Biology

Alternating deoxyribose (sugar) and phosphate groups form the backbone, linked by phosphodiester bonds between the 3'-OH of one sugar and the 5'-phosphate of the next.

The two strands run in opposite directions: one is oriented 5'→3' while its complement runs 3'→5', so their phosphodiester backbones point opposite ways.

Purines (double-ring) are adenine and guanine; pyrimidines (single-ring) are cytosine, thymine, and uracil. A purine always pairs with a pyrimidine.

In dsDNA the amount of adenine equals thymine and the amount of guanine equals cytosine (%A = %T, %G = %C), because of complementary base pairing.

DNA polymerase synthesizes only in the 5'→3' direction, adding each incoming dNTP to the free 3'-hydroxyl group of the growing strand.

Because polymerase works only 5'→3', the leading strand is copied continuously toward the moving fork, while the lagging strand is made discontinuously as Okazaki fragments away from the fork.