Nucleases and helicases are involved in numerous steps in DNA replication and repair. Nucleases act on intermediates in DNA replication created by DNA polymerases (Chapter 4) and helicases (Chapter 3). They can create substrates for repair as in Okazaki fragment processing (OFP) and homologous recombination. They can also create substrates for activation of a checkpoint response, or participate in downregulation of checkpoints. In the special case of telomere replication, they are also involved in essential processing steps (Chapter 8). Nucleases known to act during DNA replication include Dna2, Rad27, Mre11, Sae2, Exo1, RNaseH, Yen1 andMus81/Mms4. Of these, Dna2, Exo1 and Mre11 are of particular interest because they have been identified as crucial activities that initiate repair of double-strand breaks (DSBs) by homologous recombination and thus form an intrinsic link between DNA replication and repair of DSBs derived from replication fork failure. The action of the nucleases is coordinated with those of a number of helicases and is discussed here in the context of a network of their interactions that combine to maintain genome integrity during DNA replication
