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Problem 1a
How do we know that many cancer-causing agents (carcinogens) are also mutagenic?
Problem 1b
How do we know that certain chemicals and wavelengths of radiation induce mutations in DNA?
Problem 1c
How do we know that DNA repair mechanisms detect and correct the majority of spontaneous and induced mutations?
Problem 2
Write a short essay contrasting how these concepts may differ between bacteria and eukaryotes.
Problem 3
What is a spontaneous mutation, and why are spontaneous mutations rare?
Problem 4
Why would a mutation in a somatic cell of a multicellular organism not necessarily result in a detectable phenotype?
Problem 5
Most mutations are thought to be deleterious. Why, then, is it reasonable to state that mutations are essential to the evolutionary process?
Problem 6
Why is a random mutation more likely to be deleterious than beneficial?
Problem 7
Most mutations in a diploid organism are recessive. Why?
Problem 8
What is the difference between a silent mutation and a neutral mutation?
Problem 9
Describe a tautomeric shift and how it may lead to a mutation.
Problem 10
Contrast and compare the mutagenic effects of deaminating agents, alkylating agents, and base analogs.
Problem 11
Why are frameshift mutations likely to be more detrimental than point mutations, in which a single pyrimidine or purine has been substituted?
Problem 12
Why are X rays more potent mutagens than UV radiation?
Problem 13
Contrast the various types of DNA repair mechanisms known to counteract the effects of UV radiation. What is the role of visible light in repairing UV-induced mutations?
Problem 14
Mammography is an accurate screening technique for the early detection of breast cancer in humans. Because this technique uses X rays diagnostically, it has been highly controversial. Can you explain why? What reasons justify the use of X rays for such a medical screening technique?
Problem 15
A significant number of mutations in the HBB gene that cause human β-thalassemia occur within introns or in upstream noncoding sequences. Explain why mutations in these regions often lead to severe disease, although they may not directly alter the coding regions of the gene.
Problem 16
Describe how the Ames test screens for potential environmental mutagens. Why is it thought that a compound that tests positively in the Ames test may also be carcinogenic?
Problem 17
What genetic defects result in the disorder xeroderma pigmentosum (XP) in humans? How do these defects create the phenotypes associated with the disorder?
Problem 18
Compare DNA transposons and retrotransposons. What properties do they share?
Problem 19
In maize, a Ds or Ac transposon can alter the function of genes at or near the site of transposon insertion. It is possible for these elements to transpose away from their original insertion site, causing a reversion of the mutant phenotype. In some cases, however, even more severe phenotypes appear, due to events at or near the mutant allele. What might be happening to the transposon or the nearby gene to create more severe mutations?
Problem 20
It is estimated that about 0.2 percent of human mutations are due to TE insertions, and a much higher degree of mutational damage is known to occur in some other organisms. In what way might a TE insertion contribute positively to evolution?
Problem 21
In a bacterial culture in which all cells are unable to synthesize leucine (leu⁻), a potent mutagen is added, and the cells are allowed to undergo one round of replication. At that point, samples are taken, a series of dilutions are made, and the cells are plated on either minimal medium or minimal medium containing leucine. The first culture condition (minimal medium) allows the growth of only leu⁺ cells, while the second culture condition (minimal medium with leucine added) allows growth of all cells. The results of the experiment are as follows:
What is the rate of mutation at the locus associated with leucine biosynthesis?
Problem 22
Presented here are hypothetical findings from studies of heterokaryons formed from seven human xeroderma pigmentosum cell strains:
These data are measurements of the occurrence or nonoccurrence of unscheduled DNA synthesis in the fused heterokaryon. None of the strains alone shows any unscheduled DNA synthesis. Which strains fall into the same complementation groups? How many different groups are revealed based on these data? What can we conclude about the genetic basis of XP from these data?
Problem 23a
Skin cancer carries a lifetime risk nearly equal to that of all other cancers combined. Following is a graph [modified from K. H. Kraemer (1997). Proc. Natl. Acad. Sci. (USA) 94:11 14] depicting the age of onset of skin cancers in patients with or without XP, where the cumulative percentage of skin cancer is plotted against age. The non-XP curve is based on 29,757 cancers surveyed by the National Cancer Institute, and the curve representing those with XP is based on 63 skin cancers from the Xeroderma Pigmentosum Registry.
Provide an overview of the information contained in the graph.
Problem 23b
Skin cancer carries a lifetime risk nearly equal to that of all other cancers combined. Following is a graph [modified from K. H. Kraemer (1997). Proc. Natl. Acad. Sci. (USA) 94:11 14] depicting the age of onset of skin cancers in patients with or without XP, where the cumulative percentage of skin cancer is plotted against age. The non-XP curve is based on 29,757 cancers surveyed by the National Cancer Institute, and the curve representing those with XP is based on 63 skin cancers from the Xeroderma Pigmentosum Registry.
Explain why individuals with XP show such an early age of onset.
Problem 24
It has been noted that most transposons in humans and other organisms are located in noncoding regions of the genome—regions such as introns, pseudogenes, and stretches of particular types of repetitive DNA. There are several ways to interpret this observation. Describe two possible interpretations. Which interpretation do you favor? Why?
Problem 25a
Mutations in the IL2RG gene cause approximately 30 percent of severe combined immunodeficiency disorder (SCID) cases in humans. These mutations result in alterations to a protein component of cytokine receptors that are essential for proper development of the immune system. The IL2RG gene is composed of eight exons and contains upstream and downstream sequences that are necessary for proper transcription and translation. Below are some of the mutations observed. For each, explain its likely influence on the IL2RG gene product (assume its length to be 375 amino acids).
Nonsense mutation in a coding region
Problem 25b
Mutations in the IL2RG gene cause approximately 30 percent of severe combined immunodeficiency disorder (SCID) cases in humans. These mutations result in alterations to a protein component of cytokine receptors that are essential for proper development of the immune system. The IL2RG gene is composed of eight exons and contains upstream and downstream sequences that are necessary for proper transcription and translation. Below are some of the mutations observed. For each, explain its likely influence on the IL2RG gene product (assume its length to be 375 amino acids).
Insertion in Exon 1, causing frameshift
Problem 25c
Mutations in the IL2RG gene cause approximately 30 percent of severe combined immunodeficiency disorder (SCID) cases in humans. These mutations result in alterations to a protein component of cytokine receptors that are essential for proper development of the immune system. The IL2RG gene is composed of eight exons and contains upstream and downstream sequences that are necessary for proper transcription and translation. Below are some of the mutations observed. For each, explain its likely influence on the IL2RG gene product (assume its length to be 375 amino acids).
Insertion in Exon 7, causing frameshift
