EXERCISE 9.4. ALTERING THE RATE OF MITOSIS Cells can be driven to divide mitotically by a variety of internal (endogenous) and applied (exogenous) stimuli. In plants stimuli include sunlight, water, nutrients, and physical space. A number of different plant hormones (gibberellic acid, indole-3-acetic acid, and brassinosteroids) have been shown to influence stem and root elongation, germination, flowering, and the size of fruit. The number of mitotic cells in an onion root should increase when treated with these hormones. A scientist was supplied with freshly growing onion root tips derived from two plants: one grown without additional hormones as a control, and a second plant grown with additional hormones for 3 days. The in each phase of the cell cycle in Table 9.3. scientist examined the apical meristems of the roots with a microscope and recorded the number of cells TABLE 9.3. Altering Mitotic Rates of Onion Root Tips Control Cell Cycle Phase Counted Cells % of Total Hormone Treated Counted Cells % of Total Interphase 1906 95.3 1808 90.4 Prophase 43 2.15 111 5.55 Metaphase 37 1.85 46 2.30 Anaphase 13 0.65 24 1.20 Telophase 1 0.05 11 0.55 Total 2000 100 2000 100 ANALYSIS-ALTERING MITOTIC RATES 1. Which phase(s) of mitosis was/were most affected by the plant hormone? Why? 2. What factors might control the timing of an animal cell cycle? Why is this regulation important? 3. What might happen if the timing of the cell cycle was significantly altered? For example, what would happen if a cell began dividing every hour instead of every 24? 21 162 Cell Division 4. Caffeine is a known stimulant for humans. What might be the effect(s) of caffeine on human cells? Design a hypothesis and explain how to test it in the hopes of gaining knowledge about caffeine. 5. Assume that the 6 x 10" base pairs of DNA in a diploid cell is replicated during 12 hours of the S stage of interphase. How many base pairs are minimally being replicated each second?