Biol. 2320 - Fall 2000 - Fourth “Thought” Question - Due 10/6/00

This question may be done by a group of up to 4 students as long as all 4 put in equal effort and agree to accept the same score.  Group answers will be held to a somewhat higher standard of accuracy, brevity and clarity.  For group answers turn in one copy of the answer with all group members names on it.  Warning: Although this question has 4 parts, it would not be an effective strategy to have each of 4 group members do one part independent of the other members, and then combine the parts.  The answer must make sense as a whole.

Note: the information needed to answer this question comes from the lecture outlines, your lecture notes, the text and the question itself.  No further reference materials are required, and it will make the question more difficult if you try to find and use additional information.

Note further: It is highly likely you will want to ask for advice about your answer to this question.  Please make note of Dr. Rushing's and Dr. Shinkle's office hours and email addresses on your syllabus. 

Prokaryotes, plants and many animals other than placental mammals have an additional mechanism for correcting DNA damage caused by UV light beyond the one shown in figure 13.27.  The process of photo-reactivation involves an enzyme called a DNA photolyase.  The substrate for the enzyme is a pyrimidine dimer like the one in figure 13.26.  The enzyme uses light energy and a controlled reaction at the active site to reverse the cyclobutane ring formation.  Hence the enzyme "searches" DNA for pyrimidine dimers, binds to them, and reverts them to normal bases.

For the purpose of this question, assume that mismatch repair works the same way nucleotide excision repair does.

A)        [2 pts] Which of the other DNA repair processes is photo-reactivation most similar to? Explain referring to some specific aspect of the mechanism and/or presence or absence of specific enzymes.  (two sentences)

B)        [3 pts] Which of the other DNA repair processes is photo-reactivation least similar to? Explain, identifying two major points of dissimilarity, again in terms of some specific aspect of the mechanism and/or presence or absence of specific enzymes.  (two sentences)

C)        [3 pts] Identify 2 general types of enzymes (for example Pol I, II and III would all be the same type) used in all the other DNA repair processes (not including proofreading, which is simply a component of replication) that are not active in photo-reactivation.  Explain the function(s) of each enzyme when they are a part of DNA repair (4 words & three sentences). *

D)        [2 pts] The preferential pathway of nucleotide excision repair makes sense in terms of protecting those genes that are being actively used from mutations.  Explain why having this preferential repair system might also make sense because it functions where lesions are most likely to occur (1-2 sentences)

* Extra credit: There are actually 3 types of enzymes used by all the other classes of DNA repair.  Extra credit for getting all 3.  Further extra credit for naming 1 enzyme that is specific to nucleotide excision repair and base excision repair respectively, describing what each does, and whether or not it is used in photo-reactivation.