HW #6 answers

12.1: (b) Formation of crosslinked thymine dimers.

12.2: Such people are somatic mosaics, a condition that can be explained by somatic mutations in the pigmented cells of the iris of the eye or in their precursor cells.

12.5: There are 18 depurinations per minute and 1440 minutes per day, so the total number of depurinations per cell per day is 18 x 1440 = 25,920.


13.2: It is a “genetic disease” in that genetic mutations take place in cells and make them undergo uncontrolled growth and division. However, most cancers are caused by somatic mutations in the cells of affected individuals; hence, they are not inherited from the parents (familial) but rather are due to new somatic mutations.

13.11: A defect in the G1/S checkpoint allows cells with damaged chromosomes to enter DNA synthesis, when replication and repair can cause chromosome rearrangements or replication errors leading to aneuploidy.

13.19. Effectively, HPV removes all the brakes on cellular proliferation, turns on all the stimuli, and removes the infected cell’s ability to induce apoptosis to kill itself. The cell will effectively be p53−, because E6 is present to ensure that levels of p53 do not rise. This means that synthesis of Bax will not be induced, and cells will not undergo apoptosis. In addition, synthesis of p21, GADD45, and 14-3-3s will not be accelerated, so cells will not arrest in the cell cycle. Because E7 disables RB, E2F will be active at all times, resulting in new rounds of DNA synthesis and continued cellular proliferation.

13.20. For generations I and II, the band at position c is associated with the disease. In generation III, individuals 2 and 7 are at risk because of parent 5, and the individuals 21 and 22 are at risk because of parent 20. However, individual 13 is not at risk because the parent 14 with a band at position c is not affected; band c in this individual evidently originates from a nonmutant allele.

Extra Question #1:

Endogenous mutagens originate from sources inside the cell or organism; exogenous mutagens come from outside (or, environmental) sources.

Endogenous mutagen sources include nucleotide tautomerism (enol/keto), replication errors, and metabolic byproducts (such as 8-oxoguanine).

Exogenous mutagen examples include UV light, chemical base analogs (such as 5-bromouracil), and diesel fumes.

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