Regulation of DNA Replication Origin Licensing

DNA replication is a fundamental biological process that serves to create two copies of the genetic material during each cell division. Complete and precise replication enables identical sets of genes to be faithfully delivered to daughter cells during each cell division. To achieve rapid duplication of the entire genome, eukaryotic cells initiate DNA replication at multiple locations on each chromosome termed origins of DNA replication. Origin DNA is unwound and complementary DNA is then synthesized from bi-directionally moving replication forks. The replication forks eventually merge to form two identical chromosomes. The cell expends tremendous energy ensuring that a single origin of replication does not initiate replication twice within the same cell cycle. One of the most highly regulated steps in DNA replication is assembly of pre-replication complexes (pre-RCs). Pre-RC assembly begins as cells exit mitosis and continues through G1 phase, culminating in chromosomes poised for replication by the end of G1. At the onset of S phase, origins fire and replication begins. During this time, several overlapping mechanisms prevent pre-RC assembly on origins that have already fired to avoid utilizing any origins twice. An abnormal situation in which replication is triggered multiple times from the same origin during a single cell cycle is termed re-replication (Figure 1). Re-replication is detrimental to genome stability, because it generates multiple replication forks on the same DNA strand. Ultimately such structures result in double strand breaks, genome instability, and in some cases, tumorigenesis (Arentson et al., 2002; Karakaidos et al., 2004; Xouri et al., 2004; Liontos et al., 2007). This chapter focuses on mechanisms to prevent re-replication during normal and perturbed cell cycles.