that some genes do not encode polypeptides at all but instead produce
non-coding RNAs, which are never translated.
A functional unit of
gene expression consisting of a cluster of adjacent
structural genes which are collectively under the control of a single
promoter, along with one or more adjacent
regulatory sequences such as
operators which affect
transcription of the structural genes. The set of genes is transcribed together, usually resulting in a single
polycistronicmessenger RNA molecule, which may then be
translated together or undergo
splicing to create multiple mRNAs which are translated independently; the result is that the genes contained in the operon are either expressed together or not at all. Regulatory proteins, including
repressors,
corepressors, and
activators, usually bind specifically to the regulatory sequences of a given operon; by some definitions, the genes that code for these regulatory proteins are also considered part of the operon.
A spatially distinct compartment or subunit within a
cell which has a specific, specialized function. Organelles occur in both prokaryotic and eukaryotic cells. In the latter they are often separated from the
cytoplasm by being enclosed with their own
membranebilayer (whence the term membrane-bound organelles), though organelles may also be functionally specific areas or structures without a surrounding membrane; some cellular structures which exist partially or entirely outside of the cell membrane, such as
cilia and
flagella, are also referred to as organelles. There are numerous types of organelles with a wide variety of functions, including the various compartments of the
endomembrane system (e.g. the
nuclear envelope,
endoplasmic reticulum, and
Golgi apparatus),
mitochondria,
chloroplasts,
lysosomes,
endosomes, and
vacuoles, among others. Many organelles are unique to particular cell types or species.
A particular location within a DNA molecule at which
DNA replication is initiated. Origins are usually defined by the presence of a particular replicator sequence or by specific chromatin patterns.
An abnormally high level of
gene expression which results in an excessive number of copies of one or more
gene products. Overexpression produces a pronounced gene-related
phenotype.[15][16]
Also Tumor protein P53 (TP53), transformation-related protein 53 (TRP53), and cellular tumor antigen p53.
A class of regulatory proteins encoded by the TP53 gene in vertebrates which
bind DNA and
regulate gene expression in order to protect the genome from mutation and block progression through the
cell cycle if DNA damage does occur.[4] It is mutated in more than 50% of human cancers, indicating it plays a crucial role in preventing cancer formation.
A
nucleic acid sequence of a double-stranded
DNA or
RNA molecule in which the unidirectional sequence (e.g. 5' to 3') of
nucleotides on one strand matches the sequence in the same direction (e.g. 5' to 3') on the
complementary strand. In other words, a nucleotide sequence is said to be palindromic if it is equal to its own
reverse complement. Palindromic
motifs are common in most genomes and are capable of forming
hairpins.
The movement of a
solute across a
membrane by traveling down an electrochemical or concentration gradient, using only the energy stored in the gradient and not any energy from external sources.[3] Contrast active transport.
Any simple sugar or
monosaccharide containing five carbon atoms. The compounds
ribose and
deoxyribose are both pentose sugars, which, in the form of cyclic five-membered rings, serve as the central structural components of the
ribonucleotides and
deoxyribonucleotides that make up
RNA and
DNA, respectively.
Any of a class of
membrane proteins which attach only temporarily to the
cell membrane, either by penetrating the
lipid bilayer or by attaching to
other proteins which are permanently embedded within the membrane.[17] The ability to reversibly interact with membranes has made peripheral membrane proteins important in a wide variety of roles, where they commonly function as regulatory
subunits of
channel proteins and
cell surface receptors. Many protein domains undergo rearrangement, dissociation, or
conformational changes when they interact with the membrane, resulting in the activation of their biological activity.[18] In
protein purification, peripheral membrane proteins are typically more water-soluble and much easier to isolate from the membrane than integral membrane proteins.
1. The tendency of a
moving cell to continue moving in the same direction as previously; that is, even in isotropic environments, there inevitably still exists an inherent bias by which, from instant to instant, cells are more likely to not change direction than to change direction. Averaged over long periods of time, however, this bias is less obvious and cell movements are better described as a
random walk.[3]
2. The ability of some viruses to remain present and viable in cells, organisms, or populations for very long periods of time by any of a variety of strategies, including retroviral integration and immune suppression, often in a latent form which replicates very slowly or not at all.[3]
A type of cell which functions as part of the immune system by engulfing and ingesting harmful foreign molecules, bacteria, and dead or dying cells in a process known as
phagocytosis.
The process by which foreign molecules, cells, and small particulate matter is engulfed and ingested via
endocytosis by specialized cells known as
phagocytes (a class which includes
macrophages and
neutrophils).[4]
A large, intracellular, membrane-bound
vesicle formed as a result of
phagocytosis and containing whatever previously extracellular material was engulfed during that process.[4]
The complete set of
phenotypes that are or can be expressed by a
genome, cell, tissue, organism, or species; the sum of all of its manifest chemical, morphological, and behavioral characteristics or traits.
phenomic lag
A delay in the
phenotypicexpression of a genetic
mutation owing to the time required for the manifestation of changes in the affected biochemical pathways.[12]
The composite of the observable morphological, physiological, and behavioral
traits of an
organism that result from the
expression of the organism's
genotype as well as the influence of environmental factors and the interactions between the two.
Also phosphodiester backbone, sugar–phosphate backbone, and phosphate–sugar backbone.
The linear chain of alternating phosphate and sugar compounds that results from the linking of consecutive
nucleotides in the same
strand of a
nucleic acid molecule, and which serves as the structural framework of the nucleic acid. Each individual strand is held together by a repeating series of
phosphodiester bonds connecting each
phosphate group to the
ribose or
deoxyribose sugars of two adjacent nucleotides. These bonds are created by
ligases and broken by
nucleases.
A pair of
ester bonds linking a
phosphate molecule with the two
pentose rings of consecutive
nucleosides on the same strand of a
nucleic acid. Each phosphate forms a covalent bond with the
3' carbon of one pentose and the
5' carbon of the adjacent pentose; the repeated series of such bonds that holds together a long chain of nucleotides in
DNA and
RNA molecules is known as the
phosphate or phosphodiester backbone.
Any of a subclass of
lipids consisting of a central alcohol (usually
glycerol) covalently bonded to three functional groups: a negatively charged
phosphate group, and two long
fatty acid chains. This arrangement results in a highly
amphipathic molecule which in aqueous solutions tends to aggregate with other, similar molecules in a lamellar or micellar conformation with the
hydrophilic phosphate "heads" oriented outward, exposing them to the solution, and the
hydrophobic fatty acid "tails" oriented inward, minimizing their interactions with water and other polar compounds. Phospholipids are the major structural
membrane lipid in almost all biological
membranes except the membranes of some plant cells and
chloroplasts, where
glycolipids dominate instead.[3]
Any of a class of
membrane-bound organelles found in the cells of some eukaryotes such as plants and algae which are hypothesized to have evolved from
endosymbioticcyanobacteria; examples include
chloroplasts,
chromoplasts, and
leucoplasts. Plastids retain their own
circular chromosomes which replicate independently of the host cell's genome. Many contain photosynthetic pigments which allow them to perform
photosynthesis, while others have been retained for their ability to synthesize unique chemical compounds.
1. Variability in the size, shape, or
staining of cells and/or their nuclei, particularly as observed in
histology and
cytopathology, where morphological variation is frequently
an indicator of a cellular abnormality such as disease or
tumor formation.
2. In
microbiology, the ability of some microorganisms such as certain bacteria and viruses to alter their morphology, metabolism, or mode of reproduction in response to changes in their environment.
The addition of a series of multiple
adenosineribonucleotides, known as a
poly(A) tail, to the
3'-end of a
primary RNA transcript, typically a
messenger RNA. A class of
post-transcriptional modification, polyadenylation serves different purposes in different cell types and organisms. In eukaryotes, the addition of a poly(A) tail is an important step in the processing of a raw transcript into a mature mRNA, ready for export to the cytoplasm where
translation occurs; in many bacteria, polyadenylation has the opposite function, instead promoting the RNA's degradation.
Describing cells, proteins, or molecules descended or derived from more than one
clone (i.e. from more than one genome or genetic lineage) or made in response to more than one unique stimulus.
Antibodies are often described as polyclonal if they have been produced or raised against multiple distinct
antigens or multiple variants of the same antigen, such that they can recognize more than one unique
epitope.[2] Contrast monoclonal.
A
macromolecule composed of multiple repeating
subunits or
monomers; a chain or aggregation of many individual molecules of the same compound or class of compound.[2] The formation of polymers is known as
polymerization and generally only occurs when
nucleation sites are present and the concentration of monomers is sufficiently high.[3] Many of the major classes of
biomolecules are polymers, including
nucleic acids and
polypeptides.
Any of a wide variety of molecular biology methods involving the rapid production of millions or billions of copies of a specific
DNA sequence, allowing scientists to selectively
amplify fragments of a very small sample to a quantity large enough to study in detail. In its simplest form, PCR generally involves the incubation of a target DNA sample of known or unknown sequence with a reaction mixture consisting of
oligonucleotideprimers, a heat-stable
DNA polymerase, and free
deoxyribonucleotide triphosphates (dNTPs), all of which are supplied in excess. This mixture is then alternately heated and cooled to pre-determined temperatures for pre-determined lengths of time according to a specified pattern which is repeated for many cycles, typically in a
thermal cycler which automatically controls the required temperature variations. In each cycle, the most basic of which includes a
denaturation phase,
annealing phase, and
elongation phase, the copies synthesized in the previous cycle are used as
templates for synthesis in the next cycle, causing a
chain reaction that results in the exponential growth of the total number of copies in the reaction mixture. Amplification by PCR has become a standard technique in virtually all molecular biology laboratories.
The process by which
polymers are created from their constituent
monomers; a chemical reaction or series of reactions in which monomeric
subunits are covalently linked together into a polymeric chain or three-dimensional structure; e.g. the polymerization of a
nucleic acid chain by linking consecutive
nucleotides, a reaction catalyzed by a
polymerase.
A long, continuous, and unbranched
polymeric chain of
amino acidmonomers linked by covalent
peptide bonds, typically longer than a
peptide.
Proteins generally consist of one or more polypeptides
folded or arranged in a biologically functional way.
(of a cell or organism) Having more than two homologous copies of each
chromosome; i.e. any
ploidy level that is greater than
diploid. Polyploidy may occur as a normal condition of chromosomes in certain cells or even entire organisms, or it may result from errors in
cell division or mutations causing the duplication of the entire chromosome set.
The condition of a cell or organism having at least one more copy of a particular
chromosome than is normal for its
ploidy level, e.g. a
diploid organism with three copies of a given chromosome is said to show
trisomy. Every polysomy is a type of
aneuploidy.
Any effect on the
expression or functionality of a
gene or sequence that is a consequence of its location or position within a
chromosome or other DNA molecule. A sequence's precise location relative to other sequences and structures tends to strongly influence its activity and other properties, because different
loci on the same molecule can have substantially different
genetic backgrounds and physical/chemical environments, which may also change over time. For example, the
transcription of a gene located very close to a
nucleosome,
centromere, or
telomere is often
repressed or entirely prevented because the proteins that make up these structures block access to the DNA by
transcription factors, while the same gene is transcribed at a much higher rate when located in
euchromatin. Proximity to
promoters,
enhancers, and other
regulatory elements, as well as to regions of frequent
transposition by
mobile elements, can also directly affect expression; being located near the end of a chromosomal arm or to common
crossover points may affect when
replication occurs and the likelihood of
recombination. Position effects are a major focus of research in the field of
epigenetic inheritance.
A strategy for identifying and
cloning a
candidate gene based on knowledge of its
locus or position alone and with little or no information about its
products or function, in contrast to
functional cloning. This method usually begins by comparing the genomes of individuals expressing a
phenotype of unknown provenance (often a
hereditary disease) and identifying
genetic markers shared between them. Regions defined by markers flanking one or more
genes of interest are cloned, and the genes located between the markers can then be identified by any of a variety of means, e.g. by
sequencing the region and looking for
open reading frames, by comparing the sequence and expression patterns of the region in
mutant and
wild-type individuals, or by testing the ability of the putative gene to
rescue a mutant phenotype.[10]
The unprocessed, single-stranded
RNA molecule produced by the
transcription of a
DNA sequence as it exists before
post-transcriptional modifications such as
alternative splicing convert it into a mature RNA product such as an
mRNA,
tRNA, or
rRNA. A precursor mRNA or pre-mRNA, for example, is a type of primary transcript that becomes a mature mRNA ready for
translation after processing.
A short,
single-strandedoligonucleotide, typically 5–100 bases in length, which "primes" or initiates nucleic acid synthesis by
hybridizing to a complementary sequence on a
template strand and thereby provides an existing
3'-end from which a
polymerase can
extend the new strand. Natural systems exclusively use
RNA primers to initiate
DNA replication and
transcription, whereas the in vitro syntheses performed in many laboratory techniques such as
PCR often use
DNA primers. In modern laboratories, primers are carefully designed, often in "forward" and "reverse" pairs, to complement specific and unique sequences within
genomic DNA, with consideration given to their
melting and
annealing temperatures, and then purchased from commercial suppliers which create oligonucleotides on demand by
de novo synthesis.
Any
reagent used to make a single measurement in a biochemical assay such as a gene expression experiment. Molecules which have a specific affinity for one or more other molecules may be used to probe for the presence of those other molecules in samples of unknown composition. Probes are often
labelled or otherwise used as
reporters to indicate whether or not a specific chemical reaction is taking place. See also hybridization probe.
probe-set
A collection of two or more
probes designed to measure a single molecular species, such as a collection of
oligonucleotides designed to
hybridize to various parts of the
mRNA transcripts generated from a single gene.
A sequence or region of DNA, usually 100–1,000 base pairs long, to which
transcription factors bind in order to recruit
RNA polymerase to the sequence and initiate the
transcription of one or more genes. Promoters are located
upstream of the genes they transcribe, near the transcription start site.
Any of a class of
enzymes which catalyze
proteolysis, i.e. the decomposition of proteins into smaller polypeptides or individual amino acids, by cleaving
peptide bonds via
hydrolysis. Proteases are ubiquitous components of numerous
biological pathways, and therefore it is often necessary to
inhibit them in order for laboratory techniques involving protein activity to be effective.
A
polymericmacromolecule composed of one or more
long chains of
amino acids linked by
peptide bonds. Proteins are the three-dimensional structures created when these chains
fold into specific higher-order arrangements following
translation, and it is this folded structure which determines a protein's chemical activity and hence its biological function. Ubiquitous and fundamental in all living organisms, proteins are the primary means by which the activities of life are performed, participating in the vast majority of the
biochemical reactions that occur inside and outside of cells. They are often classified according to the type(s) of reaction(s) they facilitate or catalyze, by the chemical substrate(s) they act upon, or by their functional role in cellular activity; e.g. as
structural proteins,
motor proteins,
enzymes,
transcription factors, or links within
biochemical pathways.
An assembly or aggregate of multiple
proteins held together by intermolecular forces, especially one with a particular biological function. Complexes may include many of the same protein or all different proteins. Numerous cellular activities, including
DNA replication,
transcription, and
translation, rely on protein complexes.[4]
Any of the 20 canonical
amino acids which are encoded by the
standard genetic code and incorporated into
peptides and ultimately
proteins during
translation. The term may also be inclusive of an additional two amino acids encoded by non-standard
codes which can be incorporated by special translation mechanisms.
The
cleavage of
proteins by
enzymes known as
proteases. Proteolysis is an important reaction used not only for degrading and inactivating proteins but sometimes also to activate them by removing amino acid residues which inhibit their activity.[3]
The entire set of
proteins that is or can be
expressed by a particular
genome, cell, tissue, or species at a particular time (such as during a single lifespan or during a specific developmental stage) or under particular conditions (such as when compromised by a certain disease).
The biological contents enclosed within a
membrane-bound space, variously referring to the
cytoplasm, or the cytoplasm and
nucleoplasm considered collectively, and sometimes exclusive of
vacuoles.
A plant, fungal, or bacterial cell which has had its
cell wall removed by mechanical, chemical, or enzymatic means; or the complete contents (the
protoplasm) of an intact cell excluding the cell wall.
A double-ringed
heterocyclic organic compound which, along with
pyrimidine, is one of two molecules from which all
nitrogenous bases (including the
nucleobases used in
DNA and
RNA) are derived.
Adenine (A) and
guanine (G) are classified as purines. The letter R is sometimes used to indicate a generic purine; e.g. in a nucleotide sequence read, R may be used to indicate that either purine nucleobase, A or G, can be substituted at the indicated position.
putative gene
A specific
nucleotide sequence suspected to be a functional
gene based on the identification of its
open reading frame. The gene is said to be "putative" in the sense that no function has yet been described for its
products.
The irreversible condensation of
chromatin inside the
nucleus as the cell undergoes
necrosis or
apoptosis, resulting in a compact mass which
stains strongly and is conspicuous under a microscope.[10] It is followed by
karyorrhexis.
A single-ringed
heterocyclic organic compound which, along with
purine, is one of two molecules from which all
nitrogenous bases (including the
nucleobases used in
DNA and
RNA) are derived.
Cytosine (C),
thymine (T), and
uracil (U) are classified as pyrimidines. The letter Y is sometimes used to indicate a generic pyrimidine; e.g. in a nucleotide sequence read, Y may be used to indicate that either pyrimidine nucleobase – C, T, or U – can be substituted at the indicated position.
A
cell culture in which there is little or no active cell growth or replication but in which the cells nonetheless continue to survive, as observed with some
confluent cultures.[2]
A popular description of the path followed by a
locomotive cell or particle when there is no bias in movement, i.e. when the direction of movement at any given instant is not influenced by the direction of movement in the preceding instant. The essential randomness of cell movement in a uniform environment is only apparent over long periods of time, however; in the short term, cells can and do exhibit
a tendency to continue moving in the same direction.[3]
A way of dividing the
nucleotide sequence in a
DNA or
RNA molecule into a set of consecutive, non-overlapping
triplets, which is how the sequence is interpreted or "read" by
proteins and
ribosomes during
translation. In coding DNA, each triplet is referred to as a
codon and corresponds to a particular
amino acid to be added to the nascent peptide during translation. In general, only one reading frame (the so-called
open reading frame) in a given section of a nucleic acid can be used to make functional proteins, but there are exceptions in a few organisms. A
frameshift mutation results in a shift in the normal reading frame and affects all downstream codons.
The measurement and manipulation of the rate of
reannealing of
complementary strands of
DNA, generally by heating and denaturing a
double-stranded molecule into
single strands and then observing their rehybridization at a cooler temperature. Because the
base pairG+C requires more energy to anneal than the base pair A+T, the rate of reannealing between two strands depends partly on their
nucleotide sequence, and it is therefore possible to predict or estimate the sequence of the duplex molecule by the time it takes to fully hybridize. Reassociation kinetics is studied with
C0t analysis: fragments reannealing at low C0t values tend to have highly
repetitive sequences, while higher C0t values imply more unique sequences.[10]
A protein which initiates a cellular response to an external stimulus or
propagates a molecular signal by binding a specific extracellular
ligand, often a dedicated signaling molecule. Numerous types of receptors exist which serve an enormous variety of functions. Cell-surface receptors, such as those that bind
acetylcholine and
insulin, are embedded within the
cell membrane with their
binding sites exposed to the extracellular space; intracellular receptors, including many
hormone receptors, are located in the cytoplasm, where they bind ligands that have diffused across the membrane and into the cell.[4]
A type of
chromosomal translocation by which there is a reciprocal exchange of chromosome segments between two or more non-
homologouschromosomes. When the exchange of material is evenly balanced, reciprocal translocations are usually harmless.
Any
DNA molecule in which laboratory methods of
genetic recombination have brought together genetic material from multiple sources, thereby creating a
sequence that would not otherwise be found in a naturally occurring genome. Because DNA molecules from all organisms share the same basic chemical structure and properties, DNA sequences from any species, or even sequences created de novo by
artificial gene synthesis, may be incorporated into recombinant DNA molecules. Recombinant DNA technology is widely used in
genetic engineering.
The smallest unit of a DNA molecule capable of undergoing
homologous recombination, i.e. a pair of consecutive nucleotides, adjacent to each other in cis.[10]
A group of non-contiguous
genes which are
regulated as a unit, generally by virtue of having their
expression controlled by the same regulatory element or set of elements, e.g. the same
repressor or
activator. The term is most commonly used with prokaryotes, where a regulon may consist of genes from multiple
operons.
1. The process by which certain biological molecules, notably the
nucleic acidsDNA and
RNA, produce copies of themselves.
2. A technique used to estimate technical and biological variation in experiments for
statistical analysis of
microarray data. Replicates may be technical replicates, such as
dye swaps or repeated array
hybridizations, or biological replicates, biological samples from separate experiments which are used to test the effects of the same experimental treatment.
The eye-shaped structure that forms when a pair of
replication forks, each growing away from the
origin, separates the strands of the double helix during
DNA replication.
The point at which the paired strands of a
double-stranded DNA molecule are separated by
helicase during
DNA replication, breaking the hydrogen bonds between the complementary strands and thereby forming a structure with two branching single strands of DNA. Once unpaired, these strands serve as
templates from which
DNA polymerase synthesizes the
leading strand and
lagging strand. As replication proceeds, helicase moves along the DNA and continues to separate the strands, causing the replication fork to move as well.[3] A pair of replication forks forms when helicases work in opposite directions from a single
origin of replication, creating a
replication eye.
replication rate
The speed at which
deoxyribonucleotides are incorporated into an elongating chain by
DNA polymerases during
DNA replication; or more generally the speed at which any chromosome, genome, cell, or organism makes a complete, independently functional copy of itself.
The entire complex of molecular machinery that carries out the process of
DNA replication, including all proteins, nucleic acids, and other molecules which participate at an active
replication fork.
reporter
An
MIAME-compliant term to describe a
reagent used to make a single measurement in a gene expression experiment. MIAME defines it as "the nucleotide sequence present in a particular location on the array".[8] A reporter may be a segment of single-stranded DNA that is
covalently attached to the array surface. See also probe.
The restoration of a defective cell or tissue to a healthy or normal condition,[10] or the
reversion or recovery of a mutant gene to its normal functionality, especially in the context of experimental genetics, where an experiment (e.g. a drug,
cross, or gene transfer) resulting in such a restoration is said to rescue the normal
phenotype.
The spontaneous rejoining of an experimentally broken
chromosome which restores the original configuration.
restitution nucleus
A
nucleus containing twice the expected number of chromosomes owing to an error in cell division, especially an unreduced,
diploid product of
meiosis resulting from the failure of the first or second meiotic division.
Also restriction endonuclease, restriction exonuclease, or restrictase.
An
endonuclease or
exonucleaseenzyme that recognizes and cleaves a nucleic acid molecule into
fragments at or near specific recognition sequences known as
restriction sites by breaking the
phosphodiester bonds of the nucleic acid
backbone. Restriction enzymes are naturally occurring in many organisms, but are also routinely used for artificial modification of DNA in laboratory techniques such as
restriction cloning.
A diagram of known
restriction sites within a known DNA sequence, such as a
plasmidvector, obtained by systematically exposing the sequence to various
restriction enzymes and then comparing the lengths of the resulting
fragments, a technique known as restriction mapping. See also gene map.
A short, specific
sequence of nucleotides (typically 4 to 8 bases in length) that is reliably recognized by a particular
restriction enzyme. Because restriction enzymes usually bind as
homodimers, restriction sites are generally
palindromic sequences spanning both strands of a
double-stranded DNA molecule. Restriction
endonucleases cleave the
phosphate backbone between two nucleotides within the recognized sequence itself, but other types of restriction enzymes make their cuts at one end of the sequence or at a nearby sequence.
An experimental approach in
molecular genetics in which a researcher starts with a known
gene and attempts to determine its function or its effect on phenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's nucleic acid sequence or by repressing or silencing its expression and then
screening the mutated organisms for obvious changes in phenotype. When the gene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite of
forward genetics, in which a known phenotype is linked to one or more unknown genes.
A
polymericnucleic acid molecule composed of a series of
ribonucleotides which incorporate a set of four
nucleobases:
adenine (A),
guanine (G),
cytosine (C), and
uracil (U). Unlike
DNA, RNA is more often found as a
single strand folded onto itself, rather than a paired double strand. Various types of RNA molecules serve in a wide variety of essential biological roles, including
coding,
decoding,
regulating, and
expressinggenes, as well as functioning as signaling molecules and, in certain
viral genomes, as the primary genetic material itself.
A
monosaccharide sugar which, as D-ribose in its
pentose ring form, is one of three primary components of the
ribonucleotides from which
ribonucleic acid (RNA) molecules are built. Ribose differs from its structural analog
deoxyribose only at the 2' carbon, where ribose has an attached
hydroxyl group that deoxyribose lacks.
A DNA sequence that codes for
ribosomal RNA (rRNA). In many eukaryotic genomes, rDNA occupies large, highly conserved regions of multiple chromosomes and is rich in both
genes and
repeats.
A type of
non-coding RNA which is found in and is the primary constituent of
ribosomes, binding to ribosomal proteins to form the
small and
large subunits. It is ribosomal RNA which enables ribosomes to perform protein synthesis by working as a
ribozyme that catalyzes the set of reactions comprising
translation. Ribosomal RNA is transcribed from the corresponding
ribosomal DNA (rDNA), and is the predominant form of RNA found in most cells, effecting the translation of all encoded proteins despite never being translated itself.
A macromolecular complex made of both RNA and protein which serves as the site of
protein synthesis. Ribosomes have two
subunits, each of which consists of one or more strands of
ribosomal RNA bound to various
ribosomal proteins: the
small subunit, which reads the messages encoded in
messenger RNA molecules, and the
large subunit, which links
amino acids in sequence to form a
polypeptide chain. Ribosomes are fundamental and ubiquitous in all cell types and are used by all known forms of life.
An
RNA molecule with enzymatic activity,[4] i.e. one that is capable of catalyzing one or more specific biochemical reactions, similar to
proteinenzymes. Ribozymes function in numerous capacities, including in
ribosomes as part of the large subunit
ribosomal RNA.
Any of a class of
polymeraseenzymes that synthesize
RNA molecules from a
DNA template. RNA polymerases are essential for
transcription and are found in all living organisms and many viruses. They build long single-stranded polymers called
transcripts by adding
ribonucleotides one at a time in the
5'-to-
3' direction, relying on the
template provided by the
complementary strand to transcribe the nucleotide sequence faithfully.
A
ribonucleoprotein complex which works to
silence endogenous and exogenous genes by participating in various
RNA interference pathways at the transcriptional and translational levels. RISC can bind both
single-stranded and
double-stranded RNA fragments and then cleave them or use them as guides to target complementary mRNAs for degradation.
A type of
chromosomal translocation by which
double-strand breaks at or near the
centromeres of two
acrocentricchromosomes cause a reciprocal exchange of segments that gives rise to one large
metacentric chromosome (composed of the
long arms) and one extremely small chromosome (composed of the
short arms), the latter of which is often subsequently lost from the cell with little effect because it contains very few genes. The resulting
karyotype shows one fewer than the expected total number of chromosomes, because two previously distinct chromosomes have essentially fused together.
Carriers of Robertsonian translocations are generally not associated with any phenotypic abnormalities, but do have an increased risk of generating meiotically unbalanced
gametes.
A method of
DNA sequencing based on the in vitroreplication of a DNA
template sequence, during which
fluorochrome-labeled, chain-terminating
dideoxynucleotides are randomly incorporated in the elongating strand; the resulting fragments are then sorted by size with
electrophoresis, and the particular fluorochrome terminating each of the size-sorted fragments is detected by laser chromatography, thus revealing the
sequence of the original DNA template through the order of the fluorochrome labels as one reads from small-sized fragments to large-sized fragments. Though Sanger sequencing has been replaced in some contexts by
next-generation methods, it remains widely used for its ability to produce relatively long sequence reads (500+
nucleotides) and its very low error rate.
saturation hybridization
An in vitro nucleic acid
hybridization reaction in which one polynucleotide component (either
DNA or
RNA) is supplied in great excess relative to the other, causing all complementary sequences in the other polynucleotide to pair with the excess sequences and form hybrid
duplex molecules.[10]
Any genetic material (e.g. a
gene or any other DNA sequence) which can enhance its own
replication and/or transmission into subsequent generations at the expense of other genes in the genome, even if doing so has no positive effect or even a net negative effect on the
fitness of the genome as a whole. Selfish elements usually work by producing self-acting
gene products which repeatedly
copy and paste their own
coding sequences into other parts of the genome, independently of normal
DNA replication (as with
transposable elements); by facilitating the uneven swapping of chromosome segments during
genetic recombination events (as with
unequal crossing over); or by disrupting the normally equal redistribution of replicated material during
mitotis or
meiosis such that the probability that the selfish element is present in a given
daughter cell is greater than the normal 50 percent (as with
gene drives).
The standard mode of
DNA replication that occurs in all living cells, in which each of the two parental
strands of the original
double-stranded DNA molecule are used as
template strands, with
DNA polymerases replicating each strand separately and simultaneously in
antiparallel directions. The result is that each of the two double-stranded daughter molecules is composed of one of the original parental strands and one newly synthesized complementary strand, such that each daughter molecule conserves the precise sequence of information (indeed the very same atoms) from one-half of the original molecule. Contrast conservative replication and dispersive replication.
A distinction made between the individual
strands of a
double-stranded DNA molecule in order to easily and specifically identify each strand. The two
complementary strands are distinguished as sense and antisense or, equivalently, the coding strand and the template strand. It is the antisense/template strand which is actually used as the template for
transcription; the sense/coding strand merely resembles the sequence of
codons on the RNA transcript, which makes it possible to determine from the DNA sequence alone the expected amino acid sequence of any protein
translated from the RNA transcript. Which strand is which is relative only to a particular RNA transcript and not to the entire DNA molecule; that is, either strand can function as the sense/coding or antisense/template strand.
Any
codon that specifies an
amino acid, as opposed to a
stop codon, which does not specify any particular amino acid but instead signals the end of translation.
The determination of the order or
sequence of
nucleotides in a
nucleic acid molecule, or of
amino acids in a
peptide, by any means. Sequences are usually written as a linear string of letters which conveniently summarizes much of the atomic-level structure of the molecule.
In condensed
chromosomes where the positioning of the
centromere creates two segments or "arms" of unequal length, the shorter of the two arms of a
chromatid. Contrast long arm.
A type of
neutral mutation which does not have an observable effect on the organism's
phenotype. Though the term "silent mutation" is often used interchangeably with
synonymous mutation, synonymous mutations are not always silent, nor vice versa.
Missense mutations which result in a different
amino acid but one with similar functionality (e.g.
leucine instead of
isoleucine) are also often classified as silent, since such mutations usually do not significantly affect protein function.
Any
substitution of a single
nucleotide which occurs at a specific position within a
genome and with measurable frequency within a population; for example, at a specific base position in a DNA sequence, the majority of the individuals in a population may have a
cytosine (C), while in a minority of individuals, the same position may be occupied by an
adenine (A). SNPs are usually defined with respect to a "standard" reference genome; an individual human genome differs from the reference human genome at an average of 4 to 5 million positions, most of which consist of SNPs and short
indels. See also polymorphism.
Any
DNA molecule that consists of a single nucleotide polymer or
strand, as opposed to a pair of complementary strands held together by hydrogen bonds (
double-stranded DNA). In most circumstances, DNA is more stable and more common in double-stranded form, but high temperatures, low concentrations of dissolved salts, and very high or low pH can cause double-stranded molecules to decompose into two single-stranded molecules in a
denaturation process known as
melting; this reaction is exploited by naturally occurring enzymes such as those involved in
DNA replication as well as by laboratory techniques such as
polymerase chain reaction.
A pair of identical copies (
chromatids) produced as the result of the
DNA replication of a
chromosome, particularly when both copies are joined together by a common
centromere; the pair of sister chromatids is called a dyad. The two sister chromatids are ultimately separated from each other into two different cells during
mitosis or
meiosis.
A class of small
RNA molecules engineered so as to change conformation conditionally in response to cognate molecular inputs, often with the goal of controlling
signal transduction pathways in vitro or in vivo.
A subclass of
microRNAs, originally described in
nematodes, which regulate the timing of developmental events by binding to complementary sequences in the
3' untranslated regions of
messenger RNAs and inhibiting their
translation. In contrast to
siRNAs, which serve similar purposes, stRNAs bind to their target mRNAs after the initiation of translation and without affecting mRNA stability, which makes it possible for the target mRNAs to resume translation at a later time.
Any biological
cell forming the body of an organism, or, in multicellular organisms, any cell other than a
gamete,
germ cell, or undifferentiated
stem cell. Somatic cells are theoretically distinct from cells of the
germ line, meaning the
mutations they have undergone can never be transmitted to the organism's descendants, though in practice exceptions do exist.
A
molecular biology method used for detecting a specific
sequence in
DNA samples. The technique combines separation of DNA fragments by
gel electrophoresis, transfer of the DNA to a synthetic membrane, and subsequent identification of target fragments with radio-
labeled or fluorescent
hybridization probes.
Also intergenic spacer (IGS) or non-transcribed spacer (NTS).
Any sequence or region of
non-coding DNA separating neighboring
genes, whether
transcribed or not. The term is used in particular to refer to the non-coding regions between the many repeated copies of the
ribosomal RNA genes.[12] See also intergenic region.
spatially-restricted gene expression
The
expression of one or more genes only within a specific anatomical region or tissue, often in response to a
paracrine signal. The boundary between the jurisdictions of two spatially restricted genes may generate a sharp
phenotypic gradient there, as with striping patterns.
The
genetic code used by the vast majority of living organisms for
translatingnucleic acid sequences into
proteins. In this system, of the 64 possible permutations of
three-letter codons that can be made from the four
nucleotides, 61 code for one of the 20
amino acids, and the remaining three code for
stop signals. For example, the codon CAG codes for the amino acid
glutamine and the codon
UAA is a stop codon. The standard genetic code is described as
degenerate or redundant because some amino acids can be coded for by more than one different codon.
A term used to describe the end of a
double-stranded DNA molecule where one
strand is longer than the other by one or more
nucleobases, creating a single-stranded "overhang" of unpaired bases, in contrast to a so-called "
blunt end", where no such overhang exists because the terminal nucleobases on each strand are
base-paired with each other. Blunt ends and sticky ends are relevant when
ligating multiple DNA molecules, e.g. in
restriction cloning, because many
restriction enzymes cut DNA in a way that leaves behind terminal overhangs in the digested fragments. These sticky-ended molecules ligate much more readily with other sticky-ended molecules having
complementary overhangs, allowing scientists to ensure that specific DNA fragments are ligated together in specific places.
A
codon that signals the termination of protein synthesis during
translation of a
messenger RNA transcript. In the
standard genetic code, three different stop codons are used to dissociate
ribosomes from the growing amino acid chain, thereby ending translation: UAG (nicknamed "amber"), UAA ("ochre"), and UGA ("opal"). Contrast start codon.
strand
An individual chain of
nucleotides comprising a
nucleic acid polymer, existing either singly (in which case the nucleic acid molecule is said to be single-stranded) or
paired in a
duplex (in which case it is said to be double-stranded).
stringency
The effect of conditions such as temperature and pH upon the degree of
complementarity that is required for a
hybridization reaction to occur between two single-stranded nucleic acid molecules. In the most stringent conditions, only exact complements can successfully hybridize; as stringency decreases, an increasing number of
mismatches can be tolerated by the two hybridizing strands.[14]
A
protein which as its primary function contributes to the mechanical shape and structure of
cells,
organelles, or
tissues (e.g.
collagen and
actin), as distinguished from proteins which serve some other purpose, such as
enzymes. This distinction is not well-defined, however, as many proteins have both structural and non-structural roles.[2]
The subdivision of the interior of a cell into functionally distinct spaces or
compartments (e.g.
membrane-bound organelles) and the localization or delegation of particular cellular functions and activities to these particular spaces; or the determination by any of various laboratory methods (e.g. fluorescent
labelling) of the precise location(s) within a cell where a specific molecule has occupancy, or at which a specific activity occurs.
(of a linear
chromosome or chromosome fragment) Having a
centromere positioned close to but not exactly in the middle of the chromosome, resulting in
chromatid arms of slightly different lengths.[5] Compare metacentric.
1. A chemical compound or molecule upon which a particular
enzyme directly acts, often but not necessarily binding the molecule by forming one or more chemical bonds.[2] See also ligand.
2. The substance, biotic or abiotic, upon which an organism grows or lives, or by which it is supported; e.g. a particular
growth medium used in
cell culture. See also substratum.
substratum
A solid surface to which a cell or organism adheres or by which it is supported, or over which it moves.[3] See also substrate.
subunit
A single unit of a multi-unit compound or molecular aggregate; e.g. a
monomer from which a larger
polymer is composed (as with
nucleotides in
nucleic acids), or an individual
polypeptide chain in a multi-chain
protein, or an entire protein which participates alongside other proteins as part of a
protein complex.[2][4]
A type of
cell culture in which individual
cells or aggregates of cells are suspended in a gently agitated liquid
growth medium. Many prokaryotic and eukaryotic cell types readily proliferate in suspension cultures, but they are particularly useful for culturing non-adherent cell lines such as
hematopoietic cells, plant cells, and insect cells. Compare adherent culture.
Any of a class of
transmembranetransporter proteins which facilitate the transport of two or more different molecules across the membrane at the same time and in the same direction; e.g. glucose and sodium ions. Contrast antiporter and uniporter.
A
multinucleate cell, i.e. a cell containing more than one
nucleus or, in the broadest sense, more than one
nuclear genome (a meaning which is equated with
polyploidy). Syncytia may form as a result of
cell fusion between uninucleate cells, migration of a nucleus from one cell to another, or multiple nuclear divisions without accompanying
cytokinesis (forming a
coenocyte).[12] The term may also refer to cells which are interconnected by specialized membranes with
gap junctions as in some neuromuscular cell types.
Also synonymous substitution or samesense mutation.
A type of
mutation in which the
substitution of one
nucleotide base for another results, after
transcription and
translation, in an amino acid sequence which is identical to the original unmutated sequence. This is possible because of the
degeneracy of the
genetic code, which allows different
codons to code for the same amino acid. Though synonymous mutations are often considered
silent, this is not always the case; a synonymous mutation may affect the efficiency or accuracy of
transcription,
splicing,
translation, or any other process by which genes are
expressed, and thus become effectively non-silent. Contrast nonsynonymous mutation.
A pattern within a
nucleic acid sequence in which one or more
nucleobases are repeated and the repetitions are directly adjacent (i.e. tandem) to each other. An example is ATGACATGACATGAC, in which the sequence ATGAC is repeated three times.
(of a linear
chromosome or chromosome fragment) Having a
centromere positioned at the terminal end of the chromosome (near or within the
telomere), resulting in only a single arm.[5] Compare acrocentric.
A region of
repetitivenucleotide sequences at each end of a linear
chromosome which protects the end of the chromosome from deterioration and from fusion with other chromosomes. Since each round of
replication results in the shortening of the chromosome, telomeres act as disposable buffers which are sacrificed to perpetual truncation instead of nearby genes; telomeres can also be lengthened by the enzyme
telomerase.
A DNA
sequence or its RNA
complement which signals the termination of
transcription by triggering processes that ultimately arrest the activity of
RNA polymerase or otherwise cause the release of the RNA
transcript from the transcriptional complex. Terminator sequences are usually found near the ends of the
coding sequences of
genes and
operons. They generally function after being themselves transcribed into the nascent RNA strand, whereupon the part of the strand containing the sequence either directly interacts with the transcriptional complex or forms a
secondary structure such as a
hairpin loop which signals the recruitment of enzymes that promote its disassembly.[10]
One of the four standard
nucleosides used in
DNA molecules, consisting of a
thyminebase with its N9 nitrogen
bonded to the C1 carbon of a
deoxyribose sugar. The prefix deoxy- is commonly omitted, since there are no ribonucleoside analogs of thymidine used in RNA, where it is replaced with
uridine instead.
A
pyrimidinenucleobase used as one of the four standard nucleobases in
DNA molecules. Thymine forms a
base pair with
adenine. In
RNA, thymine is not used at all, and is instead replaced with
uracil.
Gene function and
expression which is restricted to a particular
tissue or cell type. Tissue-specific expression is usually the result of an
enhancer which is activated only in the proper cell type.
Any of a class of
DNA-binding enzymes which catalyze changes in the topological state of a
double-stranded DNA molecule by
nicking or
cutting the
sugar-phosphate backbone of one or both strands, relaxing the torsional stress inherent in the
double helix and unwinding or untangling the paired strands before
re-ligating the nicks. This process is usually necessary prior to
replication and
transcription. Topoisomerases thereby convert DNA between its
relaxed and
supercoiled,
linked and unlinked, and
knotted and unknotted forms without changing the sequence or overall chemical composition, such that the substrate and product molecules are structural isomers, differing only in their shape and their
twisting,
linking, and/or
writhing numbers.
A molecule or a specific atom within a molecule that has been chemically or radioactively
labelled so that it can easily be tracked or followed through a biochemical process or located in a cell or tissue.[4]
Affecting a
gene or sequence on a different nucleic acid molecule or
strand. A
locus or sequence within a particular DNA molecule such as a
chromosome is said to be trans-acting if it or its
products influence or act upon other sequences located relatively far away or on an entirely different molecule or chromosome. For example, a
DNA-binding protein acts "in trans" if it binds to or interacts with a sequence located on any strand or molecule different from the one on which it is encoded. Contrast cis-acting.
A product of
transcription; that is, any
RNA molecule which has been synthesized by
RNA polymerase using a complementary
DNA molecule as a
template. When transcription is completed, transcripts separate from the DNA and become independent
primary transcripts. Particularly in eukaryotes, multiple
post-transcriptional modifications are usually necessary for raw transcripts to be converted into stable and persistent molecules, which are then described as mature, though not all transcribed RNAs undergo maturation. Many transcripts are accidental, spurious, incomplete, or defective; others are able to perform their functions immediately and without modification, such as certain
non-coding RNAs.
The first step in the process of
gene expression, in which an
RNA molecule, known as a
transcript, is synthesized by
enzymes called
RNA polymerases using a
gene or other
DNA sequence as a
template. Transcription is a critical and fundamental process in all living organisms and is necessary in order to make use of the information encoded within a
genome. All classes of RNA must be transcribed before they can exert their effects upon a cell, though only
messenger RNA (mRNA) must proceed to
translation before a functional
protein can be produced, whereas the many types of
non-coding RNA fulfill their duties without being translated. Transcription is also not always beneficial for a cell: when it occurs at the wrong time or at a
functionless locus, or when
mobile elements or infectious pathogens utilize the host's transcription machinery, the resulting transcripts (not to mention the waste of valuable energy and resources) are often harmful to the host cell or genome.
Any
protein that controls the rate of
transcription of genetic information from
DNA to
RNA by binding to a specific
DNA sequence and
promoting or
blocking the recruitment of
RNA polymerase to nearby
genes. Transcription factors can effectively turn "on" and "off" specific genes in order to make sure they are
expressed at the right times and in the right places; for this reason, they are a fundamental and ubiquitous mechanism of
gene regulation.
The specific location within a
gene at which
RNA polymerase begins
transcription, defined by the specific nucleotide or codon corresponding to the first ribonucleotide(s) to be assembled in the
nascent transcript (which is not necessarily the same as the
first codon to be
translated). This site is usually considered the beginning of the
coding sequence and is the reference point for numbering the individual nucleotides within a gene. Nucleotides
upstream of the start site are assigned negative numbers and those
downstream are assigned positive numbers, which are used to indicate the positions of nearby sequences or structures relative to the TSS. For example, the
binding site for RNA polymerase might be a short sequence immediately upstream of the TSS, from approximately -80 to -5, whereas an
intron within the coding region might be defined as the sequence starting at nucleotide +207 and ending at nucleotide +793.
transcription unit
The segment of DNA between the
initiation site and the termination site of
transcription, containing the
coding sequences for one or more
genes. All genes within a transcription unit are transcribed together into a single transcript during a single transcription event; the resulting
polycistronic RNA may subsequently be cleaved into separate RNAs, or may be
translated as a unit and then cleaved into separate polypeptides.[10]
The intermittent nature of
transcription and
translation mechanisms. Both processes occur in "bursts" or "pulses", with periods of gene activity separated by irregular intervals.
The entire set of
RNA molecules (often referring to all types of RNA but sometimes exclusively to
messenger RNA) that is or can be
expressed by a particular
genome, cell, population of cells, or species at a particular time or under particular conditions. The transcriptome is distinct from the
exome and the
translatome.
The deliberate experimental introduction of exogenous
nucleic acids into a cell or embryo. In the broadest sense the term may refer to any such transfer and is sometimes used interchangeably with
transformation, though some applications restrict the usage of transfection to the introduction of naked or purified non-viral
DNA or
RNA into cultured eukaryotic cells (especially animal cells) resulting in the subsequent incorporation of the foreign DNA into the host
genome or the non-hereditary modification of
gene expression by the foreign RNA. As a contrast to both standard non-viral transformation and
transduction, transfection has also occasionally been used to refer to the uptake of purified viral nucleic acids by bacteria or plant cells without the aid of a viral vector.[10]
A special class of
RNA molecule, typically 76 to 90
nucleotides in length, that serves as a physical adapter allowing
mRNA transcripts to be
translated into sequences of
amino acids during protein synthesis. Each tRNA contains a specific
anticodon triplet corresponding to an amino acid that is covalently attached to the tRNA's opposite end; as translation proceeds, tRNAs are recruited to the
ribosome, where each mRNA
codon is paired with a tRNA containing the complementary anticodon. Depending on the organism, cells may employ as many as 41 distinct tRNAs with unique anticodons; because of
codon degeneracy within the
genetic code, several tRNAs containing different anticodons carry the same amino acid.
A type of RNA molecule in some bacteria which has dual
tRNA-like and
mRNA-like properties, allowing it to simultaneously perform a number of different functions during
translation.
Any
gene or other segment of genetic material that has been isolated from one organism and then transferred either naturally or by any of a variety of
genetic engineering techniques into another organism, especially one of a different species. Transgenes are usually introduced into the second organism's
germ line. They are commonly used to study gene function or to confer an advantage not otherwise available in the unaltered organism.
A
point mutation in which a
purine nucleotide is substituted for another purine (A ↔ G) or a
pyrimidine nucleotide is substituted for another pyrimidine (C ↔ T). Contrast transversion.
The entire set of
messenger RNA molecules that are
translated by a particular
genome, cell, tissue, or species at a particular time or under particular conditions. Like the
transcriptome, it is often used as a proxy for quantifying levels of
gene expression, though the transcriptome also includes many RNA molecules that are never translated.
Any of a class of self-acting enzymes capable of
binding to the flanking sequences of the
transposable element which encodes them and catalyzing its movement to another part of a genome, typically by an excision/insertion mechanism or a replicative mechanism, in a process known as
transposition.
The process by which a nucleic acid sequence known as a
transposable element changes its position within a
genome, either by excising and re-inserting itself at a different
locus (cut-and-paste) or by
duplicating itself without moving the element from its original locus (copy-paste). These reactions are catalyzed by an enzyme known as a
transposase which is encoded by a gene within the transposable element itself; thus the element's products are self-acting and can autonomously direct their own replication. Transposed sequences may re-insert at random loci or at sequence-specific targets, either on the same DNA molecule or on different molecules.
Any of a class of chemical compounds which are
ester derivatives of
glycerol, consisting of a glycerol backbone connected to any three
fatty acid substituents via ester bonds. Triglycerides are one of three major classes of esters formed by fatty acids in biological systems, along with
phospholipids and
cholesteryl esters. They are the primary constituent of
adipose tissue in vertebrates.
Any sequence in which an individual nucleotide
triplet is
repeated many times
in tandem, whether in a gene or non-coding sequence. At most
loci some degree of repetition is normal and harmless, but mutations which cause specific triplets (especially those of the form Cn
G) to increase in
copy number above the normal range are highly unstable and responsible for a variety of
genetic disorders.
triplet
A unit of three successive
nucleotides in a
DNA or
RNA molecule.[10] A triplet within a coding sequence that codes for a specific amino acid is known as a
codon.
The directional growth or movement of a cell or organism in response to a stimulus, e.g.
light,
heat, the pull of
gravity, or the presence of a particular
chemical, such that the response is dependent on the direction of the stimulus (as opposed to a non-directional
nastic response). Positive tropism is growth or movement toward the stimulus; negative tropism is away from the stimulus.[2] See also taxis and kinesis.
The
labelling of a biomolecule (often another protein) by covalently attaching a
ubiquitin protein to it, thus making it identifiable to molecules capable of recognizing ubiquitin
epitopes.
A type of
transport protein which catalyzes the movement of a single, specific solute or chemical species across a lipid membrane in either direction.[4] Contrast antiporter and symporter.
unique DNA
Also non-repetitive DNA.
A class of DNA
sequences determined by
C0t analysis to be present only once in the analyzed genome, as opposed to
repetitive sequences. Most structural genes and their introns are unique.[10]
Any
non-coding sequence which is transcribed along with a
protein-coding sequence, and thus included within a
messenger RNA, but which is not ultimately
translated during protein synthesis. A typical mRNA transcript includes two such regions: one immediately upstream of the coding sequence, known as the
5' untranslated region (5'-UTR), and one downstream of the coding sequence, known as the
3' untranslated region (3'-UTR). These regions are not removed during
post-transcriptional processing (unlike
introns) and are usually considered distinct from the
5' cap and the
3' polyadenylated tail (both of which are later additions to a primary transcript and not themselves products of transcription). UTRs are a consequence of the fact that transcription usually begins considerably upstream of the
start codon of the coding sequence and terminates long after the
stop codon has been transcribed, whereas translation is more precise. They often include motifs with regulatory functions.
upregulation
Also promotion.
Any process, natural or artificial, which increases the level of
gene expression of a certain
gene. A gene which is observed to be expressed at relatively high levels (such as by detecting higher levels of its
mRNA transcripts) in one sample compared to another sample is said to be upregulated. Contrast downregulation.
A
pyrimidinenucleobase used as one of the four standard nucleobases in
RNA molecules. Uracil forms a
base pair with
adenine. In
DNA, uracil is not used at all, and is instead replaced with
thymine.
One of the four standard
nucleosides used in
RNA molecules, consisting of a
uracilbase with its N9 nitrogen
bonded to the C1 carbon of a
ribose sugar. In
DNA, uridine is replaced with
thymidine.
Any of a class of
enclosed, fluid-filled
compartments present in many eukaryotic cells as well as bacteria, often large and conspicuous under the microscope and serving any of a huge variety of functions, including acting as a resizable reservoir for the storage of water,
metabolic waste, toxins, or foreign material; maintaining cellular
homeostasis and
hydrostatic pressure; supporting immune functions; housing symbiotic bacteria; and assisting in the
degradation and recycling of old cellular components.[2]
Variation or irregularity in a particular
phenotype, especially a conspicuous visible
trait such as color or pigmentation, occurring simultaneously in different parts of the same individual organism due to any of a variety of causes, such as
X-inactivation,
mitotic recombination,
transposable element activity,
position effects, or infection by pathogens.
Any
membrane-bound space completely enclosed by its own
membrane, which is separate though usually derived from other membranes (often the
cell membrane) either by
budding or by mechanical disruption such as
sonication.[3] The term is applied to many different structures but especially to the small, roughly spherical
compartments created during
endocytosis and
exocytosis, as well as to
lysosomes and various other small intracellular or extracellular organelles.[2]
The process of
determining the entirety or near-entirety of the DNA sequences comprising an organism's
genome with a single procedure or experiment, generally inclusive of all
chromosomal and
extrachromosomal (e.g.
mitochondrial) DNA.
The
phenotype of the typical form of a
species as it occurs in nature; a product of the standard "normal"
allele at a given
locus, as opposed to that produced by a non-standard
mutant allele.
One of two
sex chromosomes present in organisms which use the
XY sex-determination system, and the only sex chromosome in the
X0 system. The X chromosome is found in both males and females and typically contains much more
gene content than its counterpart, the
Y chromosome.
The process by which one of the two copies of the
X chromosome is silenced by being irreversibly condensed into transcriptionally inactive
heterochromatin in the cells of female therian mammals. A form of
dosage compensation, X-inactivation prevents females from producing twice as many
gene products from genes on the X chromosome as males, who only have one copy of the X chromosome. Which X chromosome is inactivated is randomly determined in the early embryo, making it possible for cell lineages with different inactive Xs to exist in the same organism.
The degree to which multiple copies of a
gene,
chromosome, or
genome have the same genetic sequence; e.g. in a diploid organism with two complete copies of its genome (one maternal and one paternal), the degree of similarity of the
alleles present in each copy. Individuals carrying two different alleles for a particular gene are said to be heterozygous for that gene; individuals carrying two identical alleles are said to be homozygous for that gene. Zygosity may also be considered collectively for a group of genes, or for the entire set of genes and genetic
loci comprising the genome.
A type of
eukaryotic cell formed as the direct result of a
fertilization event between two
gametes. In multicellular organisms, the zygote is the earliest developmental stage.
^
abcdefghijklmnopqLackie, J. M. (2013). The Dictionary of Cell and Molecular Biology (5th ed.). Amsterdam: Academic Press/Elsevier.
ISBN978-0-12-384931-1.
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abcdefghijklmnoAlberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002).
Molecular Biology of the Cell(Available from the National Center for Biotechnology Information) (4th ed.). New York: Garland Science.
^
abcKlug, William S.; Cummings, Michael R. (1986). Concepts of Genetics (2nd ed.). Glenview, Ill.: Scott, Foresman and Company.
ISBN0-673-18680-6.
^
abLewin, Benjamin (2003). Genes VIII. Upper Saddle River, NJ: Pearson Prentice Hall.
ISBN0-13-143981-2.
^"overexpression". Oxford Living Dictionary. Oxford University Press. 2017. Archived from
the original on February 10, 2018. Retrieved 18 May 2017. The production of abnormally large amounts of a substance which is coded for by a particular gene or group of genes; the appearance in the phenotype to an abnormally high degree of a character or effect attributed to a particular gene.
^"overexpress". NCI Dictionary of Cancer Terms. National Cancer Institute at the National Institutes of Health. 2011-02-02. Retrieved 18 May 2017. overexpress In biology, to make too many copies of a protein or other substance. Overexpression of certain proteins or other substances may play a role in cancer development.