DNA is the most important of the nucleic acids because it is the ultimate repository of all genetic information. It is a long-chain polymer of D-deoxyribose (right), which differs from D-ribose in having its -OH group at the 2` position replaced by -H. The polymer connection is made by esterifying a phosphate group with the 5` hydroxyl of one sugar molecule and the 3`hydroxyl of the next. The resulting polymer chain has a "sense" or a direction, with a 5` end and a 3`end as shown by the arrow (right). Ribonucleic acid is derived from a similar polymer, but uses D-ribose instead of D-deoxyribose.
In both DNA and RNA, the 1` carbon of each sugar ring is covalently bonded to one of four purine or pyrimidine bases: A, C, G, or T for DNA, and A, C, G, or U for RNA. (T differs from U only by an extra methyl group on the six-membered ring.) Genetic information is coded by the sequence of bases along a strand of DNA or RNA, with three consecutive bases containing the code for one amino acid. The three-base sequence for one amino acid is called a triplet codon. With a choice of four different bases at each of three positions, 4³ = 64 different codons are possible. Because only 20 amino acids are coded, there obviously must be redundancy within the system, with the same amino acid represented by more than one codon. This redundancy is inevitable, since a two-base codon scheme would have permitted only 4² = 16 different amino acids to be coded. Three of the 64 codons are used for "punctuation," to tell the polypeptide chain when to stop, and the other 61 represent individual amino acids.
Right: The backbone of DNA is a long polymer of alternating phosphates and deoxyribose molecules esterified at the 3` and 5` positions of the sugar.