DNA is synthesized from deoxyribonucleoside 5′-triphosphate precursors (dNTPs) in a manner analogous to RNA synthesis. Also like RNA synthesis, DNA synthesis always proceeds in the 5′→3′ direction because chain growth results from formation of a phosphoester bond between the 3′ oxygen of a growing strand and the α phosphate of a dNTP (see Figure 5-10a). As discussed earlier, an RNA polymerase can find an appropriate transcription start site on duplex DNA and initiate the synthesis of an RNA complementary to the template DNA strand (see Figure 5-11). In contrast, DNA polymerases cannot initiate chain synthesis de novo; instead, they require a short, preexisting RNA or DNA strand, called a primer, to begin chain growth. With a primer base-paired to the template strand, a DNA polymerase adds deoxyribonucleotides to the free hydroxyl group at the 3′ end of the primer as directed by the sequence of the template strand:

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When RNA is the primer, the daughter strand that is formed is RNA at the 5′ end and DNA at the 3′ end.