549 17.2 The cell-proliferation machinery of the cell cycle As different CDK-cyclin complexes appear and disap-pear, the proteins necessary for carrying out the different phases of the cell-division cycle are transcribed and translated like a well-choreographed ballet. The chore-ography requires that each active CDK-cyclin complex be “on stage” only for a limited part of the cell cycle. If an active CDK-cyclin complex is present at the wrong time, it will cause wholly inappropriate sets of genes to be transcribed or shut off. The CDK-cyclin protein complexes are composed of two subunits: a cyclin and a cyclin-dependent protein kinase (abbreviated CDK ). • Cyclins. Every eukaryote has a family of structurally and functionally related cyclin proteins. Cyclins are so named because each is present in the cell only during one or more deFned segments of the cell cycle. The appearance of a speciFc cyclin is the result of the activity of the preceding CDK-cyclin complex, which leads to the activation of a transcription factor for the new cyclin. • CDKs.
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