Definition: A functional unit of heredity that 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 gene 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 gene acting as a template. The RNA then passes into the cytoplasm and becomes oriented on the ribosomes where it in turn acts as a template to organize a chain of amino acids to form a peptide. In organisms reproducing sexually, genes 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.
[G. genos, birth]
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