Question

What role do insulator sequences play in the regulation of eukaryotic transcription?

A		They block communication between enhancers and nontargeted promoters
B		They block binding of transcription factors to cis-acting promoter elements
C		They block binding of transcription repressors to cis-acting promoter elements
D		They block binding of transcription factors to enhancers
E		They block binding of transcription factors to silencers

What would be the effect of a mutation in the lacI gene that prevented the repressor from binding to lactose?

A		The lac Z, Y, and A genes would not be expressed.
B		The lac Z, Y, and A genes would be expressed constitutively
C		The lac Z, Y, and A genes would be repressed by lactose.
D		The lac Z, Y, and A genes would be induced by lactose.
E		The lac Z and Y genes would be repressed but not the lac A gene.

Answer 1

What would be the effect of a mutation in the lacI gene that prevented the repressor from binding to lactose?

Answer: B. The lac Z, Y, and A genes would not be expressed.

[In the absence of lactose, the lac operon is switched off by binding of lac repressor to the operator region. The lac repressor inhibits the recruitment of RNA polymerase on the promoter region and thus transcription of lac Z, Y, and A genes is repressed. However, in presence of lactose, the lac repressor forms a complex with lactose that can no longer bind to operator region. So, if there is a mutation that prevents binding of lac repressor with lactose, the operator will be occupied by lac repressor. This will in turn inhibit the binding of RNA polymerase with the promoter and repress the transcription of lac genes (Z, Y and A)]

What role do insulator sequences play in the regulation of eukaryotic transcription?

Answer: A. They block communication between enhancers and nontargeted promoters

[An insulator is a stretch of double helical DNA that inhibits the interaction of enhancer with promoter (or proximal promoter) by two mechanisms: 1) by forming a loop that interferes with loops formed between promoter and enhancer 2) inhibit the transformation of silenced heterochromatin into a active chromatin]