Application Questions And Problems

* 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 15.12, is this mutation a transition or a transversion? At which position of the codon does the mutation occur?

*18. (a) If a single transition occurs in a codon that specifies Phe, what amino acids could be specified by the mutated sequence? (b) If a single transversion occurs in a codon that specifies Phe, what amino acids could be specified by the mutated sequence?

(c) If a single transition occurs in a codon that specifies Leu, what amino acids could be specified by the mutated sequence?

(d) If a single transversion occurs in a codon that specifies Leu, what amino acids could be specified by the mutated sequence?

19. Hemoglobin is a complex protein that contains four polypeptide chains. The normal hemoglobin found in adults—called adult hemoglobin—consists of two a and two p polypeptide chains, which are encoded by different loci. Sickle-cell hemoglobin, which causes sickle-cell anemia, arises from a mutation in the p 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 15.12, indicate the type and location of the mutation that gave rise to sickle-cell anemia. *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 15.12. Then, give the altered amino acid sequence of the protein that will be found in each of the following mutations.

Sequence of DNA template

Nucleotide number

(a) Mutant 1: A transition at nucleotide 11.

(b) Mutant 2: A transition at nucleotide 13.

(c) Mutant 3: A one-nucleotide deletion at nucleotide 7.

(d) Mutant 4: A T:A transversion at nucleotide 15.

(e) Mutant 5: An addition of TGG after nucleotide 6.

(f) Mutant 6: A transition at nucleotide 9.

21. 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 change that occurred in the DNA (single-base substitution, insertion, deletion) and the phenotypic effect of the mutation (nonsense mutation, missense mutation, frameshift, etc.).

(a) Mutant 1: Met-Ser-Ser-Arg-Leu-Glu-Gly

(c) Mutant 3: Met-Ser-Pro-Asp-Trp-Arg-Asp-Lys

(e) Mutant 5: Met-Ser-Pro-Arg-Leu-Leu-Glu-Gly

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

Met-Trp-His-Val-Ala-Ser-Phe.

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

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

An intragenic suppressor restores the amino acid sequence to that of the original the 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 15.12.)

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

Met-Lys-Ser-Pro-Ala-Thr-Pro

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:

Met-Lys-Cys-Pro-Ala-Thr-Pro

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

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

25. XG syndrome is a rare genetic disease that is due to an autosomal dominant gene. A complete census of a small European country reveals that 77,536 babies were born in 2000, of whom 3 had XG syndrome. In the same year, this country had a population of 5,964,321 people, and there were 35 living persons with XG syndrome. What are the mutation rate and mutation frequency of XG syndrome for this country?

*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?

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

(a) Give all the mutant sequences that may result from spontaneous depurination in this stretch of DNA.

(b) Give all the mutant sequences that may result from spontaneous deamination in this stretch of DNA.

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 by which this increase occurs?

*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?

Reversed by

Mutations

2-Amino-

Nitrous-

Hydroxy-

Acridine

produced

purine

acid

lamine

orange

by

0 0

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