Control of DNA replication by cyclin-dependent kinases in development

Abstract

Cyclin-dependent kinases (CDKs) are required for initiation of DNA replication in all eukaryotes, and appear to act at multiple levels to control replication origin firing, depending on the cell type and stage of development. In early development of many animals, both invertebrate and vertebrate, rapid cell cycling is coupled with transcriptional repression, and replication initiates at closely spaced replication origins with little or no sequence specificity. This organisation of DNA replication is modified during development as cell proliferation becomes more controlled and defined. In all eukaryotic cells, CDKs promote conversion of "licensed" pre-replication complexes (pre-RC) to active initiation complexes. In certain circumstances, CDKs may also control pre-RC formation, transcription of replication factor genes, chromatin remodelling, origin spacing, and organisation of replication origin clusters and replication foci within the nucleus. Although CDK1 and CDK2 have overlapping roles, there is a limit to their functional redundancy. Here, I review these findings and their implications for development.

Figure 10.1. Differences in organisation of DNA replication during development

In early Xenopus embryonic development, the cell cycles are rapid and synchronous, whereas after the mid-blastula transition they appear much more highly regulated. Some functional differences which have implications for replication control are highlighted.

Figure 10.2. The molecular organisation at replication origins and its control by CDKs

Replication origins are built in sequential steps : first, origins are licensed by the back-to-back loading of the double heterohexamer of MCM proteins around DNA, which requires the Orc complex, Cdc6, and a Cdt1-geminin complex. This step is inhibited by mitotic cyclin-dependent kinases. In a second step, which requires S-phase CDK activity, the pre-initiation complex (proteins in green) is loaded and DNA unwinding by the now processive helicase can occur.

Figure 10.3. Multiple potential roles for the Rb protein, a CDK substrate, in replication control

See main text for details.

Figure 10.4. Higher nuclear organisation in replication control

The organisation of replication in the nucleus is not yet well understood. In the generally accepted current model, origins of replication are clustered (bottom) and chromatin loops out from the origins. The origin clusters are probably also organised into ”factories” (middle) which can be visualised microscopically as replication foci (top). CDKs have roles at these different levels of subnuclear structure, as explained in the main text.