Exposure of DNA to X-rays results in the formation of double-strand breaks (DSBs

Question

Exposure of DNA to X-rays results in the formation of double-strand breaks (DSBs). Yeast have two pathways for repair of DSBs: homologous repair, which is dependent on the gene rad52 and other rad genes, and non-homologous end joining, which is less efficient and tends to produce many chromosomal alterations. You have two strains of yeast. train 1 is wild-type, and strain 2 is rad52-1-. You expose both strains to increasing doses of X-rays and then determine survival fraction for both strains. Which of the following epresents the expected results for this experiment? Both strains have increasing survival with increasing X-ray exposure, but the survival of strain 2 increases much more rapidly than the survival of strain 1 Both strains have decreasing survival with increasing X-ray exposure, but the survival of strain 1 decreases much more rapidly than the survival of strain 2 Both strains show extremely low survival at even the lowest X-ray exposure O Strain 1 shows 100% survival for all doses of UV, and the survival of strain 2 decreases with increasing X-ray exposure Both strains have increasing survival with increasing X-ray exposure, but the survival of strain 2 increases much more rapidly than the survival of strain 1 Strain 2 shows 100% survival for all doses of UV, and the survival of strain 1 decreases with increasing X-ray exposure Both strains have decreasing survival with increasing X-ray exposure, but the survival of strain 2 decreases much more rapidly than the survival of strain

Explanation / Answer

1. Option G is correct.

WT and mutant both exhibit decreased survival with increasing X-ray exposure.

However, WT can survive better compared to the rad mutant. This is because rad52 is involved in DNA repair. So, rad52 mutants are more susceptible to DNA damage.

2.

Option C is correct.

Teresa = Colourblind = blu7/blu7

Tommy = Colourblind: Genotype = ? (If he is blu7/blu7 or blu7/Y, there is no chance that their daughter can be normal. So, he must be a mutant at another gene which also causes colorblindness)

Terry = Normal: So, she must carry at least one allele that is responsible for normal vision (Since colorblindness is a recessive disorder).

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