Answer to Question 1 from November, 2005

a) Please draw the structure of the nucleic acid base, cytosine. Just show the structure. It is not necessary to show the numbering (4 points).

Please note that I didn't take off any points for minor errors in this structure. For full credit, you needed to draw the 6-membered ring structure with the two ring nitrogens in their correct positions, and you needed to show the keto and amino groups in their correct positions. I wasn't concerned about the positions of the ring double bonds, since these positions are a bit uncertain in any case due to resonance-mediated bond delocalization.

b) Please draw the structure of the base that results from deamination of cytosine (4 points).

Please note that I didn't take off points in this question for errors for which I had already subtracted points in the previous section. Here I was concerned with whether or not you correctly showed loss of the amino group (in cytosine) and gain of the keto group (in uracil). I subtracted points if you indicated changes that are chemically incorrect, such as addition of methyl groups or addition of two oxygens rather than one.

c) What is the name of the base that results from deamination of cytosine (1 point)?

Uracil

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