5 Eukaryotic DNA Replication Forks

Abstract

At each of the many replication origins in eukaryotic cells, two replication forks are assembled that bidirectionally replicate their portion of the chromosome with high fidelity. The protein factors involved in the assembly of the replisome and its firing have been discussed in the previous chapters. This chapter focuses on the general structure of the replication fork and on the mechanics of fork propagation. Individual subassemblies required for fork propagation are the minichromosome maintenance (MCM) helicase complex, the leading-strand replication apparatus, and the lagging-strand replication apparatus (Fig. 1). Very little information exists regarding the interactions needed between these complexes in order to allow coordinated movement of the replisome during DNA replication. There are also uncertainties with regard to the functional state of the MCM helicase and the identity of the leading-strand DNA apparatus. Recent progress in the initiation, elongation, and completion of lagging-strand DNA synthesis may make these the best-understood replication fork events that occur during movement of the replication fork. DNA POLYMERASES The three DNA polymerases responsible for normal replication fork progression are multisubunit complexes (Fig. 2). The catalytic subunit of each complex is a class B DNA polymerase (Garg and Burgers 2005). Phylogenetic studies suggest that the gene duplications that gave rise to polymerase (pol)-α, pol-δ, and pol-ɛ likely occurred prior to the split between eukaryotes and archaeal euryarchaeotes, indicating that these three paralogs are present in all eukaryotes1 (Edgell et al. 1998). The second largest subunits of each of the three polymerase complexes are also distantly related...