MCAT Biochemistry Question 98: Answer and Explanation
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Question: 98
8. When trypsin converts chymotrypsinogen to chymotrypsin, some molecules of chymotrypsin bind to a repressor, which in turn binds to the operator and prevents further transcription of trypsin. This is most similar to which of the following operons?
- A. trp operon during lack of tryptophan
- B. trp operon during abundance of tryptophan
- C. lac operon during lack of lactose
- D. lac operon during abundance of lactose
Correct Answer: B
Explanation:
The example given is a sample of repression due to the abundance of a corepressor. In other words, this is a repressible system that is currently blocking transcription. For the trp operon, an abundance of tryptophan in the environment allows for the repressor to bind tryptophan and then to the operator site. This blocks transcription of the genes required to synthesize tryptophan within the cell. The system described is a repressible system; the lac operon is an inducible system, in which an inducer binds to the repressor, thus permitting transcription.