gene expression

expression

 [eks-presh´un]

1. the aspect or appearance of the face as determined by the physical or emotional state.

2. the act of squeezing out or evacuating by pressure.

3. gene expression.

gene expression

1. the flow of genetic information from gene to protein.

2. the process, or the regulation of the process, by which the effects of a gene are manifested.

3. the manifestation of a heritable trait in an individual carrying the gene or genes that determine it.

gene ex·pres·sion

1. the detectable effect of a gene.

2. appearance of an inherited trait; for many genetic (for example, recessiveness, hypostasis, parastasis) and environmental (the absence of pertinent challenges) reasons, a gene may not be expressed at all. In those circumstances, it will have no impact on Darwinian evolution.

gene expression

the flow of genetic information from gene to protein; the process, or the regulation of the process, by which the effects of a gene are manifested; the manifestation of a heritable trait in an individual carrying the gene or genes that determine it.

gene expression

Genetics The process by which a gene's coded information is translated into the structures present and operating in the cell. See Gene, mRNA, Protein, RNA, Transcription, Translation, tRNA.

gene ex·pres·sion

(jēn eks-presh'ŭn)

1. The detectable effect of a gene.

2. Appearance of an inherited trait; for many reasons, a gene may not be expressed at all.

gene expression

the processes effecting the transfer of information encoded in the GENE to the functional gene product (PROTEIN or RNA). Generally, gene expression is equated with the processes of TRANSCRIPTION and TRANSLATION. However, where the gene product is RNA only transcription is involved. A gene that is expressed is an active gene.

gene

the unit of heredity most simply defined as a specific segment of DNA, usually in the order of 1000 nucleotides, that specifies a single polypeptide. Many phenotypic characteristics are determined by a single gene, while others are multigenic. Genes are specifically located in linear order along the single DNA molecule that makes up each chromosome. All eukaryotic cells contain a diploid (2n) set of chromosomes so that two copies of each gene, one derived from each parent, are present in each cell; the two copies often specify a different phenotype, i.e. the polypeptide will have a somewhat different amino acid composition. These alternative forms of gene, both within and between individuals, are called alleles. Genes determine the physical (structural genes), the biochemical (enzymes), physiological and behavioral characteristics of an animal.

The formation of gametes (sperm, ova) involves a process of meiosis, which allows crossing over between four pairs of chromosomes, two derived from each parent, which means that new forms of a particular chromosome are created. Gamete formation also results in cells (gametes) with a haploid (n) set of chromosomes that in fertilization creates a new individual, which is a recombinant of 2n chromosomes, half derived by way of the ovum from the mother and half via the spermatozoa from the father.

Changes in the nucleotide sequence of a gene, either by substitution of a different nucleotide or by deletion or insertion of other nucleotides, constitute mutations which add to the diversity of animal species by creating different alleles and can be used as a basis for genetic selection of different phenotypes. Some mutations, be they a single base change in a single gene or a major deletion, are lethal.

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gene action

the way in which genes exert their effects on tissues or processes, e.g. by being dominant or recessive, or partially so, being absent, being sex-linked, being involved in chromosomal aberrations.

allelic g's

different forms of a particular gene usually situated at the same position (locus) in a pair of chromosomes.

gene amplification

see gene duplication (below).

gene bank

the collection of DNA sequences in a given genome. Called also gene library.

barring gene

responsible for the barred pattern on the feathers of Barred Plymouth Rock birds.

gene box

see box (4).

gene clone

see clone.

gene cluster

a group of related genes derived from a common ancestral gene, located closely together on the same chromosome. Called also multigene family.

complementary g's

two independent pairs of nonallelic genes, neither of which is functional without the other.

gene conversion

a non-reciprocal exchange of DNA elements during meiosis which results in a functional rearrangement of chromosomal DNA.

dhfr gene

dihydrofolate reductase gene; an enzyme required to maintain cellular concentrations of H₂ folate for nucleotide biosynthesis, and which has been used as a 'selective marker'; cells lacking the enzyme only survive in media containing thymidine, glycine and purines; mutant cells (dhfr) transfected with DNA that is dhfr′ can be selectively grown in medium lacking these elements.

diversity (D) gene

genes located in diversity (D) segment; contribute to the hypervariable region of immunoglobulins.

dominant gene

one that produces an effect (the phenotype) in the organism regardless of the state of the corresponding allele. Examples of traits determined by dominant genes are short hair in cats and black coat color in dogs.

gene duplication

as a result of non-homologous recombination, a chromosome carries two or more copies of a gene.

gene expression

see expression (3).

gene frequency

the proportion of the substances or animals in the group which carry a particular gene.

holandric g's

genes located on the Y chromosome and appearing only in male offspring.

immune response (Ir) g's

genes of the major histocompatibility complex (MHC) that govern the immune response to individual immunogens.

jumping gene

see mobile dna.

gene knockout

replacement of a normal gene with a mutant allele, as in gene knockout mice.

lethal gene

one whose presence brings about the death of the organism or permits survival only under certain conditions.

gene library

see gene bank (above).

gene locus

see locus.

mutant gene

one that has undergone a detectable mutation.

non-protein encoding gene

the final products of some genes are RNA molecules rather than proteins.

overlapping g's

when more than one mRNA is transcribed from the same DNA sequence; the mRNAs may be in the same reading frame but of different size or they may be in different reading frames.

gene pool

total of all genes possessed by all members of the population which are capable of reproducing during their lifetime.

gene probe

see probe (2).

recessive gene

one that produces an effect in the organism only when it is transmitted by both parents, i.e. only when the individual is homozygous.

regulator gene, repressor gene

one that synthesizes repressor, a substance which, through interaction with the operator gene, switches off the activity of the structural genes associated with it in the operon.

reporter gene

one that produces products which can be measured and therefore used as an indicator of whether a DNA construct has successfully been transferred.

sex-linked gene

one that is carried on a sex chromosome, especially an X chromosome.

gene splicing

see splicing.

structural gene

nucleotide sequences coding for proteins.

gene therapy

the insertion of functional genes into cells of the host in order to alter its phenotype, usually used to treat an inherited defect.

gene transcription

see transcription.

gene transfer

see recombination.

tumor suppressor g's

a class of genes that encode proteins that normally suppress cell division that when mutated allow cells to continue unrestricted cell division and may result in a tumor.