The Cell Cycle

The cell cycle can be divided into four phases: Gi, S, G2, and M. G1 refers to the gap between the mitosis (M) of the previous cell division and the DNA synthesis (S) stage of the upcoming cell, and G2 refers to the gap between DNA synthesis and mitosis. Additionally, quiescent cells, which are nondividing but metabolically active and terminally differentiated, such as muscle cells or neurons, are considered to be in G0 phase.

The control of cell-cycle progression from G1 to S to G2 to M is tightly controlled by the activities of a small number of heterodimeric kinase complexes (Figure 16.1). The regulatory subunit of each dimer is required for kinase function, and the levels of these regulatory proteins oscillate throughout the cell cycle in response to upregulation by transcription factors and downregulation by the ubiquitin proteosome system. Because of the cyclic behavior of these regulatory subunits, they are called cyclins, and the kinases they regulate are called cyclin-dependent kinases (cdks).

In general, expression of G1 cyclins activates the G1-specific cyclin-cdk complex, allowing transcription of genes required for entry into S phase, including the DNA synthesis machinery; S-phase-specific cyclins; and proteins that remove inhibitors of S-phase cyclin/cdk activity. The increase in S-phase cyclin expression and function causes activation of the DNA synthesis machinery as well as the transcription of genes required for M phase, including cyclins and the mitosis machinery. Finally, M-phase cyclin-cdk complexes activate the mitotic machinery to break down the nuclear envelope, assemble the mitotic spindle to align the chromosomes during metaphase, and divide the cell during anaphase.

Because the cyclins in each phase of the cell cycle are degraded by the ubiquitin proteosome system on phase progression, once a cell enters the cell cycle, it is obligated to attempt complete cell division. Therefore, the primary control of cell-cycle entry occurs in G0 cells, when cyclins are not expressed. Exposure of G0 cells to mitogens induces the expression of cyclins and other proteins required for the cell cycle. After a certain period of exposure to mitogens, the cell reaches the restriction point, and it is irrevocably committed to entering the cell cycle. If mitogens are removed before the restriction point, the cell will exit the cell cycle and reenter

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