In Table 7-1, why are there no entries for the first four tissue sources? For the last three entries, what is the most likely explanation for the slight differences in the composition of human DNA from the three tissue sources?

In Figure 7-7, do you recognize any of the components used to make Watson and Crick’s DNA model? Where have you seen them before?

Referring to Figure 7-20, answer the following questions:

What is different about the reaction catalyzed by the green helicase in Figure 7-20 and the yellow gyrase in Figure 7-21?

In Figure 7-23(a), label all the leading and lagging strands.

Describe the types of chemical bonds in the DNA double helix.

Explain what is meant by the terms conservative and semiconservative replication.

What is meant by a primer, and why are primers necessary for DNA replication?

What are helicases and topoisomerases?

Why is DNA synthesis continuous on one strand and discontinuous on the opposite strand?

If the four deoxynucleotides showed nonspecific base pairing (A to C, A to G, T to G, and so on), would the unique information contained in a gene be maintained through round after round of replication? Explain.

If the helicases were missing during replication, what would happen to the replication process?

What would happen if, in the course of replication, the topoisomerases were unable to reattach the DNA fragments of each strand after unwinding (relaxing) the DNA molecule?

Which of the following is not a key property of hereditary material?

It is essential that RNA primers at the ends of Okazaki fragments be removed and replaced by DNA because otherwise which of the following events would result?

Polymerases usually add only about 10 nucleotides to a DNA strand before dissociating. However, during replication, DNA pol III can add tens of thousands of nucleotides at a moving fork. How is this addition accomplished?

At each origin of replication, DNA synthesis proceeds bi-directionally from two replication forks. Which of the following would happen if a eukaryotic mutant arose having only one functional fork per replication bubble? (See diagram.)

In a diploid cell in which 2n = 14, how many telomeres are there in each of the following phases of the cell cycle?

If thymine makes up 15 percent of the bases in a specific DNA molecule, what percentage of the bases is cytosine?

If the GC content of a DNA molecule is 48 percent, what are the percentages of the four bases (A, T, G, and C) in this molecule?

Bacteria called extremophiles are able to grow in hot springs such as Old Faithful at Yellowstone National Park in Wyoming. Do you think that the DNA of extremophiles would have a higher content of GC or AT base pairs? Justify your answer.

Assume that a certain bacterial chromosome has one origin of replication. Under some conditions of rapid cell division, replication could start from the origin before the preceding replication cycle is complete. How many replication forks would be present under these conditions?

A molecule of composition

is replicated in a solution containing unlabeled (not radioactive) GTP, CTP, and TTP plus adenine nucleoside triphosphate with all its phosphorus atoms in the form of the radioactive isotope ³²P. Will both daughter molecules be radioactive? Explain. Then repeat the question for the molecule

Would the Meselson and Stahl experiment have worked if diploid eukaryotic cells had been used instead?

Consider the following segment of DNA, which is part of a much longer molecule constituting a chromosome:

If the DNA polymerase starts replicating this segment from the right,

The DNA polymerases are positioned over the following DNA segment (which is part of a much larger molecule) and moving from right to left. If we assume that an Okazaki fragment is made from this segment, what will be the fragment’s sequence? Label its 5′ and 3′ ends.

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E. coli chromosomes in which every nitrogen atom is labeled (that is, every nitrogen atom is the heavy isotope ¹⁵N instead of the normal isotope ¹⁴N) are allowed to replicate in an environment in which all the nitrogen is ¹⁴N. Using a solid line to represent a heavy polynucleotide chain and a dashed line for a light chain, sketch each of the following descriptions:

If a mutation that inactivated telomerase occurred in a cell (telomerase activity in the cell = zero), what do you expect the outcome to be?

On the planet Rama, the DNA is of six nucleotide types: A, B, C, D, E, and F. Types A and B are called marzines, C and D are orsines, and E and F are pirines. The following rules are valid in all Raman DNAs:

Total marzines = total orsines = total pirines

If you extract the DNA of the coliphage ϕX174, you will find that its composition is 25 percent A, 33 percent T, 24 percent G, and 18 percent C. Does this composition make sense in regard to Chargaff’s rules? How would you interpret this result? How might such a phage replicate its DNA?

In Chapter 5 you saw that bacteria transfer DNA from one member of their species to another in a process called conjugation. Recently it has been shown that the transfer of DNA from one bacterial cell to another is not limited to members of the same species. A microbiologist studying the bacteria Diplococcus pneumonia hypothesizes that a region of its chromosome was in fact transferred from Mycobacterium tuberculosis. Based on the data presented in Table 7-1, what distinguishing feature of the transferred DNA would provide support for this hypothesis?

Given what you know about the structure and function of telomerase, provide a plausible model to explain how a species could exist with a combination of two different repeats (for example, TTAGGG and TTGTGG) on each of their telomeres.

Do bacteria require telomerase? Explain why or why not.

Watson and Crick used an approach called model building to deduce the structure of the DNA double helix. How does this differ from the more conventional experimental approach that is undertaken in a research laboratory? In this regard, why was the experiment of Meselson and Stahl considered to be of such critical importance?