In the lac operon of Escherichia coli, which event most directly causes transcription to increase when lactose becomes available in the medium?

Allolactose binds the LacI repressor, reducing its binding to the operator

Which statement best describes the trp operon when intracellular tryptophan levels are high?

Tryptophan acts as a corepressor and enables the repressor to bind the operator

In a merodiploid E. coli strain with genotype lacI- O+ Z+ / lacI+ O+ Z-, what will be the effect of the lacI+ allele on regulation of the Z+ gene copy?

It will restore inducible regulation because LacI protein acts in trans

A mutation in the trp leader peptide removes the two adjacent Trp codons but leaves the attenuator region otherwise intact. What is the most likely regulatory consequence?

The ribosome will no longer sense tryptophan availability effectively, impairing attenuation control

Why is the lac operon only weakly transcribed when lactose is present but glucose is also abundant?

High glucose lowers cAMP, reducing CAP-mediated activation of the promoter

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Molecular Biology · PCR primers · 11 formulae in the cheatsheet

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A260/A280 ≈ 1.8 → pure DNA, ≈ 2.0 → pure RNA. A low ratio means protein contamination — don’t ignore it.

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