Q13 - Beta-Clamp Processivity in DNA Polymerase III

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

DNA polymerase III holoenzyme consists of the pol III* and the beta-clamp which hold pol III* and DNA template together. You are investigating whether during lagging strand synthesis, the pol III* synthesizing the lagging strand dissociates from the beta-clamp as it finishes one Okazaki fragment and re-associates with another beta-clamp to begin making the next Okazaki fragment. You prepared a primed M13phage (a single stranded DNA and a primer) template (M13mp18) as a donor and loaded a beta- clamp and pol III* onto it. As acceptors, you then added two more primed phage DNA templates, one (M13Gori) preloaded with a beta-clamp and the other (PhiX174) lacking a beta-clamp (Figure A). You incubated the templates together under replication conditions for 90 min - long enough for the donor and acceptors to be replicated - then performed gel electrophoresis (Figure B, lane 1-4). You then repeated the experiment but now loading a beta-clamp on PhiX174 instead of M13Gori (Figure B, lane 5-8).

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. After replication has finished, Pol III* dissociates from its original beta-clamp.

B. The data are consistent with the notion that the beta-clamp is not absolutely required for replication.

C. Pol III* prefers an acceptor template preloaded with a beta-clamp.

D. Based on the results, one beta-clamp is enough to synthesize all Okazaki fragments on the lagging strand of each replication fork.

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Question reproduced from IBO 2018, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)