Answer to Question 1 from November, 2006

1. (5 points) Which specific repair pathway(s) is/are primarily responsible for repairing the following types of damage? In answering this question, you should consider non-excision pathways such as homologous recombination in addition to excision pathways such as BER.

1. AP site
2. uracil
3. thymine glycol
4. 3-methyl adenine
5. cyclobutane pyrimidine dimers
6. T-G mismatch
7. double-strand break

I understand that some students were told that I was looking for just one pathway for each type of damage. For that reason, I gave full credit to students who listed only one pathway, even though two pathways are relevant in some cases. Please note that the question uses the word "pathway(s)", indicating that in some cases more than one pathway might be relevant. Note: this question was taken verbatim from the test in November, 2002. Those of you who used my online previous tests (http://asajj.roswellpark.org/huberman/DNA_Repair/previous_exams.html) as study guides should have been able to answer this question without any effort. Here are the answers (modified from http://asajj.roswellpark.org/huberman/DNA_Repair/previous_exams.html):

1. BER. Note: AP sites are generated and then repaired as part of the complete BER pathway. When AP sites are generated by spontaneous base loss, they can easily be repaired by the downstream portion of the BER pathway.
2. BER. If the uracil in DNA is mismatched, it can also be repaired by the mismatch repair pathway.
3. BER.
4. BER. Some of you were confused by this question, because you recalled that 1-methyl adenine can also be repaired by a direct reversal process involving AlkB. But the question asked about 3-methyl adenine, not 1-methyl adenine.
5. NER, and also direct reversal by CPD photolyase.
6. T-G mismatches can be repaired by both MMR and BER.
7. Homologous recombination (HR) and non-homologous end joining (NHEJ).

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