Control of the cell cycle

Control of the cell cycle - 1 ; others stop in G 2 ....

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Control of the cell cycle Telomeres Mammalian cells typically divide only about 50 times. This limit is set by the presence of repeated sequences of DNA at the tips of the chromosomes called telomeres . In young cells, the sequence TTAGGG is repeated hundreds or thousands of times but each time the cell divides, it loses 50 to 200 of these repeats. Cells that have divided many times have fewer of these repeats left. When the telomere is reduced to a certain size, the cell will no longer divide. Telomeres are restored to their original length by an enzyme called telomerase . This enzyme contains a single strand of RNA that is used to synthesize the telomeres. Telomerase is usually found in cells involved in the production of gametes. It is not normally found in somatic cells. Cyclin-Dependent Kinases Some cells stop dividing in G
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Unformatted text preview: 1 ; others stop in G 2 . Kinases are enzymes that activate proteins by transferring a phosphate group from ATP to the protein being activated. An activated protein is needed for the cell cycle to proceed from G 1 to S. Similarly, another activated protein is needed to move the cycle from G 2 to mitosis. Kinases activate these proteins and thus stimulate the cell cycle to continue. Kinases are normally inactive and must be activated before they can activate other proteins. Cyclin-dependent kinases become activated by combining with a protein called cyclin . The activated complex is involved in stimulating the cell cycle to resume. The level of cyclin fluctuates (cycles). At low levels, kinases are not activated and the cell cycle is halted. At high levels, activation occurs and the cycle resumes....
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This note was uploaded on 12/15/2011 for the course BIO BSC1010 taught by Professor Gwenhauner during the Fall '10 term at Broward College.

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Control of the cell cycle - 1 ; others stop in G 2 ....

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