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Immunology => Gene


Gene, unit of inheritance, a piece of the genetic material that determines the inheritance of a particular characteristic, or group of characteristics. Genes are carried by chromosomes in the cell nucleus and are arranged in a line along each chromosome. Every gene occupies a place, or locus, on the chromosome. Consequently, the word locus has become loosely interchangeable with the word gene.

The genetic material is deoxyribonucleic acid, or DNA , a molecule that forms the "backbone" of the chromosome. Because the DNA in each chromosome is a single, long, thin, continuous molecule, the genes must be parts of that molecule; and because DNA is a chain of minute subunits known as nucleotide bases, each gene includes many bases. Four different kinds of bases exist in the chain-adenine, guanine, cytosine, and thymine-and their sequence in a gene determines its properties.

Genes exert their effects through the molecules they produce. The immediate products of a gene are molecules of ribonucleic acid (RNA); these are copies of the DNA, except that RNA has the base uracil instead of thymine. The RNA molecules from some genes play a direct part in the metabolism of the organism, but most are used to make protein. Proteins are chains of subunits known as amino acids, and the sequence of bases in the RNA determines the sequence of amino acids in the protein by means of the genetic code . The sequence of amino acids in a protein dictates whether it will become part of the structure of the organism, or whether it will become an enzyme for promoting a particular chemical reaction. Thus, changes in the DNA can produce changes that affect the structure or the chemistry of an organism.

The nucleotide bases in DNA that code the structure of RNAs and proteins are not the only components of genes; groups of bases adjacent to the coding sequences affect the quantities and dispositions of gene products. In higher organisms (animals and plants, rather than bacteria and viruses), the noncoding sequences outnumber the coding ones by a factor of ten or more, and the functions of these noncoding regions are largely unknown. This means that geneticists cannot yet set precise limits to the sizes of animal and plant genes.



Nucleic Acids