Analogous to RNA, DNA is synthesized from deoxynucleo-side 5'-triphosphate precursors (dNTPs). Also like RNA synthesis, DNA synthesis always proceeds in the 5'n3'
direction because chain growth results from formation of a phosphoester bond between the 3' oxygen of a growing strand and the a phosphate of a dNTP (see Figure 4-9). 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 4-10). 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 deoxynucleotides 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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