What happens when replication goes wrong?
It is clearly important that synthesis of a complementary second strand of DNA should occur with complete accuracy, but occasionally, a non-complementary nucleotide is inserted; this may happen as frequently as once in every 10 000 nucleotides. Cells, however, are able to have a second attempt to incorporate the correct base because of the proofreading activity of the enzymes DNA polymerase I and III. These are able to cut out the ‘wrong’ nucleotide and replace it with the correct one. As a result of this monitoring, mistakes are very rare; they are thought to occur at a frequency of around one in every billion (109) nucleotides copied. Mistakes that do slip through the net result in mutations.