ch22 - Chapter 22 22.1 Many cancers seem to involve...

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Chapter 22 22.1 Many cancers seem to involve environmental factors. Why, then, is cancer called a genetic disease? ANS: Cancer has been called a genetic disease because it results from mutations of genes that regulate cell growth and division. Nonhereditary forms of cancer result from mutations in somatic cells. These mutations, however, can be induced by environmental factors including tobacco smoke, chemical pollutants, ionizing radiation, and UV light. Hereditary forms of cancer also frequently involve the occurrence of environmentally- induced somatic mutations. FEEDBACK: 22.1 DIFF: easy 22.2. Both embryonic cells and cancer cells divide quickly. How can these two types of cells be distinguished from each other? ANS: Cancer cells do not display contact inhibition—they pile up on top of each other— whereas embryonic cells spread out in flat sheets. Cancer cells are frequently aneuploid; embryonic cells are euploid. FEEDBACK: 22.1 DIFF: easy 22.3 Most cancer cells are aneuploid. Suggest how aneuploidy might contribute to deregulation of the cell cycle. ANS: Aneuploidy might involve the loss of functional copies of tumor suppressor genes, or it might involve the inappropriate duplication of proto-oncogenes. Loss of tumor suppressor genes would remove natural brakes on cell division, and duplication of proto- oncogenes would increase the abundance of factors that promote cell division. FEEDBACK: 22.1 DIFF: easy 22.4. Would you ever expect to find a tumor-inducing retrovirus that carried a processed cellular tumor suppressor gene in its genome? ANS: No. A virus that carried a processed copy of a tumor suppressor gene would not
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be expected to induce tumor formation because the product of the tumor suppressor gene would help to restrain cell growth and division. FEEDBACK: 22.1 DIFF: medium 22.5 How do we know that normal cellular oncogenes are not simply integrated retroviral oncogenes that have acquired the proper regulation? ANS: They possess introns. FEEDBACK: 22.2 DIFF: easy 22.6. How might the absence of introns in a retroviral oncogene explain that gene’s overexpression in the tissues of an infected animal? ANS: The absence of introns might speed up the expression of the gene’s protein product because there would be no need for splicing. In addition, some introns contain sequences called silencers that negatively regulate transcription. Removal of these sequences might cause transcription to occur when it otherwise would not. FEEDBACK: 22.2 DIFF: medium 22.7 When cellular oncogenes are isolated from different animals and compared, the amino acid sequences of the polypeptides they encode are found to be very similar. What does this suggest about the functions of these polypeptides? ANS:
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ch22 - Chapter 22 22.1 Many cancers seem to involve...

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