CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

9. Cell Division—Proliferation and Reproduction

9.4. Controlling Mitosis

The cell-division process is regulated so that it does not interfere with the activities of other cells or of the whole organism. To determine if cell division is appropriate, many cells gather information about themselves and their environment. Checkpoints are times during the cell cycle when cells determine if they are prepared to move forward with cell division.

At these checkpoints, cells use proteins to evaluate their genetic health, their location in the body, and a need for more cells. Poor genetic health, the wrong location, and crowded conditions are typically interpreted as signals to wait. Good genetic health, the correct location, and uncrowded conditions are interpreted as signals to proceed with cell division.

The cell produces many proteins to gather this information and assess if cell division is appropriate. These proteins are made by one of two classes of genes. Proto-oncogenes code for proteins that encourage cell division. Tumor-suppressor genes code for proteins that discourage cell division. A healthy cell receives signals from both groups of proteins about how appropriate it is to divide. The balance of information provided by these two groups of proteins allows for controlled cell division.

One tumor-suppressor gene is p53. Near the end of G1, the protein produced by the p53 gene identifies if the cell’s DNA is damaged. If the DNA is healthy, p53 allows the cell to divide (figure 9.12a). If the p53 protein detects damaged DNA, it triggers other proteins to become active and repair the DNA. If the damage is too extensive for repair, the p53 protein triggers an entirely different response from the cell. The p53 protein causes the cell to self-destruct. Apoptosis is the process whereby a cell digests itself from the inside out. You might think of it as cellular suicide. In this scenario, apoptosis prevents mutated cells from continuing to grow. Other healthy cells will undergo cell division to replace the lost cell.

FIGURE 9.12. The Function of p53 Protein

(a) Normal p53 protein stops cell division until damaged DNA is repaired. If the DNA is unrepairable, the p53 protein causes cell death. (b) Mutated p53 protein allows cells with damaged DNA to divide.

Consider the implications of a mutation within the p53 gene. If the p53 protein does not work correctly, then cells with damaged DNA may move through cell division. As these cells move through many divisions, their inability to detect damaged DNA disposes them to accumulate more mutations than do other cells. These mutations may occur in their proto-oncogenes and other tumor-suppressor genes. As multiple mutations occur in the genes responsible for regulating cell division, the cell is less likely to control cell division appropriately. When a cell is unable to control cell division, cancer can develop.

9.4. CONCEPT REVIEW

12. What are checkpoints?

13. What role does p53 have in controlling cell division?