Question 18.17

A codon that specifies the amino acid Gly undergoes a single-base substitution to become a nonsense mutation. In accord with the genetic code given in Figure 15.10, is this mutation a transition or a transversion? At which position of the codon does the mutation occur?

Question 18.18

• a. If a single transition occurs in a codon that specifies Phe, what amino acids can be specified by the mutated sequence?
• b. If a single transversion occurs in a codon that specifies Phe, what amino acids can be specified by the mutated sequence?
• c. If a single transition occurs in a codon that specifies Leu, what amino acids can be specified by the mutated sequence?
• d. If a single transversion occurs in a codon that specifies Leu, what amino acids can be specified by the mutated sequence?

Question 18.19

Hemoglobin is a complex protein that contains four polypeptide chains. The normal hemoglobin found in adults—called adult hemoglobin—consists of two alpha and two beta polypeptide chains, which are encoded by different loci. Sickle-
cell hemoglobin, which causes sickle-cell anemia, arises from a mutation in the beta chain of adult hemoglobin. Adult hemoglobin and sickle-cell hemoglobin differ in a single amino acid: the sixth amino acid from one end in adult hemoglobin is glutamic acid, whereas sickle-cell hemoglobin has valine at this position. After consulting the genetic code provided in Figure 15.10, indicate the type and location of the mutation that gave rise to sickle-cell anemia.

Question 18.20

The following nucleotide sequence is found on the template strand of DNA. First, determine the amino acids of the protein encoded by this sequence by using the genetic code provided in Figure 15.10. Then, give the altered amino acid sequence of the protein that will be found in each of the following mutations:

Question 18.21

Draw a hairpin turn like that shown in Figure 18.5 for the repeated sequence found in Fragile X syndrome (see Table 18.1).

Question 18.22

A polypeptide has the following amino acid sequence:

The amino acid sequence of this polypeptide was determined in a series of mutants listed in parts a through e. For each mutant, indicate the type of mutation that occurred in the DNA (single-base substitution, insertion, deletion) and the phenotypic effect of the mutation (nonsense mutation, missense mutation, frameshift, etc.).

Question 18.23

A gene encodes a protein with the following amino acid sequence:

A mutation occurs in the gene. The mutant protein has the following amino acid sequence:

An intragenic suppressor restores the amino acid sequence to that of the original protein:

Give at least one example of base changes that could produce the original mutation and the intragenic suppressor. (Consult the genetic code in Figure 15.10.)

Question 18.24

A gene encodes a protein with the following amino acid sequence:

A nonsense mutation from a single-base-pair substitution occurs in this gene, resulting in a protein with the amino acid sequence Met-Lys. An intergenic suppressor mutation allows the gene to produce the full-length protein. With the original mutation and the intergenic suppressor present, the gene now produces a protein with the following amino acid sequence:

Give the location and nature of the original mutation and of the intergenic suppressor.

Question 18.25

Can nonsense mutations be reversed by hydroxylamine? Why or why not?

Question 18.26

The following nucleotide sequence is found in a short stretch of DNA:

If this sequence is treated with hydroxylamine, what sequences will result after replication?

Question 18.27

The following nucleotide sequence is found in a short stretch of DNA:

• a. Give all the mutant sequences that can result from spontaneous depurination in this stretch of DNA.
• b. Give all the mutant sequences that can result from spontaneous deamination in this stretch of DNA.

Question 18.28

In many eukaryotic organisms, a significant proportion of cytosine bases are naturally methylated to 5-methylcytosine. Through evolutionary time, the proportion of AT base pairs in the DNA of these organisms increases. Can you suggest a possible mechanism for this increase?

Question 18.29

A chemist synthesizes four new chemical compounds in the laboratory and names them PFI1, PFI2, PFI3, and PFI4. He gives the PFI compounds to a geneticist friend and asks her to determine their mutagenic potential. The geneticist finds that all four are highly mutagenic. She also tests the capacity of mutations produced by the PFI compounds to be reversed by other known mutagens and obtains the following results. What conclusions can you make about the nature of the mutations produced by these compounds?

Question 18.30

Mary Alexander studied the effects of radiation on mutation rates in the sperm of Drosophila melanogaster. She irradiated Drosophila larvae with either 3000 roentgens (r) or 3975 r, collected the adult males that developed from irradiated larvae, mated them with unirradiated females, and then counted the number of mutant F₁ flies produced by each male. All mutant flies that appeared were used in subsequent crosses to determine if their mutant phenotypes were genetic. She obtained the following results (M. L. Alexander. 1954. Genetics 39:409-428):

• a. Calculate the mutation rates of the control group and the two groups of irradiated flies.
• b. On the basis of these data, do you think radiation has any effect on mutation? Explain your answer.

Question 18.31

What conclusion would you make if the number of bacterial colonies in Figure 18.22 were the same on the control plate and the treatment plate? Explain your reasoning.

Question 18.32

A genetics instructor designs a laboratory experiment to study the effects of UV radiation on mutation in bacteria. In the experiment, the students expose bacteria plated on petri plates to UV light for different lengths of time, place the plates in an incubator for 48 hours, and then count the number of colonies that appear on each plate. The plates that have received more UV radiation should have more pyrimidine dimers, which block replication; thus, fewer colonies should appear on the plates exposed to UV light for longer periods of time. Before the students carry out the experiment, the instructor warns them that while the bacteria are in the incubator, the students must not open the incubator door unless the room is darkened. Why should the bacteria not be exposed to light?

Question 18.33

A particular transposable element generates flanking direct repeats that are 4 bp long. Give the sequence that will be found on both sides of the transposable element if this transposable element inserts at the position indicated on each of the following sequences.

• a.
• 
  
• b.

Question 18.34

White eyes in Drosophila melanogaster result from an X-linked recessive mutation. Occasionally, white-eyed mutants give rise to offspring that possess white eyes with small red spots. The number, distribution, and size of the red spots are variable. Explain how a transposable element could be responsible for this spotting phenomenon.

Question 18.35

What factor might potentially determine the length of the flanking direct repeats that are produced in transposition?

Question 18.36

Which of the following pairs of sequences might be found at the ends of an insertion sequence?

Question 18.37

Explain why the corn kernel in Figure 18.34d is variegated, with some areas colored and some areas lacking pigment.

Question 18.38

Two different strains of Drosophila melanogaster are mated in reciprocal crosses. When strain A males are crossed with strain B females, the progeny are normal. However, when strain A females are crossed with strain B males, there are many mutations and chromosome rearrangements in the gametes of the F₁ progeny and the F₁ generation is effectively sterile. Explain these results.

Question 18.39

An insertion sequence contains a large deletion in its transposase gene. Under what circumstances would this insertion sequence be able to transpose?

Question 18.40

Zidovudine (AZT) is a drug used to treat patients with AIDS. AZT works by blocking the reverse transcriptase enzyme used by the human immunodeficiency virus (HIV), the causative agent of AIDS. Do you expect that AZT would have any effect on transposable elements? If so, what type of transposable elements would be affected and what would be the most likely effect?

Question 18.41

A transposable element is found to encode a reverse transcriptase enzyme. On the basis of this information, what conclusions can you make about the likely structure and method of transposition of this element?

Question 18.42

A geneticist examines an ear of corn in which most kernels are yellow, but he finds a few kernels with purple spots, as shown here. Give a possible explanation for the appearance of the purple spots in these otherwise yellow kernels, accounting for their different sizes. (Hint: See the section on Ac and Ds elements in maize.)

Question 18.43

Which DNA repair mechanism would most likely correct the incorporated error labeled by balloon 2 in Figure 18.11?

Question 18.44

A plant breeder wants to isolate mutants in tomatoes that are defective in DNA repair. However, this breeder does not have the expertise or equipment to study enzymes in DNA-repair systems. How can the breeder identify tomato plants that are deficient in DNA repair? What are the traits to look for?