Quality control in the initiation of eukaryotic DNA replication

Abstract

Origins of DNA replication must be regulated to ensure that the entire genome is replicated precisely once in each cell cycle. In human cells, this requires that tens of thousands of replication origins are activated exactly once per cell cycle. Failure to do so can lead to cell death or genome rearrangements such as those associated with cancer. Systems ensuring efficient initiation of replication, while also providing a robust block to re-initiation, play a crucial role in genome stability. In this review, I will discuss some of the strategies used by cells to ensure once per cell cycle replication and provide a quantitative framework to evaluate the relative importance and efficiency of individual pathways involved in this regulation.

Figure 1.

Stepwise assembly of DNA replication complexes. The individual steps leading to the assembly of bidirectional replisomes is outlined. Names associated with each of the complexes are shown on the right: pre-RC, pre-replication complex; pre-IC, pre-initiation complex; RPC, replisome progression complex. Cell cycle phases permissive for the individual steps are shown on the left. For simplicity, some of the protein names have been abbreviated: 11, Dpb11; 3, Sld3; 7, Sld7; 2, Sld2; G, GINS; 45, Cdc45; 4, Ctf4. The shapes of many of the individual components are loosely based on three-dimensional reconstructions from electron micrographs: ORC and Cdc6 are from Chen et al. [17], Mcm double hexamer is from Remus et al. [18], Cdc45, Mcm2-7, GINS (CMG) are from Costa et al. [19], DNA polymerase epsilon (polɛ) is from earlier studies [20,21], and DNA polymerase α (polα) is from Klinge et al. [21]. The roles of CTF4 and Mcm10 in the RPC are inferred from earlier studies [22,23]. Cyclin-dependent kinase (CDK) phosphorylations are shown in red, Dbf4-dependent kinase (DDK) phosphorylations are shown in blue. The order of DDK and CDK in activating replication comes from earlier studies [–26].