Hereditary information is stored in the form of double-stranded deoxyribonucleic acid (DNA). This genetic material is required to be stable and capable of accurate transmission but also to contain the potential for limited alteration, giving scope for evolutionary change. DNA consists of a linear array of four "bases", adenine (A), guanine (G),cytosine (C) and thymine (T). A and G have a double carbon ring structure, C and T a single carbon ring. The bases are linked by deoxyribose sugars and phosphate groups, in a chain of nucleotides. Ribonucleic acid (RNA) is the other major form of nucleic acid involved in the processing of genetic information.
RNA is located in the cytoplasm as well as the nucleus and also differs from DNA in that it contains uracil (U) in place of thymine, and the linking sugar is ribose. The sugar / phosphate components of the nucleotides join together by a 5' to 3' (referring to the positions of the participating carbon atoms) phosphodiester linkage forming the characteristic structural backbone of DNA. Projecting from this backbone at the 1' position of each sugar are the four bases completing the polynucleotide chain. All linear strands of DNA have a free 5' phosphate group at one end and a free 3' hydroxyl group at the otber, commonly termed the 5' (five prime) and 3 (three prime) ends.
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