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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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