Worked Problems

1. If a fertilized Drosophila egg is punctured at the anterior end and a small amount of cytoplasm is allowed to leak out, what will be the most likely effect on the development of the fly embryo?

The egg-polarity genes determine the major axes of development in the Drosophila embryo. One of these genes is bicoid, which is transcribed in the maternal parent. As bicoid mRNA passes into the egg, the mRNA becomes anchored to the anterior end of the egg. After the egg is laid, bicoid mRNA is translated into Bicoid protein, which forms a concentration gradient along the anterior-posterior axis of the embryo. The high concentration of Bicoid protein at the anterior end induces the development of anterior structures such as the head of the fruit fly. If the anterior end of the egg is punctured, cytoplasm containing high concentrations of Bicoid protein will leak out, reducing the concentration of Bicoid protein at the anterior end. The result will be that the embryo fails to develop head and thoracic structures at the anterior end.

2. In some cancer cells, a specific gene has become duplicated many times. Is this gene likely to be an oncogene or a tumor-suppressor gene? Explain your reasoning.

The gene is likely to be an oncogene. Oncogenes stimulate cell proliferation and act in a dominant manner. Therefore, extra


* 1. What experiments suggested that genes are not lost or permanently altered in development? 2. Briefly explain how the Dorsal protein is redistributed in the formation of the Drosophila embryo and how this copies of an oncogene will result in cell proliferation and cancer. Tumor-suppressor genes, on the other hand, suppress cell proliferation and act in a recessive manner; a single copy of a tumor-suppressor gene is sufficient to prevent cell proliferation. Therefore extra copies of the suppressor gene will not lead to cancer.

3. The immunoglobulin molecules of a particular mammalian species has kappa and lambda light chains and heavy chains. The kappa gene consists of 250 V and 8 J segments. The lambda gene contains 200 V and 4 J segments. The gene for the heavy chain consists of 300 V, 8 J, and 4 D segments. Considering just somatic recombination and random combinations of light and heavy chains, how many different types of antibodies can be produced by this species?

For the kappa light chain, there are 250 X 8 = 2000 combinations; for the lambda light chain, there are 200 X 4 = 800 combinations; so a total of 2800 different types of light chains are possible. For the heavy chains, there are 300 X 8 X 4 = 9600 possible types. Any of the 2800 light chains can combine with any of the 9600 heavy chains; so there are 2800 X 9600 = 26,880,000 different types of antibodies possible from somatic recombination and random combination alone. Junctional diversity and somatic hypermutation would greatly increase this diversity.

redistribution helps to establish the dorsal-ventral axis of the early embryo. * 3. Briefly describe how the bicoid and nanos genes help to determine the anterior-posterior axis of the fruit fly.

* 4. List the three major classes of segmentation genes and outline the function of each. 5. What role do homeotic genes play in the development of fruit flies?

* 6. What is apoptosis and how is it regulated?

* 7. Explain how each of the following processes contributes to antibody diversity.

(a) Somatic recombination.

(b) Junctional diversity.

(c) Hypermutation.

8. What is the function of the MHC antigens? Why are the genes that encode these antigens so variable?

* 9. Outline Knudson's multistage theory of cancer and describe how it helps to explain unilateral and bilateral cases of retinoblastoma.

10. Briefly explain how cancer arises through clonal evolution.

*11. What is the difference between an oncogene and a tumor-suppressor gene? Give some examples of the functions of proto-oncogenes and tumor suppressers in normal cells.

12. Why do mutations in genes that encode DNA repair enzymes and chromosome segregation often produce a predisposition to cancer?

*13. What role do telomeres and telomerase play in cancer progression?

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