Question

Toettcher et al. (2009) examined the impact that gamma-irradiation and DNA damage had on cell cycle arrest in wild-type and p21-/- mutant cells (cells that lack the protein p21). They ran an experiment where cells were exposed to gamma-irradiation and then measured the DNA content by FACS at the indicated time-points. A. WT (normal) cells IK 2Non % of cells 1000 0 500 10000 500 10000 500 1000 500 DNA content B. p21-/- cells on 2N AN 48h % of cells 10000 500 0 500 DNA content 10000 500 1000 0 500 1000 Toettcher J E et al. PNAS 2009;106:785-790

Answer 1

Let's analize the charts first. The results show that wild type cells appeared in the beginning as both 2N and 4N DNA content, this means some cells had already passed Synthesis stage and were on G2, while others were still on G1. As hours and days pass by, we can see there is no change in that, the same percentage of 4N cells stay in place, meaning they were unable to jump to M stage, they stayed at G2. We can guess p21 protein has a role in this, as the p21 -/- were not affected like we found in the wild type and proceeded to M stage without ay problem.

The answer is h. B and E, which leads to b. Cells were arrested in G2, and e. C and D, which leads to c. Cells failed to enter M phase, and d. p21 became mutated