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Ch. 15 - Gene Mutation, DNA Repair, and Transposition
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 15 - Gene Mutation, DNA Repair, and TranspositionProblem 18
Chapter 15, Problem 18

What genetic defects result in the disorder xeroderma pigmentosum (XP) in humans? How do these defects create the phenotypes associated with the disorder?

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Understand that xeroderma pigmentosum (XP) is caused by mutations in genes responsible for nucleotide excision repair (NER), a DNA repair pathway that fixes damage caused by ultraviolet (UV) light.
Identify the key genes involved in XP, such as XPA, XPB, XPC, XPD, XPE, XPF, and XPG, which encode proteins that recognize and excise UV-induced DNA lesions like thymine dimers.
Explain that defects in these genes impair the NER pathway, leading to an inability to properly repair UV-induced DNA damage, resulting in accumulation of mutations in skin cells.
Connect the accumulation of unrepaired DNA damage to the clinical phenotypes of XP, including extreme sensitivity to sunlight, freckling, pigmentation abnormalities, and a high predisposition to skin cancers.
Summarize that the genetic defects cause a failure in DNA repair mechanisms, which leads to cellular damage and the characteristic symptoms of XP due to increased mutagenesis and cell death in UV-exposed tissues.

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Key Concepts

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

Nucleotide Excision Repair (NER) Pathway

NER is a DNA repair mechanism that removes bulky DNA lesions, such as those caused by UV light-induced thymine dimers. It involves recognition, excision, and resynthesis of damaged DNA segments. Defects in NER genes impair this process, leading to accumulation of DNA damage.
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Repair Pathways

Genetic Mutations in XP-Related Genes

Xeroderma pigmentosum results from mutations in genes encoding proteins essential for NER, such as XPA, XPB, XPC, and others. These mutations reduce or eliminate repair activity, causing cells to be hypersensitive to UV-induced DNA damage.
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Mutations and Phenotypes

Phenotypic Consequences of DNA Repair Defects

Failure to repair UV-induced DNA damage leads to mutations, genomic instability, and cell death. Clinically, this manifests as extreme UV sensitivity, skin abnormalities, pigmentation changes, and a high risk of skin cancers characteristic of XP patients.
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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?

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Compare DNA transposons and retrotransposons. What properties do they share?

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Speculate on how improved living conditions and medical care in the developed nations might affect human mutation rates, both neutral and deleterious.

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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?

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