GATE Life Sciences Practice Quiz

Question 1

In the E. coli lac operon, which condition gives the highest transcription of lacZYA?

Accepted Answer

Lactose present, glucose absent

Question 2

A mutation in the lacI gene produces a repressor that cannot bind allolactose but can still bind the operator. What phenotype is expected for the lac operon?

Accepted Answer

Permanent repression even in the presence of lactose

Question 3

In the ara operon, AraC can both repress and activate transcription. Which mechanism explains repression in the absence of arabinose?

Accepted Answer

AraC binds distant sites and forms a DNA loop that hinders transcription initiation

Question 4

A point mutation changes the two adjacent Trp codons in the trpL leader peptide to Ala codons without altering the RNA sequences that form hairpins. What is the most likely effect under tryptophan starvation?

Accepted Answer

Increased attenuation, because ribosome no longer stalls specifically in response to low tryptophan

Question 5

In a merodiploid E. coli strain with genotype I+ Oc Z+ / I+ O+ Z-, what will be the expression pattern of functional beta-galactosidase?

Accepted Answer

Constitutive, because Oc acts only in cis on the linked Z+ gene

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