DNA, or deoxyribonucleic acid, consists, like RNA, of long polynucleotide chains assembled from four different kinds of mononucleotide units. There are two differences. First, ribose is replaced in DNA by 2-deoxyribose, which corresponds to ribose minus the oxygen atom in position 2 of the molecule. The second difference is the replacement of uracil by thymine, which is uracil with an added methyl group (CH3). This modification does not affect the ability of the molecule to pair with adenine.

In contrast to RNA, almost all the DNA in nature is in double-stranded form, known worldwide as the double helix. Single-stranded DNA is found only in a few rare viruses. Intriguingly, the exact opposite occurs with RNA, which is almost always single-stranded, except in some rare viruses.

The replication of DNA takes place in the same way as that of RNA. The precursors are the deoxyribonucleotide triphosphates, which are distinguished from their ribonucleotide congeners by the prefix d: dATP, dGTP, dCTP, and dTTP. Base pairing rules the process, as for RNA, except that A pairs with T instead of with U. The double-stranded nature of the template does, however, introduce a number of complications, which are solved by the participation of unwinding and rewinding enzymes (helicases, gyrases, topoisomerases). In addition, because the two DNA strands have opposite polarities and replication obligatorily follows the template in only one direction (from its 3′ end to its 5′ end), one strand is replicated by a continuous process, generating what is called the leading strand, whereas the other strand is replicated discontinuously into a series of short stretches (Okazaki fragments) that are assembled in a direction opposite to the direction of assembly of the leading strand and are later joined together by a ligase, to form the lagging strand.