19.1: Introduction
Mitosis is the condensation of chromosomes from chromatin and their separation into dividing cells. Cytokinesis is the process that divides a cell into two new cells after duplicated chromosomes are safely on opposite sides of the cell. Mitosis and Cytokinesis together are a relatively short time in the cell cycle . While cell cycle times vary, imagine a cell that divides every 20 hours. Mitosis and cytokinesis would last about 1-1.5 hours in the life of this cell. Mitosis is divided into 4-5 phases (depending on whose text you are reading!), the last of which overlaps cytokinesis. Mitosis takes about an hour and cytokinesis about 30 minutes in this example.
The rest of a 20-hour cell cycle is spent in interphase , so-called because 19th century microscopists saw nothing happening in cells when they were not in mitosis or actually dividing. However, by the 1970s, experiments had revealed that interphase itself could be divided into discrete phases of cellular activity, called G 1 , S and G 2 , occurring in that order. It turns out that kinases regulate progress through the cell cycle, catalyzing timely protein phosphorylations . The early experiments led to the discovery of mitosis-promoting factor ( MPF ), one of these kinases.
Kinase-regulated events are checkpoints that cells must pass through in order to enter the next step in the cell cycle. As you might guess, the failure of a checkpoint can have serious consequences. Carcinogenesis , the runaway proliferation of cancer cells, is one such consequence that we will consider in this chapter. We will also look at the fate of differentiating cells and at details of cellular end-of-life events, including apoptosis , or programed cell death ).
Learning Objectives
When you have mastered the information in this chapter, you should be able to:
1. Describe the phases of the cell cycle and what occurs in each.
2. Interpret experiments leading to our understanding of the separation of chromosomal events from duplication of the DNA contained in those chromosomes.
3. Describe the role of cyclin and cdk (cyclin-dependent kinases) in MPF.
4. Compare the roles of different cyclins and cdks in regulating the cell cycle.
5. Define cell-cycle checkpoints that monitor cell cycle activities.
6. Explain the molecular interactions between DNA damage, cell cycle checkpoints (arrest of the cell cycle if vital activities are blocked) and apoptosis.
7. State an hypotheses for how cell cycling errors can transform normal cells into cancer cells.
8. List some examples of apoptosis in humans and other organisms.
9. Compare and contrast examples of apoptosis and necrosis .
10. Formulate an hypothesis to account for the degradation of cyclin after mitosis.
11. Research and explain how different chemotherapeutic agents work and the biochemical or molecular basis of their side effects.