Leading strand synthesis

Elongation The elongation phase of replication includes two distinct but related operations leading strand synthesis and lagging strand synthesis. Several enzymes at the replication fork are important to the synthesis of both strands. Parent DNA is first unwound by DNA helicases, and the resulting topological stress is relieved by topo-isomerases. Each separated strand is then stabilized by... 

Poll polA Prime removal and gap filling Initial leading strand synthesis on ColEl... 

Fisher, K. J., Gao, G. P., Weitzman, M. D., De Matteo, R., Burda, J. F. and Wilson, J. M. (1996). Transduction with recombinant adeno-associated virus for gene therapy is limited by leading-strand synthesis. J. Virol. 70, 520-532. 

DNA synthesis requires a primer usually made of RNA. A primase synthesizes the ribonucleotide primer ranging from 4 to 12 nucleotides in length. DNA polymerase then incorporates a dNMP onto the 3 end of the primer initiating leading strand synthesis. Only one primer is required for the initiation and propagation of leading strand synthesis. 

Eukaryotic DNA polymerases have also been isolated and characterized as listed in Table 22.1. Based on studies of SV40 DNA replication in vitro, it has been found that DNA polymerase d has high processivity and is required for leading-strand synthesis, making it analogous to E. coli DNA pol III. DNA polymerase d requires ATP and is stimulated by two additional DNA replication proteins, RF-C and PCNA. DNA polymerase a serves the same role as E. coli DNA pol I in that DNA polymerase a is necessary for lagging-strand synthesis. In addition to DNA polymerase a and d, three other DNA polymerizing activities have been identified. DNA polymerase I is involved in DNA repair and is most similar to E. coli DNA pol II. DNA polymerase b is also a repair enzyme, and DNA polymerase g is required for mitochondrial DNA synthesis. 

Fig. 5. Two alternative models for error-free PRR via recombinational processes. (A) A strand exchange model, and (B) a template switching model. Both models propose that progression of leading strand synthesis in the presence of replicationblocking DNA damage (represented by a triangle) requires the association of the two nascent DNA strands, followed by resolution of the intermediary structure via (A) cleavage of the Holliday junction or (B) reverse branch migration. Adapted from Broomfield et al. (2001).

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