550 Chapter 17 • Genetic Regulation of Cell Number: Normal and Cancer Cells a change in the shape of Rb so that it can no longer bind to the E2F protein. The unattached E2F protein is then free to promote the transcription of certain genes that en-code the enzymes vital for DNA synthesis and other as-pects of chromosome replication. It also activates gene expression of the next cyclin to be expressed, cyclin B. With cyclin B’s appearance, the next phase of the cell cycle—S phase—can proceed. Rb and E2F are in fact representatives of two fami-lies of related proteins. In mammals, different CDK-cyclin complexes selectively phosphorylate different proteins of the Rb family, each of which in turn releases a speci±c E2F family member to which it is bound. The different E2F transcription factors then promote the transcription of different genes that execute different aspects of the cell cycle. Checkpoints as brakes on cell-cycle progression Proper choreography of the cell cycle’s events is crucial to the production of progeny cells with the proper number of intact chromosomes. For example, attempting to con-dense chromosomes and move them to metaphase (re-
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.