AP Biology : DNA, RNA, and Proteins

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #32 : Transcription

The trp operon is normally active or turned “on.” However, when tryptophan is present, it binds to a repressor molecule, activating it, and causing inhibition of transcription of the genes. What type of molecule is tryptophan, and what type of operon is the trp operon?

Possible Answers:

corepressor; repressible

repressor; repressible

corepressor; inducible

inducer; inducible

Correct answer:

corepressor; repressible

Explanation:

The trp operon is normally on, meaning that the genes are normally being transcribed by RNA polymerase. Thus, this is a repressible operon. The operon can be turned off or repressed if another molecule (called a corepressor) binds to a repressor and causes the repressor to bind to the operator region (in the case of the trp operon this molecule is tryptophan; it binds to a repressor causing the repressor to bind to the operator region and prevents RNA polymerase from transcribing the genes).

 

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