Replication is semiconservative: DNA’s two nucleotide strands separate, and each serves as a template on which a new strand is synthesized.
All DNA synthesis is in the 5′→3′ direction. Because the two nucleotide strands of DNA are antiparallel, replication takes place continuously on one strand (the leading strand) and discontinuously on the other (the lagging strand).
Replication begins when initiator proteins bind to an origin of replication and unwind a short stretch of DNA, to which DNA helicase attaches. DNA helicase unwinds the DNA at the replication fork, single-
During replication, primase synthesizes short primers consisting of RNA nucleotides, providing a 3′-OH group to which DNA polymerase can add DNA nucleotides.
DNA polymerases add new nucleotides to the 3′ end of a growing polynucleotide strand. Bacteria have two DNA polymerases that have primary roles in replication: DNA polymerase III, which synthesizes new DNA on the leading and lagging strands, and DNA polymerase I, which removes and replaces primers.
DNA ligase seals the breaks that remain in the sugar–phosphate backbones when the RNA primers are replaced by DNA nucleotides.
Several mechanisms ensure the high rate of accuracy in replication, including precise nucleotide selection, proofreading, and mismatch repair.
Precise replication at multiple origins in eukaryotes is ensured by a licensing factor that must attach to an origin before replication can begin.
The ends of linear eukaryotic DNA molecules are replicated by the enzyme telomerase.
Homologous recombination takes place through alignment of homologous DNA segments, breaks in nucleotide strands, and rejoining of the strands.