How DNA replication is organised and controlled to ensure precise chromosome duplication

Chromosome replication is a key event in the eukaryotic cell division cycle. During S phase the entire genome must be faithfully duplicated with the minimum of errors. The many thousands of replication forks involved in this process must be co-ordinated to ensure that despite the very large quantities of DNA involved, no section of DNA is left unreplicated and no section of DNA is replicated more than once. Cells achieve this by having a distinct stage that occurs prior to S phase when replication origins are "licensed" for replication. At the onset of S phase, replication forks are initiated only at these licensed replication origins. As initiation occurs at each origin, the licence is removed, thereby ensuring that it fires only once in each cell cycle. Mistakes made in this process may cause irreversible genetic modifications that could ultimately lead to the cells becoming cancerous. Many early stage cancer cells have lost the ability to correctly down-regulate the licensing system, suggesting that it is an important control system for cancer cells to evade.

The aim of our work is to understand at a molecular level the way that chromosome replication is regulated, and to apply this knowledge to improving  the diagnosis and treatment of cancer. We use a number of different model systems to study cell cycle control of DNA replication. Cell-free extracts of Xenopuseggs, which support all the nuclear events of the early embryonic cell cycle, provide a powerful system for studying DNA replication control at a biochemical level. We then apply insights gained from the Xenopus system to the study of replication in human tissue culture cells which have important advantages for studying DNA replication using cell biological and molecular genetic approaches. We are also using C. elegans as a model system to study DNA replication in a genuine physiological setting. Current research projects are addressing: a) how the licensing system is regulated at different stages of the cell division cycle; b) how the Cdc7 and cyclin-dependent kinases trigger the initiation of DNA replication; c) the way that replication is co-ordinated with other cell cycle events; d) how replication origins are physically organised on chromosomal DNA; and e) how different cellular stresses affect the replication programme through checkpoint signals.