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Ch. 14 - Gene Mutation, DNA Repair, and Transposition
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
All textbooksKlug 10th EditionCh. 14 - Gene Mutation, DNA Repair, and TranspositionProblem 23b
Chapter 14, 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.

Verified step by step guidance
1
Understand that Xeroderma Pigmentosum (XP) is a genetic disorder characterized by a defect in nucleotide excision repair, a DNA repair mechanism responsible for fixing damage caused by ultraviolet (UV) light.
Recognize that UV light from sunlight causes DNA damage in skin cells, primarily forming thymine dimers and other lesions that distort the DNA helix.
In individuals without XP, the nucleotide excision repair system efficiently repairs this UV-induced DNA damage, preventing mutations that could lead to skin cancer, which results in a later age of onset.
In individuals with XP, the defective repair system means that UV-induced DNA damage accumulates rapidly, leading to mutations in critical genes that control cell growth and division, causing skin cancers to develop at a much earlier age.
Therefore, the early age of onset of skin cancer in XP patients is due to their inability to repair UV-induced DNA damage effectively, leading to faster accumulation of mutations and earlier tumor formation.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Xeroderma Pigmentosum (XP)

XP is a rare genetic disorder characterized by a defect in nucleotide excision repair, a DNA repair mechanism. Individuals with XP cannot effectively repair UV-induced DNA damage, leading to a high accumulation of mutations. This defect causes extreme sensitivity to sunlight and a dramatically increased risk of developing skin cancers at a young age.

DNA Damage and Repair Mechanisms

DNA repair systems, such as nucleotide excision repair, correct damage caused by environmental factors like UV radiation. When these repair pathways fail, DNA mutations accumulate, increasing cancer risk. In normal individuals, efficient repair delays cancer onset, whereas defective repair in XP leads to early and frequent skin cancers.
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Repair Pathways

Cumulative Risk and Age of Onset in Cancer

Cumulative risk refers to the total probability of developing cancer over time. In the general population, skin cancer risk increases gradually with age due to accumulated DNA damage. In XP patients, impaired repair accelerates mutation accumulation, causing skin cancers to appear much earlier, as reflected by the steep early rise in the cumulative incidence curve.
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Cancer Characteristics
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Textbook Question

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?

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Textbook Question

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?

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Textbook Question

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. 

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Textbook Question

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?

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Textbook Question

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

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Textbook Question

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

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