Concept 28 Some types of mutations are automatically repaired.

HI! Xeroderma pigmentosum (XP) is a recessive hereditary disease that causes extreme sensitivity to ultraviolet light. Most patients die at a young age after exposure to the sun causes skin cancer. James Cleaver hypothesized that a defect in a DNA repair system caused the disease. Cleaver removed skin cells from people with XP and people without XP and grew them in petri dishes. He exposed both skin cell cultures to UV light. He then adapted the density gradient centrifugation procedure from Meselson and Stahl�€™s semi-conservative DNA replication experiment. Remember that in Meselson and Stahl's experiment, cells grown in 15N and then switched to 14N for one generation produced DNA with an intermediate density. Instead of nitrogen, Cleaver added a base labeled with "heavy" hydrogen (3H) to the culture. After one generation, Cleaver found the following: Which statement is true? UV-exposed XP cells replicate DNA but not as well as unexposed XP cells. (Yes, but there's more.) Unexposed XP cells replicate DNA as well as unexposed normal cells. (Yes, but there's more.) Both of the above. (That is correct.) UV-exposed XP cells can not replicate DNA. (No, that is incorrect.) UV inhibits DNA replication in both XP and normal cells. Cleaver also found another band that represented repaired DNA in the UV-exposed normal cells. If repaired DNA has the same density as unlabeled DNA, which picture illustrates his results? XP + UV NORMAL + UV (That is correct.) XP + UV NORMAL+ UV (That is incorrect.) XP + UV NORMAL+ UV (That is incorrect.) XP + UV NORMAL+ UV (That is incorrect.) Repaired DNA will be lighter than the semi-conservatively replicated DNA, which was labeled with 3H. Based on this result, what is your conclusion? People with XP can�€™t repair DNA damaged by X-rays. (No, Cleaver didn�€™t expose the cell cultures to X-rays.) People with XP can repair DNA damaged by UV light. (No, XP cells could not repair DNA damaged by UV light.) Both of the above. (No, that is incorrect.) People with XP can�€™t repair DNA. (Yes, but only certain types of damage.) People with XP can�€™t repair DNA damaged by UV light. (That is correct.) Cleaver found that cells exposed to UV light could not repair DNA. He didn�€™t test any other mutagens in this experiment. In a different experiment, Cleaver exposed the cell cultures to X-rays. This graph shows the amount of repaired DNA in normal and XP cell cultures at different X-ray doses. What do the results show? XP cells can�€™t repair X-ray damage. (No, that is incorrect.) XP cells can�€™t repair X-ray damage as well as the normal cells. (No, only tiny differences exist between XP and normal cells.) Normal and XP cells repair X-ray damage equally well. (That is correct.) Neither cell can repair X-ray damage. (No, that is incorrect.) Normal cells can�€™t repair X-ray damage. (No, that is incorrect.) Unlike the UV experiment, Cleaver found no difference between the normal and XP cultures in repair of DNA damaged by X-rays. When X-rays damage DNA, they break the strands and bases are removed. The damage is repaired by DNA synthesis. When UV damages DNA, bases are altered and must be removed before DNA synthesis proteins can insert new bases. What is the most likely explanation for Cleaver�€™s findings that XP cells can repair X-ray damage but not UV damage? XP patients have a mutation in a gene required for DNA synthesis. (No, they can repair X-ray damage, which requires DNA synthesis.) XP patients have a mutation in a gene required for DNA excision. (That is correct.) XP patients have mutations in genes for DNA synthesis and excision. (No, they can repair X-ray damage, which requires DNA synthesis.) XP patients have no mutations. (No, that is incorrect.) Because UV damage must be removed from DNA during repair, they probably have a mutation in a gene required for DNA excision. Researchers have so far identified seven different genes involved in excision repair that cause XP when mutated. CONGRATULATIONS!!! YOU'RE SO SMART!