Question

At the promoter for the lac operon, how does the DNA loop block transcription?

A. by blocking the binding of CAP

B. by creating torsional strain in the DNA at the promoter which prevents the transition from the closed to open promoter complex.

C. by blocking access to the core promoter by RNA polymerase

D. causes excessive abortive cycling

Answer 1

The DNA loop at the promoter for the lac operon blocks transcription by creating torsional strain in the DNA at the promoter, which prevents the transition from the closed to open promoter complex. When cAMP-bound CAP binds to the promoter, it causes the DNA to bend, making it easier for RNA polymerase to bind and initiate transcription.

Step-by-step explanation:

The DNA loop at the promoter for the lac operon blocks transcription by creating torsional strain in the DNA at the promoter, which prevents the transition from the closed to open promoter complex.

This torsional strain hampers the binding of RNA polymerase to the promoter, thereby blocking transcription. Essentially, when cAMP-bound CAP binds to the promoter, it causes the DNA to bend, making it easier for RNA polymerase to bind and initiate transcription.