BIOL355 Lecture Notes - Lecture 16: G1 Phase, Dna Replication, P21

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For cells to proceed from one phase to next is highly regulated. Normal cell checkpoints; follow the progress of the cell cycle and allow it to proceed from one stage to the next. Hayflick found that dividing cultures contain a % of senescent (growth-arrested cells); this % increases progressively until all cells stopped dividing. Senescent cells are growth arrested in the transition from phage. G1 to phase s; irreversible -> cannot stimulate the cells to divide anymore. Dna replication is initiated by nuclear proteins known as cyclins. > bind to the promotor region of the cyclin genes. > cyclin binds to inactive form of cdk (kinase) in nucleus. > activate and phosphorylates other proteins involved in checkpoint. > cells enter s phase and proteases breakdown the cyclin bound to the cdk. Checkpoints monitor dna damage before entering s phase. Prevents damaged dna from being duplicated in s (-> cause shortening of telomeres)

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