31.A conditional mutation is one that expresses its mutant phenotype only under certain conditions (the restrictive conditions) and expresses the normal phenotype under other conditions (the permissive conditions). One type of conditional mutation is a temperature-sensitive mutation, which expresses the mutant phenotype only at certain temperatures.

Strains of E. coli have been isolated that contain temperature-sensitive mutations in the genes encoding different components of the replication machinery. In each of these strains, the protein produced by the mutated gene is nonfunctional under the restrictive conditions. These strains are grown under permissive conditions and then abruptly switched to the restrictive condition. After one round of replication under the restrictive condition, the DNA from each strain is isolated and analyzed. What characteristics would you expect to see in the DNA isolated from each strain with a temperature-sensitive mutation in its gene that encodes in the following proteins?

32. DNA topoisomerases play important roles in DNA replication and supercoiling (see Chapter 8). These enzymes are also the targets for certain anticancer drugs. Eric Nelson and his colleagues studied m-AMSA, an anticancer compound that acts on topoisomerases. They found that m-AMSA stabilizes an intermediate produced in the course of the topoisomerase’s action. The intermediate consists of the topoisomerase bound to the broken ends of the DNA (E. M. Nelson, K. M. Tewey, and L. F. Liu. 1984. Proceedings of the National Academy of Sciences of the United States of America 81:1361–1365). Breaks in DNA that are produced by anticancer compounds such as m-AMSA inhibit the replication of the cellular DNA and thus stop cancer cells from proliferating. Explain how m-AMSA and other anticancer agents that target topoisomerase enzymes taking part in replication might lead to DNA breaks and chromosome rearrangements.