The question
Name: ______________________ Date: ____________ A checkpoint gene and tumor growth (9 points)
Data table
Mice with zero, one or two working copies of gene T
Gene T codes for a protein needed at the G2 checkpoint: it stops cells with damaged DNA from entering mitosis. Researchers bred mice with two working copies of gene T, one working copy, or no working copies (both copies disabled). Experiment 1: 30 mice of each kind were checked every month for tumors. Experiment 2: skin cells from each kind of mouse were grown in culture, exposed to a dose of X-rays that breaks DNA, and the percentage of cells entering mitosis in the next 2 hours was measured in 4 cultures each. Unexposed cells were measured in the same way.
| Working copies of gene T | Tumors by 6 months (%) | Tumors by 12 months (%) | Tumors by 18 months (%) | Cells entering mitosis after X-rays (%) | Cells entering mitosis, unexposed (%) |
|---|---|---|---|---|---|
| Two | 0 | 3 | 7 | 0.4 ± 0.3 | 5.8 ± 0.7 |
| One | 0 | 13 | 47 | 0.9 ± 0.4 | 6.0 ± 0.8 |
| None | 57 | 90 | 100 | 6.2 ± 0.8 | 6.1 ± 0.7 |
(a) (i) Describe what the G2 checkpoint checks before a cell enters mitosis.
(ii) Explain how cyclin and a cyclin-dependent kinase (CDK) together move a cell from G2 into mitosis. [2 points](b) (i) Identify the independent variable in experiment 1.
(ii) Identify the purpose of measuring unexposed cells in experiment 2.
(iii) State the null hypothesis for comparing cells with two working copies and cells with no working copies of gene T after X-rays in experiment 2. [3 points](c) (i) Calculate how many more mice with no working copies than mice with two working copies had a tumor by 12 months.
(ii) Using the error bars (±2 SE), determine whether the percentage of X-rayed cells entering mitosis differs significantly between cells with one and two working copies of gene T. Support your answer. [2 points](d) (i) Support the claim that gene T is a tumor suppressor gene, using evidence from both experiments.
(ii) Mice with one working copy develop tumors later than mice with none, even though their cells hold at the checkpoint almost as well as cells with two copies. Explain why tumors still develop in these mice, and why later. [2 points]