Detailed structure of Deoxyribonucleic Acid (DNA)

DNA is a double-stranded molecule with base pairs inside, held together by hydrogen bonds. Each outer strand has a 5'->3' directionality, determined by the numbering of the deoxyribose sugar.  Successive sugars are joined by a 3'-5' phospho-diester bond. The 5' terminus of the strand is also called the phosphate end, and the -OH group on the 3' terminus the hydroxyl end. The 5'->3' orientation of the two strands runs in opposite directions. Bases are attached to the 1' carbon. There are four bases, A C G & T (the color convention is that used in automated DNA sequencers). C & T are single-ring pyrimidines, A & G are double-ring purines. C & G pair by means of three hydrogen bonds, A & T by means of two bonds. Either base pair therefore comprises three rings, and have the same molecular shape. This allows base pairs to occur between the strands in any order: the sequence is aperiodic. If the strands are separated, the base pair rules mean that the strands are complementary, and either single-strand contains the information present in the double-stranded molecule.