Overview

To reproduce, a cell must first copy its genome via DNA replication. In DNA replication, double-stranded DNA (dsDNA) is copied to yield two daughter molecules, each identical to the parent molecule. As in transcription, the new polymer grows 5′→3′. Unlike transcription, whose product is a free, single-stranded RNA molecule, replication yields two double-stranded B-form DNA helices. DNA replication is semiconservative, as each daughter helix consists of one parental strand and one newly synthesized strand.

In cells, a large complex of proteins, the replisome, replicates DNA. The replisome includes DNA-dependent DNA polymerase, which, unlike RNA polymerase, cannot begin with a single nucleotide, but rather adds nucleotides to the 3′ end of a primer, a short chain of RNA synthesized by primase. DNA synthesis is faster and much more accurate than RNA synthesis.

Chain Growth

In DNA synthesis, as in RNA synthesis, the polymer is built by making a phosphodiester bond between the α phosphate of a nucleotide triphosphate (dNTP) and the 3′ oxygen of the growing polynucleotide chain, lengthening the chain by one and producing diphosphate. Required are the four dNTPs (dATP, dGTP, dCTP, and dTTP), a template strand, a primer (a short nucleotide chain that base pairs with the template) to which nucleotides are added, and DNA polymerase. Each of the two strands of a DNA double helix is a template for the synthesis of a new, complementary strand.