1. (5 points) Draw a diagram of a cyclobutyl pyrimidine dimer (CPD). There's no need to draw the sugar and phosphate groups in detail; just call them "sugar" and "phosphate" or other abbreviation. Do show the pyrimidine ring structure in sufficient detail to make clear which ring constituents are nitrogens and which are carbons, and do show the cyclobutane ring connected to the correct positions on the pyrimidine rings.
Below at the left is the diagram from the lecture notes. Although the question specifically stated that all of this detail wasn't necessary, some of you provided all the detail shown here anyway. To the right of the complicated diagram is a much simpler diagram showing the bare minimum that was sufficient to answer this question. When you're taking an exam, it's always a good idea to provide only the minimum information necessary to answer the question. Providing additional information simply wastes your time.
2. (2 points) Which DNA bases can form CPDs? Which bases are found in the most common type of CPD?
The pyrimidine bases, C and T, can form CPDs. Thymine (TT) dimers are most common.
3. (2 points) Which of the following pathways is/are responsible for repairing CPDs? (a) photolyase; (b) mismatch repair; (c)base excision repair; (d) nucleotide excision repair; (e) homology-based double-strand break repair; (f) non-homologous end joining; (g) damage bypass
(a) Photolyase and (d) nucleotide excision repair. Although damage bypass pathways respond to CPDs, they don't repair CPDs; they allow replication forks to bypass CPDs.
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