n.
Unit of heredity that occupies a fixed position on a chromosome .
Genes achieve their effects by directing protein synthesis. They are composed of DNA , except in some viruses that contain RNA instead. The sequence of nitrogenous bases along a strand of DNA determines the genetic code . When the product of a particular gene is needed, the portion of the DNA molecule that contains that gene splits, and a complementary strand of RNA, called messenger RNA (mRNA), forms and then passes to ribosomes , where proteins are synthesized. A second type of RNA, transfer RNA (tRNA), matches up the mRNA with specific amino acids , which combine in series to form polypeptide chains, the building blocks of proteins. Experiments have shown that many of the genes within a cell are inactive much or even all of the time, but they can be switched on and off. Mutations occur when the number or order of bases in a gene is disrupted. See also genetic engineering , genetics , Hardy-Weinberg law , Human Genome Project , linkage group .