Cell multiplication

摘要

It is now more that 10 years since it became apparent that the cell cycle could be explained in terms of the periodic activation of complexes containing cyclins and cyclin-dependent kinases (CDKs). This concept has held firm during a period of unprecedented advance in our under-standing of the way that the cell cycle is controlled. The G_1 cell cycle stage is characterised by low CDK activity, with replication origins being assembled into the 'licensed' or 'pre-replicative' state, meaning that theyare competent to undergo DNA replication in the subsequent S phase. Activation of G_1/S CDK activity drives the cell into a new state, where replication can be initiated but replication origins cannot be relicensed. The persistence of this block to origin licensing is responsible for ensuring that chromosomal DNA replicates only once per cell cycle. After the completion of S phase, the activation of cyclin-B-Cdk1 initiates entry into mitosis and the complex programme of chromosome segregation. Exit from mitosis, with attendant shutdown of CDK activity, is then necessary for the daughter cells to return to the G_1 state.