Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

Exercise 4

Still More DNA Replication. Suppose you are given a new temperature-sensitive bacterial mutant that grows normally at 37°C but cannot replicate its chromosomes properly at 42°C. To investigate the nature of the underlying defect, you incubate the cells at 42°C with radioactive substrates required for DNA synthesis. After one hour you find that the cell population has doubled its DNA content, suggesting that DNA replication can still occur at 42°C. Moreover, centrifugation reveals that all of this DNA has the same large molecular weight as does the original DNA present in the cells. When the DNA is denatured, however, you discover that 75% of the resulting single-stranded DNA has a molecular weight that is half that of the original double-stranded DNA, and the remaining 25% of the DNA has a much lower molecular weight. Based on the pre ceding results, what gene do you think is defective in these cells Explain how such a defect would account for the experimental results that you observed.