16.Suppose that a future scientist explores a distant planet and discovers a novel form of double-stranded nucleic acid. When this nucleic acid is exposed to DNA polymerases from E. coli, replication takes place continuously on both strands. What conclusion can you make about the structure of this novel nucleic acid?

17.Phosphorus is required to synthesize the deoxyribonucleoside triphosphates used in DNA replication. A geneticist grows some E. coli in a medium containing nonradioactive phosphorus for many generations. A sample of the bacteria is then transferred to a medium that contains a radioactive isotope of phosphorus (³²P). Samples of the bacteria are removed immediately after the transfer and after one and two rounds of replication. Assume that newly synthesized DNA contains ³²P and that the original DNA contains nonradioactive phosphorus. What will be the distribution of radioactivity in the DNA of the bacteria in each sample? Will radioactivity be detected in neither strand, one strand, or both strands of the DNA?

*18.A line of mouse cells is grown for many generations in a medium with ¹⁵N. Cells in G₁ are then switched to a new medium that contains ¹⁴N. Draw a pair of homologous chromosomes from these cells at the following stages, showing the two strands of DNA molecules found in the chromosomes. Use different colors to represent strands with ¹⁴N and ¹⁵N. (See Chapter 2 for a review of the stages of the cell cycle and meiosis.)

*19.A circular molecule of DNA contains 1 million base pairs. If the rate of DNA synthesis at a replication fork is 100,000 nucleotides per minute, how much time will theta replication require to completely replicate the molecule, assuming that theta replication is bidirectional?

20.A bacterium synthesizes DNA at each replication fork at a rate of 1000 nucleotides per second. If this bacterium completely replicates its circular chromosome by theta replication in 30 minutes, how many base pairs of DNA will its chromosome contain?

21.In Figure 9.7, which is the leading strand and which is the lagging strand?

*22.The following diagram represents a DNA molecule that is undergoing replication. Draw in the strands of newly synthesized DNA and identify (a) through (d):

*23.What would be the effect on DNA replication of mutations that destroyed each of the following activities of DNA polymerase I?

24.How would DNA replication be affected in a bacterial cell that is lacking DNA gyrase?

*25.DNA polymerases are not able to prime replication, yet primase and other RNA polymerases can. Some geneticists have speculated that the inability of DNA polymerase to prime replication is due to its proofreading function. This hypothesis argues that proofreading is essential for the faithful transmission of genetic information and that, because DNA polymerases have evolved the ability to proofread, they cannot prime DNA synthesis. Explain why proofreading and priming functions in the same enzyme might be incompatible.

26.Arrange the following components of replication in the order in which they first act in the replication process: ligase, DNA polymerase I, helicase, gyrase, primase, single-strand-binding protein, initiator proteins.

27.A number of scientists who study cancer treatment have become interested in telomerase. Why? How might anticancer therapies that target telomerase work?

*28.The enzyme telomerase is part protein and part RNA. What would be the most likely effect of deleting the gene that encodes the RNA part of telomerase? How would the function of telomerase be affected?

29. Dyskeratosis congenita (DKC) is a rare genetic disorder characterized by abnormal fingernails and skin pigmentation, the formation of white patches on the tongue and cheek, and progressive failure of the bone marrow. An autosomal dominant form of DKC results from mutations in the gene that encodes the RNA component of telomerase. Tom Vulliamy and his colleagues examined 15 families with autosomal dominant DKC (T. Vulliamy et al. 2004. Nature Genetics 36:447–449). They observed that the median age of onset of DKC in parents was 37 years, whereas the median age of onset in the children of affected parents was 14.5 years. Thus, DKC in these families arose at progressively younger ages in successive generations, a phenomenon known as anticipation. The researchers measured the telomere length of members of these families; the measurements are given in accompanying table. Telomeres normally shorten with age, so telomere length was adjusted for age. Note that the age-adjusted telomere length of all members of these families is negative, indicating that their telomeres are shorter than normal. For age-adjusted telomere length, the more negative the number, the shorter the telomere.

30.An individual is heterozygous at two loci (Ee Ff), and the two genes are in repulsion (see Chapter 5). Assume that single-strand breaks and branch migration occur at the positions shown below. Using different colors to represent the two homologous chromosomes, draw the noncrossover recombinant and crossover recombinant DNA molecules that will result from homologous recombination. (Hint: See Figure 9.16.)