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gene (jen)

A functional unit of heredity which occupies a specific place (locus) on a chromosome, is capable of reproducing itself exactly at each cell division, and directs the formation of an enzyme or other protein. The g. as a functional unit consists of a discrete segment of a giant DNA molecule containing the purine (adenine and guanine) and pyrimidine (cytosine and thymine) bases in the correct sequence to code the sequence of amino acids of a specific peptide. Protein synthesis is mediated by molecules of messenger-RNA formed on the chromosome with the g. acting as template. The RNA then passes into the cytoplasm and becomes oriented on the ribosomes where it in turn acts as template to organize a chain of amino acids to form a peptide. G.'s normally occur in pairs in all cells except gametes, as a consequence of the fact that all chromosomes are paired except the sex chromosomes (X and Y) of the male.factor (3) ; [G. genos, birth]
allelic g. See allele, dominance of traits.
autosomal g. a g. located on any chromosome other than the sex chromosomes (X or Y).
C g. the g. coding for the constant regions of immunoglobulin chains.
codominant g. a set of two or more alleles, each expressed phenotypically in the presence of the other.
control g. See operator g., regulator g.
dominant g. See dominance of traits.
extrachromosomal g. a g. located outside of the nucleus (e.g., mitochondrial genes).
H g. histocompatibility g
histocompatibility g. in laboratory animals, a g. which can elicit an immune response and thereby cause rejection of a homograft when tissue is transplanted from one individual to another; in humans, histocompatibility g.'s control HLA antigens.H g;
holandric g. Y-linked g
homeotic g.'s a group of g.'s that regulate the development of the body parts by defining the boundaries of the several regions.
housekeeping g.'s g.'s that are generally always expressed and thought to be involved in routine cellular metabolism.
immune response g.'s g.'s in the HLA-D region of the histocompatibility complex of human chromosome 6 which control the immune response to specific antigens.
jumping g. a g. associated with transposable elements. See transposon.
lethal g. a g. that produces a genotype that leads to death of the organism before reproduction is possible or that precludes reproduction; for a recessive g. the homozygous or hemizygous state is lethal.
mimic g.'s nonallelic (independent) g.'s with closely similar effects, e.g., elliptocytosis.
mitochondrial g. a functioning g. located not in the nucleus of a cell but in the mitochondrial chromosome.
modifier g. a nonallelic g. that controls or changes the manifestation of a g. by interfering with its transcription.
mutant g. a g. that has been changed from an ancestral type, not necessarily in the current generation. See also mutant, mutation.
operator g. a g. with the function of activating the production of messenger RNA by one or more adjacent structural loci; part of the feedback system for determining the rate of production of an enzyme.
pleiotropic g. a g. that has multiple, apparently unrelated, phenotypic manifestations.polyphenic g;
polyphenic g. pleiotropic g
regulator g. a g. that produces a repressor substance that inhibits an operator g. when combined with it. It thus prevents production of a specific enzyme. When the enzyme is again in demand, a specific regulatory metabolite inhibits the repressor substance.
repressor g. a g. that prevents a nonallele from being transcribed.
SOS g.'s a group of g.'s involved in DNA repair, often induced by damage severe enough to cause stoppage of DNA synthesis.
split g.'s g.'s where the genomic sequences are interrupted by intervening sequences (introns) that are spliced out of the mRNA prior to translation.
structural g. a g. that codes for a specific protein or peptide.
transfer g.'s g.'s carried by a conjugative plasmid, essential for fertility and establishment of the bacterial donor state.
transforming g. oncogene
V g. the g. coding for the major part of the variable region of an immunoglobulin chain.
X-linked g. a g. located on an X chromosome.
Y-linked g. a g. located on a Y chromosome.holandric g;
Z g. the structural g. for beta-galactosidase.


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