Figure 513

Mitosis and cytokinesis. (Only 4 of the 46 chromosomes in a human cell are illustrated.) (a) During interphase, chromatin exists in the nucleus as long, extended chains. The chains are partially coiled around clusters of histone proteins, producing a beaded appearance. (b) Prior to the onset of mitosis, DNA replicates, forming two identical sister chromatids that are joined at the centromere. A second pair of centrioles is formed at this time. As mitosis begins, the chromatids become highly condensed and (c) become attached to spindle fibers. (d) The two chromatids of each chromosome separate and move toward opposite poles of the cell (e) as the cell divides (cytokinesis) into two daughter cells.

centrosomes, while others connect the centrioles to the chromosomes. The spindle fibers and centrosomes constitute the mitotic apparatus.

As mitosis proceeds, the sister chromatids of each chromosome separate at the centromere and move toward opposite centrioles (Figure 5-13d). Cytokinesis begins as the sister chromatids separate. The cell begins to constrict along a plane perpendicular to the axis of the mitotic apparatus, and constriction continues until the cell has been pinched in half, forming the two daughter cells (Figure 5-13e), each having half the volume of the parent cell. Following cytokinesis, in each daughter cell, the spindle fibers dissolve, a nuclear envelope forms, and the chromatids uncoil.

The forces producing the movements associated with mitosis and cytokinesis are generated by (1) contractile proteins similar to those producing the forces generated by muscle cells (described in Chapter 11) and (2) the chemical kinetics associated with the elongation and shrinkage of microtubular filaments.

There are two critical checkpoints in the cell cycle, at which special events must occur in order for a cell to progress to the next phase (see Figure 5-12). One is at the boundary between Gi and S, and the other between G2 and M. For example, if some of the chromosomes have not completed their DNA replication during S phase, the cell will not begin mitosis until the replication is complete. To take another example, if DNA has been damaged, by x-rays for example, the cell will not enter M phase until the DNA has been repaired.

Two classes of proteins are the major players in timing cell division and the progression through these checkpoints—cell division cycle kinases (cdc kinases) and cyclins. Cyclins act as modulator molecules to activate the cdc kinases. The concentration of cyclins progressively increases during interphase and then rapidly falls during mitosis. Once activated, the kinase enzymes phosphorylate, and thus activate or inhibit a variety of proteins necessary for division, including an enzyme that digests cyclin and thus prepares the cell to begin the next division cycle. Signals generated by DNA damage or its failure to replicate inhibit cdc ki-nases, thus stopping the division process.

As we have noted, different types of cells progress through the cell cycle at different rates, some remaining for long periods of time in interphase. In order to progress to DNA replication, most cells must receive an external signal delivered by one or more of a group of proteins known as growth factors. Growth factors bind to their specific receptors in the cell membrane to generate intracellular signals; these signals activate various transcription factors that control the synthesis of key proteins involved in the division process and the checkpoint mechanisms. At least 50 growth factors have been identified. Many are secreted by one cell and stimulate other specific cell types to divide; others stimulate division in the cell that secretes them. Growth factors also influence various aspects of metabolism and cell differentiation. In the absence of the appropriate growth factor, most cells will not divide.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Genetic Information and Protein Synthesis CHAPTER FIVE

Genetic Information and Protein Synthesis CHAPTER FIVE

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