Glossary of Genetics

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3'-end

Also rendered as three-prime end.

The end of a single strand of DNA or RNA at which the chain of nucleotides terminates at the third carbon atom in the furanose ring of deoxyribose or ribose (i.e. the terminus at which the 3' carbon is not attached to another nucleotide via a phosphodiester bond; in vivo, the 3' carbon is often still bonded to a hydroxyl group). By convention, sequences and structures positioned nearer to the 3'-end relative to others are referred to as downstream. Contrast 5'-end.

A ribose ring with the carbon atoms numbered 1' through 5' according to chemical convention. The 5' carbon is said to be upstream; the 3' carbon is said to be downstream. Bonds to a generic base and a phosphate group are also shown.

5' cap

Also rendered as five-prime cap.

A specially altered nucleotide attached to the 5'-end of some primary RNA transcripts as part of the set of post-transcriptional modifications which convert raw transcripts into mature RNA products. The precise structure of the 5' cap varies widely by organism; in eukaryotes, the most basic cap consists of a methylated guanine nucleoside bonded to the triphosphate group that terminates the 5'-end of an RNA sequence. Among other functions, capping helps to regulate the export of mature RNAs from the nucleus, prevent their degradation by exonucleases, and promote translation in the cytoplasm. Mature mRNAs can also be decapped.

5'-end

Also rendered as five-prime end.

The end of a single strand of DNA or RNA at which the chain of nucleotides terminates at the fifth carbon atom in the furanose ring of deoxyribose or ribose (i.e. the terminus at which the 5' carbon is not attached to another nucleotide via a phosphodiester bond; in vivo, the 5' carbon is often still bonded to a phosphate group). By convention, sequences and structures positioned nearer to the 5'-end relative to others are referred to as upstream. Contrast 3'-end.

activator

A type of transcription factor that increases the transcription of a gene or set of genes. Most activators work by binding to a specific sequence located within or near an enhancer or promoter and facilitating the binding of RNA polymerase and other transcription machinery in the same region. See also coactivator; contrast repressor.

adenine

Abbreviated in shorthand with the letter A.

One of the four main nucleobases present in DNA and RNA. Adenine forms a base pair with thymine in DNA and with uracil in RNA.

affected relative pair

Any pair of organisms which are related genetically and both affected by the same trait. For example, two cousins who both have blue eyes are an affected relative pair since they are both affected by the allele that codes for blue eyes.

allele

One of multiple alternative versions of an individual gene, each of which is a viable DNA sequence occupying a given position, or locus, on a chromosome. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.

allele frequency

The relative frequency with which a particular allele of a given gene (as opposed to other alleles of the same gene) occurs at a particular locus in the members of a population; more specifically, it is the proportion of all chromosomes within a population that carry a particular allele, expressed as a fraction or percentage. Allele frequency is distinct from genotype frequency, although they are related.

allosome

Also called a sex chromosome, heterochromosome, or idiochromosome.

Any chromosome that differs from an ordinary autosome in size, form, or behavior and which is responsible for determining the sex of an organism. In humans, the X chromosome and the Y chromosome are sex chromosomes.

alternative splicing

Also called differential splicing or simply splicing.

A regulated phenomenon of eukaryotic gene expression in which specific exons or parts of exons from the same primary transcript are variably included within or removed from the final, mature messenger RNA transcript. A class of post-transcriptional modification, alternative splicing allows a single gene to code for multiple protein isoforms and greatly increases the diversity of proteins that can be produced by an individual genome. See also RNA splicing.

amino acid

An organic compound containing amine and carboxyl functional groups, as well as a side chain specific to each individual amino acid. Out of nearly 500 known amino acids, a set of 20 are coded for by the standard genetic code and incorporated in sequence as the building blocks of polypeptides and hence of proteins. The specific sequence of amino acids in the polypeptide chains that form a protein are ultimately responsible for determining the protein's structure and function.

anaphase

The stage of mitosis and meiosis that occurs after metaphase and before telophase, when the replicated chromosomes are segregated and each of the sister chromatids are moved to opposite sides of the cell.

aneuploidy

The condition of a cell or organism having an abnormal number of one or more specific individual chromosomes (but excluding abnormal numbers of complete sets of chromosomes, which instead is known as euploidy).

anticipation

A phenomenon by which the symptoms of a genetic disorder become apparent (and often more severe) at an earlier age in affected individuals with each generation that inherits the disorder.

anticodon

A series of three consecutive nucleotides within a transfer RNA which complement the three nucleotides of a codon within an mRNA transcript. During translation, each tRNA recruited to the ribosome contains a single anticodon triplet that pairs with one or more complementary codons from the mRNA sequence, allowing each codon to specify a particular amino acid to be added to the growing peptide chain. Anticodons containing inosine in the first position are capable of pairing with more than one codon due to a phenomenon known as wobble base pairing.

antiparallel

The orientation of two strands of a double-stranded nucleic acid (and more generally any pair of biopolymers) which are parallel to each other but with opposite directionality. For example, the two complementary strands of a DNA molecule run side-by-side but in opposite directions, with one strand oriented 5'-to-3' and the other 3'-to-5'.

antisense

See [template strand](https://en.wikipedia.org/wiki/Glossary_of_genetics#template_strand).

autosome

Any chromosome that is not an allosome and hence is not involved in the determination of the sex of an organism. Unlike the sex chromosomes, the autosomes in a diploid cell exist in pairs, with the members of each pair having the same structure, morphology, and genetic loci.

back mutation

A mutation that reverses the effect of a previous mutation which had inactivated a gene, thus restoring wild-type function.[[2]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-Lewin-2)

backcrossing

Also called testcrossing.

The breeding of a hybrid organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type of selective breeding, with the aim of producing offspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as a backcross, often abbreviated in genetics shorthand with the symbol BC.

base pair (bp)

A pair of two nucleobases on complementary DNA or RNA strands which are bonded to each other by hydrogen bonds. The ability of consecutive base pairs to stack one upon another contributes to the long-chain double helix structures observed in both double-stranded DNA and double-stranded RNA molecules.

baseline

A measure of the gene expression level of a gene or genes prior to a perturbation in an experiment, as in a negative control. Baseline expression may also refer to the expected or historical measure of expression for a gene.

canalisation

The ability of a population to consistently produce the same phenotype regardless of the variability of its environment or the genetic variation within its genome. The concept is most often used in developmental biology to interpret the observation that developmental pathways are frequently shaped by natural selection such that developing cell lineages are "guided" or "canalized" towards a single, definite fate, regardless of any minor variations that may disturb the cells during development.

candidate gene

A gene whose location on a chromosome is associated with a particular phenotype (often a disease-related phenotype), and which is therefore suspected of causing or contributing to the phenotype. Candidate genes are often selected for study based on a priori knowledge or speculation about their functional relevance to the trait or disease being researched.

carrier

An individual who has inherited a recessive allele for a genetic trait or mutation but in whom the trait is not usually expressed or observable in the phenotype. Carriers are usually heterozygous for the recessive allele and therefore still able to pass the allele onto their offspring, where the associated phenotype may reappear if the offspring inherits another copy of the allele. The term is commonly used in medical genetics in the context of a disease-causing recessive allele.

cellular reprogramming

The conversion of a cell from one tissue-specific cell type to another. This involves dedifferentiation to a pluripotent state; an example is the conversion of mouse somatic cells to an undifferentiated embryonic state, which relies on the transcription factors Oct4, Sox2, Myc, and Klf4.[[3]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-3)

centimorgan (cM)

Also called a map unit (m.u.).

A unit for measuring genetic linkage defined as the distance between chromosomal loci for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. Though it is not an actual measure of physical distance, it is used to infer the distance between two loci based on the apparent likelihood of a crossover occurring between them.

centromere

The part of a chromosome that links a pair of sister chromatids. During mitosis, spindle fibers attach to the centromere via kinetochores.

chimerism

The presence of two or more populations of cells with distinct genotypes in an individual organism, known as a chimera, which has developed from the fusion of cells originating from separate zygotes; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical issues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result from grafting or errors in cell division. It is similar to but distinct from mosaicism.

chromatid

One copy of a newly copied chromosome, which is joined to the original chromosome by a centromere.

chromatin

A complex of DNA, RNA, and protein found in eukaryotic cells that is the primary substance comprising chromosomes. Chromatin functions as a means of packaging very long DNA molecules into highly organized and densely compacted shapes, which prevents the strands from becoming tangled, reinforces the DNA during cell division, helps to prevent DNA damage, and plays an important role in regulating gene expression and DNA replication.

chromosomal duplication

The duplication of an entire chromosome, as opposed to a segment of a chromosome or an individual gene.

chromosome

A DNA molecule containing part or all of the genetic material of an organism. Chromosomes may be considered a sort of molecular "package" for carrying DNA within the nucleus of cells and, in most eukaryotes, are composed of long strands of DNA coiled with packaging proteins which bind to and condense the strands to prevent them from becoming an unmanageable tangle. Chromosomes are most easily distinguished and studied in their completely condensed forms, which only occur during cell division. Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA.

cis-dominant mutation

A mutation occurring within a cis-regulatory element (such as an operator) which alters the functioning of a nearby gene or genes on the same strand of DNA. Cis-dominant mutations affect the expression of genes because they occur at sites that control transcription rather than within the genes themselves.

cis-regulatory element (CRE)

Any region of non-coding DNA which regulates the transcription of nearby genes, typically by serving as a binding site for one or more transcription factors. Contrast [trans-regulatory element](https://en.wikipedia.org/wiki/Glossary_of_genetics#trans-regulatory_element).

classical genetics

The branch of genetics based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of molecular biology. Contrast [molecular genetics](https://en.wikipedia.org/wiki/Glossary_of_genetics#molecular_genetics).

cloning

The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of asexual reproduction, and cells that undergo mitosis produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in molecular cloning, copies of DNA fragments or other molecules.

coactivator

A type of coregulator that increases the expression of one or more genes by binding to an activator.

coding strand

Also sense strand, positive (+) sense strand, and nontemplate strand.

The strand of a double-stranded DNA molecule whose nucleotide sequence corresponds directly to that of the RNA transcript produced during transcription (except that thymine bases are substituted with uracil bases in the RNA molecule). Though it is not itself transcribed, the coding strand is by convention the strand used when displaying a DNA sequence because of the direct analogy between its sequence and the codons of the RNA product. Contrast [template strand](https://en.wikipedia.org/wiki/Glossary_of_genetics#template_strand); see also [sense](https://en.wikipedia.org/wiki/Glossary_of_genetics#sense).

codon

A series of three consecutive nucleotides in a coding region of a nucleic acid sequence. Each of these triplets codes for a particular amino acid or stop signal during protein synthesis. DNA and RNA molecules are each written in a language using four "letters" (four different nucleobases), but the language used to construct proteins includes 20 "letters" (20 different amino acids). Codons provide the key that allows these two languages to be translated into each other. In general, each codon corresponds to a single amino acid (or stop signal), and the full set of codons is called the genetic code.

cofactor

Any non-protein organic compound that is bound to an enzyme. Cofactors are required for the initiation of catalysis.

complementarity

A property of nucleic acid biopolymers whereby two polymeric chains (or "strands") aligned antiparallel to each other will tend to form base pairs consisting of hydrogen bonds between the individual nucleobases comprising each chain, with each of the four types of nucleobase pairing exclusively with one other type of nucleobase; e.g. in double-stranded DNA molecules, A pairs only with T and C pairs only with G. Strands that are paired in such a way, and the bases themselves, are said to be complementary. The degree of complementarity between two strands strongly influences the stability of the duplex molecule; certain sequences may also be internally complementary, which can result in a single strand binding to itself. Complementarity is fundamental to the mechanisms governing DNA replication, transcription, and DNA repair.

complementary DNA (cDNA)

DNA that is synthesized from a single-stranded RNA template (typically mRNA or miRNA) in a reaction catalyzed by the enzyme reverse transcriptase. cDNA is produced both naturally by retroviruses and artificially in certain laboratory techniques, particularly molecular cloning. In bioinformatics, the term may also be used to refer to the sequence of an mRNA transcript expressed as its DNA coding strand counterpart (i.e. with thymine replacing uracil).

complex trait

See [quantitative trait](https://en.wikipedia.org/wiki/Glossary_of_genetics#quantitative_trait).

conditional expression

The controlled, inducible expression of a transgene, either [in vitro](https://en.wikipedia.org/wiki/In_vitro) or [in vivo](https://en.wikipedia.org/wiki/In_vivo).

consensus sequence

Also called a canonical sequence.

A calculated order of the most frequent residues (of either nucleotides or amino acids) found at each position in a common sequence alignment and obtained by comparing multiple closely related sequence alignments.

conservation genetics

An interdisciplinary branch of population genetics which applies genetic methods and concepts in an effort to understand the dynamics of genes in populations, principally in order to avoid extinctions and to conserve and restore biodiversity.

conserved sequence

A nucleic acid or protein sequence that is highly similar or identical across many species or within a genome, indicating that it has remained relatively unchanged through a long period of evolutionary time.

constitutive expression

The continuous transcription of a gene, as opposed to facultative expression, in which a gene is only transcribed as needed. A gene that is transcribed continuously is called a constitutive gene.

contig

A continuous stretch of genomic DNA generated by assembling cloned fragments by means of their overlaps.[[2]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-Lewin-2)

copy-number variation (CNV)

A phenomenon in which sections of a genome are repeated and the number of repeats varies between individuals in the population, usually as a result of duplication or deletion events that affect entire genes or sections of chromosomes. Copy-number variations play an important role in generating genetic variation within a population.

coregulator

A protein that works together with one or more transcription factors to regulate gene expression.

corepressor

A type of coregulator that reduces (represses) the expression of one or more genes by binding to and activating a repressor.

crossbreeding

Also called [outbreeding](https://en.wikipedia.org/wiki/Glossary_of_genetics#outbreeding).

The breeding of purebred parents belonging to two different breeds, varieties, or populations, often intentionally as a type of selective breeding, with the aim of producing offspring which share traits of both parent lineages or which show heterosis. In animal breeding, the progeny of a cross between breeds of the same species is called a crossbreed, whereas the progeny of a cross between different species is called a hybrid.

cytogenetics

The branch of genetics that studies how chromosomes influence and relate to cell behavior and function, particularly during mitosis and meiosis.

cytosine

Abbreviated in shorthand with the letter C.

One of the four main nucleobases present in DNA and RNA. Cytosine forms a base pair with guanine.

degeneracy

The redundancy of the genetic code, exhibited as the multiplicity of different codons that specify the same amino acid. For example, in the standard genetic code, the amino acid serine is specified by six unique codons (UCA, UCG, UCC, UCU, AGU, and AGC). Codon degeneracy accounts for the existence of synonymous mutations.

deletion

Denoted in shorthand with the symbol *[Δ](https://en.wikipedia.org/wiki/Delta_(letter%29)*.

A type of mutation in which one or more bases are removed from a nucleic acid sequence.

deoxyribonucleic acid (DNA)

A polymeric nucleic acid molecule composed of a series of deoxyribonucleotides which incorporate a set of four nucleobases: adenine (A), guanine (G), cytosine (C), and thymine (T). DNA is most often found in the form of a "double helix", which consists of two paired complementary DNA molecules resembling a ladder that has been twisted. The "rungs" of the ladder are made of pairs of nucleobases.

diploid

Denoted in shorthand with the somatic number *2n.

(of a [cell](https://en.wikipedia.org/wiki/Cell_(biology%29) or organism) Having two [homologous](https://en.wikipedia.org/wiki/Glossary_of_genetics#homologous_chromosomes) copies of each [chromosome](https://en.wikipedia.org/wiki/Glossary_of_genetics#chromosome). Contrast haploid and polyploid*.

distance measure

Any quantity used to measure the dissimilarity between the gene expression levels of different genes.[[4]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-Priness-4)

DNA condensation

The process of compacting very long DNA molecules into densely packed, orderly configurations such as chromosomes, either in vivo or in vitro.

DNA microarray

A high-throughput technology used to measure expression levels of mRNA transcripts or to detect certain changes in the nucleotide sequence. It consists of an array of thousands of microscopic spots of DNA oligonucleotides, called features, each containing picomoles of a specific DNA sequence. This can be a short section of a gene or other DNA element that is used as a probe to hybridize a cDNA, cRNA or genomic DNA sample (called a target) under high-stringency conditions. Probe-target hybridization is usually detected and quantified by fluorescence-based detection of fluorophore-labeled targets.

DNA polymerase

Any of a class of enzymes that synthesizes DNA molecules from individual deoxyribonucleotides. DNA polymerases are essential for DNA replication and usually work in pairs to create identical copies of the two strands of an original double-stranded molecule. They build long chains of DNA by adding nucleotides one at a time to the 3'-end of a DNA strand, usually relying on the template provided by the complementary strand to copy the nucleotide sequence faithfully.

DNA repair

The collection of processes by which a cell identifies and corrects structural damage or mutations in the DNA molecules that encode its genome. The ability of a cell to repair its DNA is vital to the integrity of the genome and the normal functionality of the organism.

DNA replication

The process by which a DNA molecule copies itself, producing two identical copies of one original DNA molecule.

DNA sequencing

The process of determining, by any of a variety of different methods and technologies, the order of the bases in the long chain of nucleotides that constitutes a sequence of DNA.

dominance

A relationship between the alleles of a gene in which one allele produces an effect on phenotype that overpowers or "masks" the contribution of another allele at the same locus; the first allele and its associated phenotypic trait are said to be dominant, and the second allele and its associated trait are said to be [recessive](https://en.wikipedia.org/wiki/Glossary_of_genetics#recessiveness). Often, the dominant allele codes for a functional protein while its recessive counterpart does not. Dominance is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes; it is possible for one allele to be simultaneously dominant over a second allele, recessive to a third, and codominant to a fourth. In genetics shorthand, dominant alleles are often represented by a single uppercase letter (e.g. "A", in contrast to the recessive "a").

dosage compensation

Any mechanism by which organisms neutralize the large difference in gene dosage caused by the presence of differing numbers of sex chromosomes in the different sexes, thereby equalizing the expression of sex-linked genes so that the members of each sex receive the same or similar amounts of the products of such genes. An example is X-inactivation in female mammals.

double-stranded DNA (dsDNA)

Any DNA molecule that is composed of two antiparallel, complementary nucleotide polymers, or "strands", which are bonded together by hydrogen bonds between the complementary nucleobases. Though it is possible for DNA to exist as a single strand, it is generally more stable and more common in double-stranded form. In most cases, the complementary base pairing causes the twin strands to coil around each other in the shape of a double helix.

downregulation

Also called repression or suppression.

Any process, natural or artificial, which decreases the level of gene expression of a certain gene. A gene which is observed to be expressed at relatively low levels (such as by detecting lower levels of its mRNA transcripts) in one sample compared to another sample is said to be downregulated. Contrast [upregulation](https://en.wikipedia.org/wiki/Glossary_of_genetics#upregulation).

downstream

Towards or closer to the 3'-end of a chain of nucleotides. Contrast [upstream](https://en.wikipedia.org/wiki/Glossary_of_genetics#upstream).

ecological genetics

The study of genetics as it pertains to the ecology and fitness of natural populations of living organisms.

emergenesis

The quality of genetic traits that results from a specific configuration of interacting genes, rather than simply their combination.

endonuclease

Any enzyme whose activity is to cleave phosphodiester bonds within a chain of nucleotides, including those that cleave relatively nonspecifically (without regard to sequence) and those that cleave only at very specific sequences (so-called restriction endonucleases). When recognition of a specific sequence is required, endonucleases make their cuts in the middle of the sequence. Contrast [exonuclease](https://en.wikipedia.org/wiki/Glossary_of_genetics#exonuclease).

enhancer

A region of DNA near a gene that can be bound by an activator to increase gene expression or by a repressor to decrease expression.

episome

1. Another name for a plasmid, especially one that is capable of integrating into a chromosome.
2. In eukaryotes, any non-integrated extrachromosomal circular DNA molecule that is stably maintained and replicated in the nucleus simultaneously with the rest of the host cell. Such molecules may include viral genomes, bacterial plasmids, and aberrant chromosomal fragments.

epistasis

The collective action of multiple genes interacting during gene expression. A form of gene action, epistasis can be either additive or multiplicative in its effects on specific phenotypic traits.

euploidy

The condition of a cell or organism having an abnormal number of complete sets of chromosomes, possibly excluding the sex chromosomes. Euploidy differs from aneuploidy, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.

evolution

The change in the heritable characteristics of biological populations over successive generations. In the most traditional sense, it occurs by changes in the frequencies of alleles in a population's gene pool.

exome

The entire set of exons within a particular genome, including untranslated regions of mature mRNAs as well as coding regions.

exon

Any part of a gene that encodes a part of the final mature mRNA produced by that gene after introns have been removed by alternative splicing. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts.

exonuclease

Any enzyme whose activity is to cleave phosphodiester bonds within a chain of nucleotides, including those that cleave only upon recognition of a specific sequence (so-called restriction exonucleases). Exonucleases make their cuts at either the 3' or 5'-end of the sequence (rather than in the middle, as with endonucleases).

expression vector

Also called an expression construct.

A type of vector, usually a plasmid or viral vector, designed specifically for the expression of a transgene insert in a target cell, rather than for some other purpose such as cloning.

expressivity

For a given genotype associated with a variable non-binary phenotype, the proportion of individuals with that genotype who show or express the phenotype to a specified extent, usually given as a percentage. Because of the many complex interactions that govern gene expression, the same allele may produce a wide variety of possible phenotypes of differing qualities or degrees in different individuals; in such cases, both the phenotype and genotype may be said to show variable expressivity. Expressivity attempts to quantify the range of possible levels of phenotypic variation in a population of individuals expressing the phenotype of interest. Compare [penetrance](https://en.wikipedia.org/wiki/Glossary_of_genetics#penetrance).

extrachromosomal DNA

Also called extranuclear DNA or cytoplasmic DNA.

Any DNA that is not found in chromosomes or in the nucleus of a cell and hence is not genomic DNA. This may include the DNA contained in plasmids or organelles such as mitochondria or chloroplasts, or, in the broadest sense, DNA introduced by viral infection. Extrachromosomal DNA usually shows significant structural differences from nuclear DNA in the same organism.

facultative expression

The transcription of a gene only as needed, as opposed to constitutive expression, in which a gene is transcribed continuously. A gene that is transcribed as needed is called a facultative gene.

fixation

The process by which a single allele for a particular gene with multiple different alleles increases in frequency in a given population such that it becomes permanently established at 100% frequency – that is, the only allele at that locus within the population's gene pool. In the absence of mutation and heterozygote advantage, any given allele is eventually destined to become either permanently fixed over all other variants or completely lost from the population, though how long this takes depends on selection pressures and chance fluctuations in allele frequencies.

frameshift mutation

A type of mutation in a nucleic acid sequence caused by the insertion or deletion of a number of nucleotides that is not divisible by three. Because of the triplet nature by which nucleotides code for amino acids, a mutation of this sort causes a shift in the reading frame of the nucleotide sequence, resulting in the sequence of codons downstream of the mutation site being completely different from the original.

Functional Genomics Data (FGED) Society

Formerly known by the abbreviation MGED.

An organization that works with others "to develop standards for biological research data quality, annotation and exchange" as well as software tools that facilitate their use.[[5]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-urlFGED_Homepage-5)

G banding

Also Giemsa banding or G-banding.

A technique used in cytogenetics to produce a visible karyotype by staining the condensed chromosomes with Giemsa stain. The staining produces consistent and identifiable patterns of dark and light "bands" in regions of chromatin, which allows specific chromosomes to be easily distinguished.

gene

Any segment or set of segments of a nucleic acid molecule that contains the information necessary to produce a functional RNA transcript in a controlled manner. In living organisms, genes are often considered the fundamental units of heredity and are typically encoded in DNA. A particular gene can have multiple different versions, or alleles, and a single gene can result in a gene product that influences many different phenotypes.

gene dosage

The number of copies of a particular gene present in a genome. Gene dosage directly influences the amount of gene product a cell is able to express, though a variety of controls have evolved which tightly regulate gene expression. Changes in gene dosage caused by mutations include copy-number variations.

gene duplication

Also called gene amplification.

A type of mutation defined as any duplication of a region of DNA that contains a gene. Compare [chromosomal duplication](https://en.wikipedia.org/wiki/Glossary_of_genetics#chromosomal_duplication).

gene expression

The process by which the information encoded in a gene is converted into a form useful for the cell. The first step is transcription, which produces a messenger RNA molecule complementary to the DNA molecule in which the gene is encoded. For protein-coding genes, the second step is translation, in which the messenger RNA is read by the ribosome to produce a protein.

Gene Expression Omnibus (GEO)

A database for gene expression managed by the National Center for Biotechnology Information.

gene mapping

Any of a variety of methods used to precisely identify the location of a particular gene within a DNA molecule (such as a chromosome) and/or the physical or linkage distances between it and other genes.

gene pool

The sum of all of the various alleles shared by the members of a single population.

gene product

Any of the biochemical material resulting from the expression of a gene, most often interpreted as the functional mRNA transcript produced by transcription of the gene or the fully constructed protein produced by translation of the transcript. A measurement of the quantity of a given gene product that is detectable in a cell or tissue is sometimes used to infer how active the corresponding gene is.

gene regulation

The broad range of mechanisms used by cells to increase or decrease the production or expression of specific gene products, such as RNA or proteins. Gene regulation increases an organism's versatility and adaptability by allowing its cells to express different gene products when required by changes in its environment. In multicellular organisms, the regulation of gene expression also drives cellular differentiation and morphogenesis in the embryo, enabling the creation of a diverse array of cell types from the same genome.

gene silencing

Any mechanism of gene regulation which drastically reduces or completely prevents the expression of a particular gene. Gene silencing may occur naturally during either transcription or translation. Laboratory techniques often exploit natural silencing mechanisms to achieve gene knockdown.

gene trapping

A high-throughput technology used to simultaneously inactivate, identify, and report the expression of a target gene in a mammalian genome by introducing an insertional mutation consisting of a promoterless reporter gene and/or a selectable genetic marker flanked by an upstream splice site and a downstream polyadenylated termination sequence.

genetic association

The co-occurrence within a population of one or more alleles or genotypes with a particular phenotypic trait more often than might be expected by chance alone; such statistical correlation may be used to infer that the alleles or genotypes are responsible for producing the given phenotype.

genetic code

A set of rules by which information encoded within nucleic acids is translated into proteins by living cells. These rules define how sequences of nucleotide triplets called codons specify which amino acid will be added next during protein synthesis. The vast majority of living organisms use the same genetic code (sometimes referred to as the "standard" genetic code) but variant codes do exist.

genetic counseling

The process of advising individuals or families who are affected by or at risk of developing genetic disorders in order to help them understand and adapt to the physiological, psychological, and familial implications of genetic contributions to disease. Genetic counseling integrates genetic testing, genetic genealogy, and genetic epidemiology.[[6]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-pmid16761103-6)

genetic distance

A measure of the genetic divergence between species, populations within a species, or individuals, used especially in phylogenetics to express either the time elapsed since the existence of a common ancestor or the degree of differentiation in the DNA sequences comprising the genomes of each population or individual.

genetic diversity

Sometimes used interchangeably with [genetic variation](https://en.wikipedia.org/wiki/Glossary_of_genetics#genetic_variation).

The total number of genetic traits or characteristics in the genetic make-up of a population, species, or other group of organisms. It is often used as a measure of the adaptability of a group to changing environments. Genetic diversity is similar to, though distinct from, genetic variability.

genetic drift

Also called allelic drift or the Sewall Wright effect.

A change in the frequency with which an existing allele occurs in a population due to random variation in the distribution of alleles from one generation to the next. It is often interpreted as the role that random chance plays in determining whether a given allele becomes more or less common with each generation, regardless of the influence of natural selection. Genetic drift may cause certain alleles, even otherwise advantageous ones, to disappear completely from the gene pool, thereby reducing genetic variation, or it may cause initially rare alleles, even neutral or deleterious ones, to become much more frequent or even fixed.

genetic engineering

Also called genetic modification or genetic manipulation.

The direct, deliberate manipulation of an organism's genetic material using any of a variety of biotechnology methods, including the insertion or removal of genes, the transfer of genes within and between species, the mutation of existing sequences, and the construction of novel sequences using artificial gene synthesis. Genetic engineering encompasses a broad set of technologies by which the genetic composition of individual cells, tissues, or entire organisms may be altered for various purposes, commonly in order to study the functions and expression of individual genes, to produce hormones, vaccines, and other drugs, and to create genetically modified organisms for use in research and agriculture.

genetic genealogy

The use of genealogical DNA testing in combination with traditional genealogical methods to infer the level and type of genetic relationships between individuals, find ancestors, and construct family trees, genograms, or other genealogical charts.

genetic hitchhiking

Also called genetic draft or the hitchhiking effect.

A type of linked selection by which the positive selection of an allele undergoing a selective sweep causes alleles for different genes at nearby loci to change frequency as well, allowing them to "hitchhike" to fixation along with the positively selected allele. If selection at the first locus is strong enough, neutral or even slightly deleterious alleles within the same linkage group may undergo the same positive selection because the physical distance between the nearby loci is small enough that a recombination event is unlikely to occur between them. Genetic hitchhiking is often considered the opposite of background selection.

genetic marker

A specific, easily identifiable, and usually highly polymorphic gene or other DNA sequence with a known location on a chromosome that can be used to identify the individual or species possessing it.

genetic recombination

Any reassortment or exchange of genetic material within an individual organism or between individuals of the same or different species, especially that which creates genetic variation. In the broadest sense, the term encompasses a diverse class of naturally occurring mechanisms by which nucleic acid sequences are copied or physically transferred into different genetic environments, including homologous recombination during meiosis or mitosis or as a normal part of DNA repair; horizontal gene transfer events such as bacterial conjugation, viral transduction, or transformation; or errors in DNA replication or cell division. Artificial recombination is central to many genetic engineering techniques which produce recombinant DNA.

genetic regulatory network (GRN)

A graph that represents the regulatory complexity of gene expression. The vertices (nodes) are represented by various regulatory elements and gene products while the edges (links) are represented by their interactions. These network structures also represent functional relationships by approximating the rate at which genes are transcribed.

genetic testing

Also called DNA testing or genetic screening.

A broad class of various procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or ancestry, diagnose vulnerabilities to heritable diseases, or detect mutant alleles associated with increased risks of developing genetic disorders. Genetic testing is widely used in human medicine, agriculture, and biological research.

genetic variability

Sometimes used interchangeably with [genetic variation](https://en.wikipedia.org/wiki/Glossary_of_genetics#genetic_variation).

The formation or the presence of individuals differing in genotype within a population or other group of organisms, as opposed to individuals with environmentally induced differences, which cause only temporary, non-heritable changes in phenotype. Barring other limitations, a population with high genetic variability has a greater potential for successful adaptation to changing environmental conditions than a population with low genetic variability. Genetic variability is similar to, though distinct from, genetic diversity.

genetic variation

Sometimes used interchangeably with [genetic diversity](https://en.wikipedia.org/wiki/Glossary_of_genetics#genetic_diversity) and [genetic variability](https://en.wikipedia.org/wiki/Glossary_of_genetics#genetic_variability).

The genetic differences both within and between populations, species, or other groups of organisms. It is often visualized as the variety of different alleles in the gene pools of different populations.

genetically modified organism (GMO)

Any organism whose genetic material has been altered using genetic engineering techniques, particularly in a way that does not occur naturally by mating or by natural genetic recombination.

genetics

The field of biology that studies genes, genetic variation, and heredity in living organisms.

genome

The entire complement of genetic material contained within the chromosomes of an organism, organelle, or virus. The term is also used to refer to the collective set of genetic loci shared by every member of a population or species, regardless of the different alleles that may be present at these loci in different individuals.

genome size

The total amount of DNA contained within one copy of a genome, typically measured by mass (in picograms or daltons) or by the total number of base pairs (in kilobases or megabases). For diploid organisms, genome size is often used interchangeably with C-value.

genomic DNA (gDNA)

Also called chromosomal DNA.

The DNA contained in chromosomes, as opposed to the extrachromosomal DNA contained in separate structures such as plasmids or organelles such as mitochondria or chloroplasts.

genomic imprinting

An epigenetic phenomenon that causes genes to be expressed in a manner dependent upon the particular parent from which the gene was inherited. It occurs when epigenetic marks such as DNA or histone methylation are established or "imprinted" in the germ cells of a parent organism and subsequently maintained through cell divisions in the somatic cells of the organism's progeny; as a result, a gene in the progeny that was inherited from the father may be expressed differently than another copy of the same gene that was inherited from the mother.

genomics

An interdisciplinary field that studies the structure, function, evolution, mapping, and editing of entire genomes, as opposed to individual genes.

genotoxicity

The ability of certain chemical agents to cause damage to genetic material within a living cell (e.g. through single- or double-stranded breaks, crosslinking, or point mutations), which may or may not result in a permanent mutation. Though all mutagens are genotoxic, not all genotoxic compounds are mutagenic.

genotype

The entire complement of alleles present in a particular individual's genome, which gives rise to the individual's phenotype.

genotyping

The process of determining differences in the genotype of an individual by examining the DNA sequences in the individual's genome using bioassays and comparing them to another individual's sequences or a reference sequence.

germ cell

Any biological cell that gives rise to the gametes of an organism that reproduces sexually. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from somatic cells, which are entirely separate from the germ line.

germ line

1. In multicellular organisms, the population of cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from somatic cells. The cells of the germ line are called [germ cells](https://en.wikipedia.org/wiki/Glossary_of_genetics#germ_cell).
2. The lineage of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.

guanine

Abbreviated in shorthand with the letter G.

One of the four main nucleobases present in DNA and RNA. Guanine forms a base pair with cytosine.

guanine-cytosine content

Also abbreviated GC-content.

The proportion of nitrogenous bases in a nucleic acid that are either guanine (G) or cytosine (C), typically expressed as a percentage. DNA and RNA molecules with higher GC-content are generally more thermostable than those with lower GC-content due to molecular interactions that occur during base stacking.[[7]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-Yakovchuk2006-7)

haplodiploidy

A type of sex-determination system in which sex is determined by the number of sets of chromosomes an individual possesses: offspring which develop from fertilized eggs are females and diploid, while offspring which develop from unfertilized eggs are males and haploid, with half as many chromosomes as the females. Haplodiploidy is common to all members of the insect order Hymenoptera and several other insect taxa.

haploid

Denoted in shorthand with the somatic number *n.

(of a [cell](https://en.wikipedia.org/wiki/Cell_(biology%29) or organism) Having one copy of each chromosome, with each copy not being part of a pair. Contrast diploid and polyploid*.

haplotype

A set of alleles in an individual organism that were inherited together from a single parent.

hemizygous

In a diploid organism, having just one allele at a given genetic locus (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a chromosome is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is deleted, or when a gene is located on a sex chromosome in the heterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost all X-linked genes are said to be hemizygous because there is only one X chromosome and few of the same genes exist on the Y chromosome.

heredity

Also called inheritance.

The passing on of phenotypic traits from parents to their offspring, either through sexual or asexual reproduction. Offspring cells or organisms are said to inherit the genetic information of their parents.

heritability

1. The ability to be inherited.
2. A statistic used in quantitative genetics that estimates the proportion of variation within a given phenotypic trait that is due to genetic variation between individuals in a particular population. Heritability is estimated by comparing the individual phenotypes of closely related individuals in the population.

heterochromosome

See allosome.

heterologous expression

The expression of a foreign gene or any other DNA sequence within a host organism which does not naturally contain the same gene. Insertion of foreign transgenes into heterologous hosts using recombinant vectors is a common biotechnology method for studying gene structure and function.

heterozygous

In a diploid organism, having two different alleles at a given genetic locus. In genetics shorthand, heterozygous genotypes are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating a dominant allele) and a lowercase letter (indicating a recessive allele), such as "Aa" or "Bb". Contrast homozygous.

histone

Any of a class of highly alkaline proteins responsible for packaging nuclear DNA into structural units called nucleosomes in eukaryotic cells. Histones are the chief protein components of chromatin, where they associate into complexes which act as "spools" around which the linear DNA molecule winds. They play a major role in gene regulation and expression.

homologous chromosomes

Also called homologs.

A set of two matching chromosomes, one maternal and one paternal, which pair up with each other inside the nucleus during meiosis. They have the same genes at the same loci, but may have different alleles.

homologous recombination

A type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical ("homologous") molecules of DNA, especially that which occurs between homologous chromosomes. The term may refer to the recombination that occurs as a part of any of a number of distinct cellular processes, most commonly DNA repair or chromosomal crossover during meiosis in eukaryotes and horizontal gene transfer in prokaryotes. Contrast nonhomologous recombination.

homozygous

In a diploid organism, having two identical alleles at a given genetic locus. In genetics shorthand, homozygous genotypes are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrast heterozygous.

housekeeping gene

Any constitutive gene that is transcribed at a relatively constant level across many or all known conditions. Such a gene's products typically serve functions critical to the maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions.

hybrid

The offspring that results from combining the qualities of two organisms of different genera, species, breeds, or varieties through sexual reproduction. Hybrids may occur naturally or artificially, as during selective breeding of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in speciation.

hybridization

1. The process by which a hybrid organism is produced from two organisms of different genera, species, breeds, or varieties.
2. The process by which a single-stranded DNA or RNA preparation is added to an array surface, in solution, and potentially anneals to the complementary probe. Note that with respect to a gene expression assay, hybridization refers to a step in the experimental paradigm, while in molecular biology or genetics, the term refers to the chemical process.

idiochromosome

See allosome.

inbreeding

Sexual reproduction between breeds or individuals that are closely related genetically. Inbreeding results in homozygosity, which can increase both the probability of offspring being affected by deleterious recessive traits and the probability of fixing beneficial traits within the breeding population. Contrast outbreeding.

indel

A term referring to either an insertion or a deletion of one or more bases in a nucleic acid sequence.

inducer

A protein that binds to a repressor (to disable it) or to an activator (to enable it).

inducible gene

A gene whose expression is either responsive to environmental change or dependent on its host cell's position within the cell cycle.

inheritance

See heredity.

insertion

A type of mutation in which one or more bases are added to a nucleic acid sequence.

insulator

A specific DNA sequence that prevents a gene from being influenced by the activation or repression of nearby genes.

intergenic region (IGR)

Any sequence of non-coding DNA that is located between functional genes.

interphase

All stages of the cell cycle excluding cell division. A typical cell spends most of its life in interphase, during which it conducts everyday metabolic activities as well as the complete replication of its genome in preparation for mitosis or meiosis.

introgression

Also called introgressive hybridization.

The movement of a gene from the gene pool of one population or species into that of another population by the repeated backcrossing of hybrids of the two populations with one of the parent populations. Introgression is a ubiquitous and important source of genetic variation in natural populations, but may also be practiced intentionally in the cultivation of domesticated plants and animals.

intron

Any nucleotide sequence within a gene that is removed by RNA splicing during post-transcriptional modification of the mRNA primary transcript and is therefore absent from the final mature mRNA. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts. Contrast exon.

isochromosome

A type of abnormal chromosome in which the arms of the chromosome are mirror images of each other. Isochromosome formation is equivalent to simultaneous duplication and deletion events such that two copies of either the long arm or the short arm comprise the resulting chromosome.

karyotype

The number and appearance of chromosomes within the nucleus of a eukaryotic cell, especially as depicted in an organized photomicrograph known as a karyogram or idiogram (in pairs and ordered by size and by position of the centromere). The term is also used to refer to the complete set of chromosomes in a species or individual organism or to any test that detects this complement or measures the chromosome number.

The karyotype of a human male as visualized in a karyogram using Giemsa staining

knockdown

A genetic engineering technique by which the normal rate of expression of one or more of an organism's genes is reduced, either through direct modification of a DNA sequence or through treatment with a reagent such as a short DNA or RNA oligonucleotide with a sequence complementary to either an mRNA transcript or a gene.

knockout

A genetic engineering technique in which an organism is modified to carry genes that have been made inoperative ("knocked out"), such that their expression is disrupted at some point in the pathway that produces their gene products and the organism is deprived of their normal effects. Contrast knockin.

lagging strand

On the lagging strand template, a primase "reads" the template DNA and initiates synthesis of a short complementary RNA primer. A DNA polymerase extends the primed segments, forming Okazaki fragments. The RNA primers are then removed and replaced with DNA, and the fragments of DNA are joined together by DNA ligase.

Law of Dominance and Uniformity

One of three fundamental principles of Mendelian inheritance, which states that different alleles of the same gene may be dominant or recessive relative to others, and that an organism with at least one dominant allele will uniformly display the phenotype associated with the dominant allele.

Law of Independent Assortment

One of three fundamental principles of Mendelian inheritance, which states that genes responsible for different phenotypic traits are segregated independently during meiosis. Linked genes are a notable exception to this rule.

Law of Segregation

One of three fundamental principles of Mendelian inheritance, which states that during meiosis, the alleles of each gene segregate from each other such that each resulting gamete carries only one allele of each gene.

linkage

The tendency of DNA sequences which are physically near to each other on the same chromosome to be inherited together during meiosis. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (often loci or genetic markers) will be separated on to different chromatids during chromosomal crossover is statistically very low; such loci are then said to be more linked than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectly unlinked. The standard unit for measuring genetic linkage is the centimorgan (cM).

locus

Plural loci.

A specific, fixed position on a chromosome where a particular gene or genetic marker resides.

long arm

Denoted in shorthand with the symbol *q.

In condensed chromosomes where the positioning of the centromere creates two segments or "arms" of unequal length, the longer of the two arms of a chromatid. Contrast short arm*.

lyonization

See X-inactivation.

map unit (m.u.

See centimorgan.

meiosis

A specialized type of cell division that occurs exclusively in sexually reproducing eukaryotes, 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, also include various genetic recombination events between homologous chromosomes.

Mendelian inheritance

A theory of biological inheritance based on a set of principles originally proposed by Gregor Mendel in 1865 and 1866. Mendel derived three generalized laws about the genetic basis of inheritance which, together with several theories developed by later scientists, are considered the foundation of classical genetics.

metagenomics

Also called environmental genomics, ecogenomics, and community genomics.

The study of genetic material recovered directly from environmental samples, as opposed to organisms cultivated in laboratory cultures.

metaphase

The stage of mitosis and meiosis that occurs after prometaphase and before anaphase, during which the centromeres of the replicated chromosomes align along the equator of the cell, with each kinetochore attached to the mitotic spindle.

MicroArray and Gene Expression (MAGE)

A group that "aims to provide a standard for the representation of DNA microarray gene expression data that would facilitate the exchange of microarray information between different data systems".[[8]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-pmid17087822-8)

microchromosome

A type of very small chromosome, generally less than 20,000 base pairs in size, present in the karyotypes of some organisms.

Minimum information about a microarray experiment (MIAME)

A commercial standard developed by FGED and based on MAGE in order to facilitate the storage and sharing of gene expression data.[[9]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-pmid18629115-9)[[10]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-pmid19484163-10)

Minimal information about a high-throughput sequencing experiment (MINSEQE)

A commercial standard developed by FGED for the storage and sharing of high-throughput sequencing data.[[11]](https://en.wikipedia.org/wiki/Glossary_of_genetics#cite_note-minseqe-11)

missense mutation

A type of point mutation which results in a codon that codes for a different amino acid than in the unmutated sequence.

mitosis

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 stage of interphase, when the cell's DNA is replicated, and either occurs simultaneously with or is followed by cytokinesis, when the cytoplasm and cell membrane are divided into two new daughter cells.

mobile genetic element (MGE)

Any genetic material that can move between different parts of a genome or be transferred from one species or replicon to another within a single generation. The many types of MGEs include transposable elements (transposons), bacterial plasmids, bacteriophage elements which integrate into host genomes by viral transduction, and self-splicing introns.

molecular genetics

A branch of genetics that employs methods of molecular biology to study the structure and function of genes and gene products at the molecular level.

monosomy

The abnormal and frequently pathological presence of only one chromosome of a normal diploid pair. It is a type of aneuploidy.

mosaicism

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, endoreplication, or mutations in individual stem cell lineages during the early development of the embryo. Mosaicism is similar to but distinct from chimerism.

mutagen

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.

mutagenesis

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.

mutation

Any permanent change in the nucleotide sequence of a strand of DNA or RNA. Mutations play a role in both normal and abnormal biological processes, including evolution. They can result from replication errors, molecular damage, or manipulations by mobile genetic elements. Repair mechanisms have evolved in many organisms to correct them.

neutral mutation

1. Any mutation of a nucleic acid sequence that is neither beneficial nor detrimental to the ability of an organism to survive and reproduce.
2. Any mutation in which natural selection does not affect the spread of the mutation within a population.

nitrogenous base

Sometimes used interchangeably with nucleobase or simply base.

Any organic compound containing a nitrogen atom that has the chemical properties of a base. A set of five distinct nitrogenous bases – adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U) – are especially relevant to biology because they are components of nucleotides, which in turn are the primary monomers that make up nucleic acids.

non-coding DNA

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.

non-coding RNA (ncRNA)

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.

noncoding strand

See template strand.

nondisjunction

The failure of homologous chromosomes or sister chromatids to separate properly during cell division. Nondisjunction results in daughter cells that are aneuploid, containing abnormal numbers of one or more specific chromosomes. It may be caused by any of a variety of factors.

nonsense mutation

Also called a point-nonsense mutation.

A type of point mutation which results in a premature stop codon in the transcribed mRNA sequence, thereby causing the premature termination of translation, which results in a truncated, incomplete, and often non-functional protein.

nonsynonymous mutation

Also called a 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.

nuclear membrane

Also called the nuclear envelope.

A sub-cellular barrier consisting of two lipid bilayer membranes that surrounds the nucleus in eukaryotic cells.

nucleic acid

A long, polymeric macromolecule 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 used in biological systems to encode the genetic information governing the construction, development, and ordinary processes of all living organisms. The order, or sequence, of the nucleotides in DNA and RNA molecules contains information that is translated into proteins, which direct all of the chemical reactions necessary for life.

nucleic acid sequence

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 nitrogenous bases, nucleobases, nucleotides, or base pairs, and they correspond directly to sequences of codons and amino acids.

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