Mispaired bases

The mispaired 3 -OH end of the growing strand blocks further elongation. DNA polymerase slides back to position the mispaired base in the 3 —>5 exonuclease active site. 

Mismatch repair was an essential component of the Holliday model for meiotic recombination in fungi (Holliday, 1965a). This states that recombination depends on the formation of relatively short stretches of hybrid DNA between homologous chromatids within which, if they spanned a region of heterozygosity, a mispaired base-pair would appear. Mismatch repair would determine whether or not aberrant segregations would be found in the tetrads formed on completion of meiosis. 

E.E. Kryachko, J.R. Sabin, Quantum Chemical Study of the Hydrogen-Bonded Patterns in the AT Base Pair of DNA The Origins of Tautomeric Mispairs, Base Flipping, and Watson-Crick Hoogsteen Conversion, Int. J. Quantum Chem. 91 (2003) 695. 

Proofreading depends on the 3 5 exonuclease activity of some DNA polymerases. When an incorrect base is incorporated during DNA synthesis, the polymerase pauses, then transfers the 3 end of the growing chain to the exonuclease site, where the incorrect mispaired base is removed (Figure 23-24). Then the 3 end is transferred back to the polymerase site, where this region is copied correctly. All three E. coli DNA polymerases have proofreading activity, as do the two DNA polymerases, S and e, used for DNA replication in animal cells. It seems likely that proofreading is indispensable for all cells to avoid excessive mutations. 

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