Application Questions And Problems

14. If telomeres are normally shortened after each round of replication in somatic cells, what prediction would you make about the length of telomeres in Dolly, the first cloned sheep?

*15. Give examples of genes that affect development in fruit flies by regulating gene expression at the level of (a) transcription and (b) translation.

16. What would be the most likely effect on development of puncturing the posterior end of a Drosophila egg, allowing a small amount of cytoplasm to leak out, and then injecting that cytoplasm into the anterior end of another egg?

*17. What would be the most likely result of injecting bicoid mRNA into the posterior end of a Drosophila embryo and inhibiting the translation of nanos mRNA?

18. What would be the most likely effect of inhibiting the translation of hunchback mRNA throughout the embryo?

*19. Molecular geneticists have performed experiments in which they altered the number of copies of the bicoid gene in flies, affecting the amount of Bicoid protein produced.

(a) What would be the effect on development of an increased number of copies of the bicoid gene?

(b) What would be the effect of a decreased number of copies of bicoid?

Justify your answers.

20. What would be the most likely effect on fruit-fly development of a deletion in the nanos gene?

21. Give an example of a gene found in each of the categories of genes (egg-polarity, gap, pair-rule, and so forth) listed in Figure 21.12.

*22. In a particular species, the gene for the kappa light chain has 200 V gene segments and 4 J segments. In the gene for the lambda light chain, this species has 300 V segments and 6 J segments. Considering only the variability arising from somatic recombination, how many different types of light chains are possible?

23. In the fictional book Chromosome 6 by Robin Cook, a biotechnology company genetically engineers individual bonobos (a type of chimpanzee) to serve as future organ donors for clients. The genes of the bonobos are altered so that no tissue rejection takes place when their organs are transplanted into a client. What genes would need to be altered for this scenario to work? Explain your answer.

*24. A couple has one child with bilateral retinoblastoma. The mother is free from cancer, but the father had unilateral retinoblastoma and he has a brother who has bilateral retinoblastoma.

(a) If the couple has another child, what is the probability that this next child will have retinoblastoma?

(b) If the next child has retinoblastoma, is it likely to be bilateral or unilateral?

(c) Propose an explanation for why the father's case of retinoblastoma was unilateral, whereas his sons and brother's cases are bilateral.

25. Some cancers are consistently associated with the deletion of a particular part of a chromosome. Does the deleted region contain an oncogene or a tumor-suppressor gene? Explain why.

26. Cells in a tumor contain mutated copies of a particular gene that promotes tumor growth. Gene therapy can be used to introduce a normal copy of this gene into the tumor cells. Would you expect this therapy to be effective if the mutated gene were an oncogene? A tumor-suppressor gene? Explain your reasoning.

CHALLENGE QUESTIONS]_

27. As we have learned in this chapter, the Nanos protein inhibits the translation of hunchback mRNA, thus lowering the concentration of Hunchback protein at the posterior end of a fruit-fly embryo and stimulating the differentiation of posterior characteristics. The results of experiments have demonstrated that the action of Nanos on hunchback mRNA depends on the presence of an 11-base sequence that is located in the 3' untranslated region of hunchback mRNA. This sequence has been termed the Nanos response element (NRE). There are two copies of NREs in the trailer of hunchback mRNA. If a copy of NRE is added to the 3' untranslated region of another mRNA produced by a different gene, the mRNA now becomes repressed by Nanos. The repression is greater if several NREs are added. On the basis of these observations, propose a mechanism for how Nanos inhibits Hunchback translation.

28. Offer a possible explanation for the widespread distribution of Hox genes among animals.

29. Many cancer cells are immortal (will divide indefinitely) because they have mutations that allow telomerase to be expressed. How might this knowledge be used to design anticancer drugs?

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