Chapter 14

Why are X rays more potent mutagens than UV radiation?

Contrast the various types of DNA repair mechanisms known to counteract the effects of UV radiation. What is the role of visible light in repairing UV-induced mutations?

Mammography is an accurate screening technique for the early detection of breast cancer in humans. Because this technique uses X rays diagnostically, it has been highly controversial. Can you explain why? What reasons justify the use of X rays for such a medical screening technique?

A significant number of mutations in the $HBB$ gene that cause human $\beta$ -thalassemia occur within introns or in upstream noncoding sequences. Explain why mutations in these regions often lead to severe disease, although they may not directly alter the coding regions of the gene.

Describe how the Ames test screens for potential environmental mutagens.Why is it thought that a compound that tests positively in the Ames test may also be carcinogenic?

What genetic defects result in the disorder xeroderma pigmento$\mathrm{sum}(\mathrm{XP})$ in humans? How do these defects create the phenotypes associated with the disorder?

Compare DNA transposons and retrotransposons. What properties do they share?

In maize, a $Ds$ or $Ac$ transposon can alter the function of genes at or near the site of transposon insertion. It is possible for these elements to transpose away from their original insertion site, causing a reversion of the mutant phenotype. In some cases, however, even more severe phenotypes appear, due to events at or near the mutant allele. What might be happening to the transposon or the nearby gene to create more severe mutations?

It has been noted that most transposons in humans and other organisms are lacated in noncoding regions of the genome regions such as introns, pseudogenes, and stretches of particular types of repetitive DNA. There are several ways to interpret this observation. Describe two possible interpretations. Which interpretation do you favor? Why?