DNA polymerases synthesize DNA in the 5' : 3' direction by adding new nucleotides to the 3' end of a growing nucleotide strand.

DNA ligase After DNA polymerase III attaches a DNA nucleotide to the 3'-OH group on the last nucleotide of the RNA primer, each new DNA nucleotide then provides the 3'-OH group needed for the next DNA nucleotide to be added. This process continues as long as template is available (I Figure 12.14a). DNA polymerase I follows DNA polymerase III and, using its 5':3' exonuclease activity, removes the RNA primer. It then uses its 5':3' polymerase activity to replace the RNA nucleotides with DNA nucleotides. DNA polymerase I attaches the first nucleotide to the OH group at the 3' end of the preceding Okazaki fragment and then continues, in the 5': 3' direction along the nucleotide strand, removing and replacing, one at a time, the RNA nucleotides of the primer (i Figure 12.14b).

Template strand

Template strand


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