12.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 11.5, is this mutation a transition or a transversion? At which position of the codon does the mutation occur?

*13.Refer to the genetic code in Figure 11.5 to answer the following questions:

14.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 11.5, indicate the type and location of the mutation that gave rise to sickle-cell anemia.

*15.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 11.5. Then give the altered amino acid sequence of the protein that will be found in each of the following mutations:

16.Draw a hairpin structure like that shown in Figure 13.5 for the repeated sequence found in fragile-X syndrome (see Table 13.1).

*17.A polypeptide has the following amino acid sequence.

Met-Ser-Pro-Arg-Leu-Glu-Gly

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.).

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

Met-Trp-His-Arg-Ala-Ser-Phe

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

Met-Trp-His-Ser-Ala-Ser-Phe

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

Met-Trp-His-Arg-Ala-Ser-Phe

Give at least one example of base changes that could produce the original mutation and the intragenic suppressor. (Consult the genetic code in Figure 11.5).

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

5′—ATGT—3′

3′—TACA—5′

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

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

5′—AG—3′

3′—TC—5′

21. Mary Alexander studied the effects of radiation on mutation rates in the sperm of Drosophila melanogaster. She irradiated Drosophila larvae with either 3000 roentgens (3000 r) or 3975 r, collected the adult males that developed from irradiated larvae, mated them with nonirradiated females, and then counted the number of mutant F₁ offspring 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):

22.What conclusion would you draw if the number of bacterial colonies in Figure 13.21 were the same on the control plate and the treatment plate? Explain your reasoning.

*23.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.

24.What factor determines the length of the flanking direct repeats that are produced in a transposition?

25.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?

26.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 method of transposition of this element?

*27.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?