6a. In the absence of external mutagens (no chemicals, no radiation), what is the most common type of DNA damage (3 points)?
Base loss, especially depurination (thousands of purines lost from each of our cells per day!).
6b. What DNA repair pathway(s) are used to repair that damage (3 points)?
The post-DNA-glycosylase portion of the base excision repair pathway is used to repair the AP sites that are generated by spontaneous base loss.
If you didn't answer "base loss" to question 6a, then I graded you for question 6b on the appropriateness of your answer (in 6b) to the type of DNA damage you mentioned in 6a.
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