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Chapter 15

This activity contains 34 questions.

What is the difference between a chromosomal mutation and a gene mutation?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Discuss the importance of mutations to the successful study of genetics.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Describe the technique for the detection of nutritional mutants in Neurospora.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Most mutations are thought to be deleterious. Why, then, is it reasonable to state that mutations are essential to the evolutionary process?

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Why is a random mutation more likely to be deleterious than beneficial?

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Most mutations in a diploid organism are recessive. Why?

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What is meant by a conditional lethal mutation?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Contrast the concerns about mutation in somatic and gametic tissue.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

In the attached-X technique for detection of mutations in Drosophila, (a) explain the rationale behind detection. (b) What type of mutation may be detected?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

In an experiment using the attached-X system, a student irradiated wild-type males and subsequently scored 500 F₁ cultures, finding that all males in each culture were normal. What conclusions can you draw about the induced mutation rate?

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In Drosophila, induced mutations on chromosome 2 that are recessive lethals may be detected by using a second chromosomal stock, Curly Lobe/Plum (Cy L /Pm). These alleles are all dominant and lethal in the homozygous condition. Detection is performed by crossing Cy L /Pm females to wild-type males that have been subjected to a mutagen. Three generations are required. In the F₁ of each cross, Cy L males are selected and individually backcrossed to Cy L /Pm females. In the F₂ of each cross, flies expressing Cy L are mated to produce a series of F₃ generations. Diagram these crosses, and predict how an F₃ culture would vary if a recessive lethal were induced in the original test male compared with the case wherein no lethal mutation resulted. In which F₃ flies would a recessive morphological mutation be expressed?

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Describe a tautomeric shift and how it may lead to mutation.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Contrast and compare the mutagenic effects of deaminating agents, alkylating agents, and base analogs.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Acridine dyes induce frameshift mutations. Is such a mutation likely to be more detrimental than a point mutation in which a single pyrimidine or purine has been substituted?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Why are X-rays a more potent mutagen than UV radiation?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Contrast the induction of mutations by UV radiation and X-rays.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Contrast the various types of DNA repair mechanisms known to counteract the effects of UV radiation and other DNA damage. What is the role of visible light in photoreactivation?

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Mammography is an accurate screening technique for the early detection of breast cancer in humans. Because this technique uses diagnostic X-rays, it has been highly controversial. Can you explain why? What reasons justify the use of X-rays for this type of medical diagnosis?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Compose a short essay that relates the molecular basis of fragile X syndrome, myotonic dystrophy, and Huntington disease to the severity of the disorders, as well as to genetic anticipation.

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Describe how the Ames assay screens for potential environmental mutagens. Why is it thought that a compound that tests positively in the Ames assay may also be carcinogenic?

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Describe the general approach used in site-directed mutagenesis.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

What genetic defect results in xeroderma pigmentosum (XP) in humans? How does this defect relate to the phenotype associated with this disorder?

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Differentiate between point mutations that are transitions and those that are transversions. Using the DNA bases (A, T, C, and G), list the four types of transitions and the eight types of transversions.

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In a bacterial culture in which all cells were unable to synthesize leucine (lue^-), a potent mutagen was added and the cells were allowed to undergo one round of replication. At that point, samples were taken, a series of dilutions was made, and the cells were plated on either minimal medium or leucine-enhanced minimal medium. The first culture condition (minimal medium) allowed the detection of mutations from lue^- to lue⁺ while the second culture condition (minimal medium plus leucine) allowed the determination of the total cells, since all bacteria can grow. From the results in the table below, (a) determine the frequency of mutant cells. (b) What is the rate of mutation at the locus involved with leucine biosynthesis?
Please refer to the table in your textbook on page 348 for help in answering this question.

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Contrast the various transposable genetic elements in bacteria, maize, Drosophila, and humans. What properties do they share?

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Ty, a transposable genetic element in yeast, has been found to contain an open reading frame (ORF) that encodes the enzyme reverse transcriptase. This enzyme synthesizes DNA from an RNA template. Speculate on the role of this gene product in the transposition of Ty within the yeast genome.

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Hypothetical findings from studies of heterokaryons formed from seven human xeroderma pigmentosum cell strains are presented below. Complementing groups are marked +, noncomplementing groups are marked -. These data represent the occurrence of unscheduled DNA synthesis in the fused heterokaryon when neither of the strains alone showed synthesis. (a) What does unscheduled DNA synthesis represent? (b) Which strains fall into the same complementation groups? (c) How many different groups are revealed based on these limited data? (d) How do we interpret the presence of these complementation groups?
Please refer to the table in your textbook on page 348 for help in answering this question.

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Imagine yourself as one of the team of geneticists who launched the study of the genetic effects of high-energy radiation on the surviving Japanese population immediately following the atomic-bomb attacks at Hiroshima and Nagasaki in 1945. Demonstrate your insights into both chromosomal and gene mutation by outlining comprehensive short-term and long-term studies that address this topic. Be sure to include strategies for considering the effects on both somatic and germline tissues.

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Cystic fibrosis (CF) is a severe autosomal recessive disorder in humans that results from a chloride ion–channel defect in epithelial cells. Over 500 sequence alterations have been identified in the 24 exons of the responsible gene CFTR (cystic fibrosis transmembrane regulator), including dozens of different missense mutations and frameshift mutations, as well as numerous splicing defects. Although all affected CF individuals demonstrate chronic obstructive lung disease, there is variation in pancreatic enzyme insufficiency (PI). Speculate which types of observed mutations are likely to give rise to less severe symptoms of CF, including only minor PI. A number of the 500 sequence alterations within the exon regions of the CFTR gene do not give rise to cystic fibrosis. Using your accumulated knowledge of the genetic code, gene expression, protein function, and mutation, explain to a freshman biology major how this might be.

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Electrophilic oxidants are known to create the highly deleterious lesion in DNA named 7,8-dihydro-8-oxoguanine (oxoG). (Bruner, S. D. et al. 2000. Nature 403: 859-62.) Normally, guanine base-pairs with cytosine, but oxoG base-pairs with either cytosine or adenine. (a) What are the sources of reactive oxidants within cells that cause this type of lesion? (b) Drawing on your knowledge of nucleotide chemistry, draw the molecular structure of oxoG, and, below it, draw guanine. Opposite guanine, sketch cytosine, including the hydrogen bonds that allow these two molecules to base-pair. Does the structure of oxoG, in contrast to guanine, provide any hint as to why it base pairs with adenine? (c) Assume that an unrepaired oxoG lesion is present in the helix of DNA opposite cytosine. Predict the type of mutation that will occur after several rounds of replication. (d) Drawing on your knowledge of DNA repair mechanisms, consider which cellular approaches might work to counteract an oxoG lesion and determine which of them is likely to be most effective.

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Among the Betazoids in the world of Star Trek^®, the ability to read minds is under the control of a mindreader (mr) gene. Most Betazoids can read minds, but rare recessive mutations in the mr gene result in two alternative phenotypes: delayed receivers and insensitives. Delayed receivers have some mindreading ability but perform the task much more slowly than normal Betazoids; insensitives cannot read minds at all. Betazoid genes do not have introns, so the gene contains only coding DNA. It is 3332 nucleotides in length, arranged in a four- letter genetic code.
The table below shows data from unrelated mr mutations. For each mutation, provide a plausible explanation for why it gives rise to its associated phenotype and not to the other phenotype. For example, hypothesize why the mr-1 nonsense mutation in codon 829 gives rise to the milder delayed-receiver phenotype rather than the more severe insensitive phenotype. (More than one explanation is possible, so be creative within plausible bounds!)
Please refer to the table in your textbook on page 349 for help in answering this question.

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Approximately 5% of live-born human offspring will have a genetic disorder, and of these, 20% are due to new germ-line mutations. Below are data derived from a study by Walter and colleagues (1998. Proc. Natl. Acad. Sci. (USA) 95: 10,015-19.) of the spontaneous mutation frequency for an introduced gene in mouse spermatids at various life stages. (a) What conclusions can be drawn from the data? (b) Given the nature of spermatogenesis, what factors might contribute to the results?
Please refer to the table in your textbook on page 349 for help in answering this question.

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Skin cancer carries a lifetime risk nearly equal to all other cancers combined. Below is a graph (Modified from Kraemer. 1997. Proc. Natl. Acad. Sci. (USA) 94:11-14.) depicting the age of onset of skin cancers in patients with or without xeroderma pigmentosum (XP), where cumulative percentage 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 Xeroderm Pigmentosum Registry. (a) Provide an overview of the information contained in the graph. (b) Explain why individuals with XP show such an early age of onset.
Please refer to the table in your textbook on page 349 for help in answering this question.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

Flower color in morning glories is primarily dependent on a complex flavonoid biosynthetic pathway leading to the production of anthocyanin. Flower colors appear to be significantly influenced by multiple genes involving epistasis and typical dominant/recessive patterns. Flowers may be pigmented in shades of red/magenta or variegated with pigmented patches randomly or uniformly distributed. The table below outlines the known causes of variations in morning glory flower pigmentation. (Clegg and Durbin. 2000. Proc. Natl. Acad. Sci. (USA) 97: 7016-23.) If these mutations represent virtually all the known types of mutation causing variation in morning glory pigmentation, what general conclusions can you reach from the data? Defend your conclusions.
Please refer to the table in your textbook on page 350 for help in answering this question.

To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

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Problems and Discussion Questions

What is the difference between a chromosomal mutation and a gene mutation?

Discuss the importance of mutations to the successful study of genetics.

Describe the technique for the detection of nutritional mutants in Neurospora.

Most mutations are thought to be deleterious. Why, then, is it reasonable to state that mutations are essential to the evolutionary process?

Why is a random mutation more likely to be deleterious than beneficial?

Most mutations in a diploid organism are recessive. Why?

What is meant by a conditional lethal mutation?

Contrast the concerns about mutation in somatic and gametic tissue.

In the attached-X technique for detection of mutations in Drosophila, (a) explain the rationale behind detection. (b) What type of mutation may be detected?

In an experiment using the attached-X system, a student irradiated wild-type males and subsequently scored 500 F1 cultures, finding that all males in each culture were normal. What conclusions can you draw about the induced mutation rate?

Describe a tautomeric shift and how it may lead to mutation.

Contrast and compare the mutagenic effects of deaminating agents, alkylating agents, and base analogs.

Acridine dyes induce frameshift mutations. Is such a mutation likely to be more detrimental than a point mutation in which a single pyrimidine or purine has been substituted?

Why are X-rays a more potent mutagen than UV radiation?

Contrast the induction of mutations by UV radiation and X-rays.

Contrast the various types of DNA repair mechanisms known to counteract the effects of UV radiation and other DNA damage. What is the role of visible light in photoreactivation?

Mammography is an accurate screening technique for the early detection of breast cancer in humans. Because this technique uses diagnostic X-rays, it has been highly controversial. Can you explain why? What reasons justify the use of X-rays for this type of medical diagnosis?

Compose a short essay that relates the molecular basis of fragile X syndrome, myotonic dystrophy, and Huntington disease to the severity of the disorders, as well as to genetic anticipation.

Describe how the Ames assay screens for potential environmental mutagens. Why is it thought that a compound that tests positively in the Ames assay may also be carcinogenic?

Describe the general approach used in site-directed mutagenesis.

What genetic defect results in xeroderma pigmentosum (XP) in humans? How does this defect relate to the phenotype associated with this disorder?

Differentiate between point mutations that are transitions and those that are transversions. Using the DNA bases (A, T, C, and G), list the four types of transitions and the eight types of transversions.

Contrast the various transposable genetic elements in bacteria, maize, Drosophila, and humans. What properties do they share?

In an experiment using the attached-X system, a student irradiated wild-type males and subsequently scored 500 F₁ cultures, finding that all males in each culture were normal. What conclusions can you draw about the induced mutation rate?