Question 1

Which of the following best describes the difference between cis-elements and trans-acting factors in prokaryotic operons?

Accepted Answer

Cis-elements are DNA sequences that regulate gene expression, while trans-acting factors are proteins or RNAs that bind to these sequences to influence transcription.

Question 2

Given the following -35 region sequences from 7 E. coli genes: 5’-TCCTTAAA-3’, 5’-CCCCTGAA-3’, 5’-CCCTCGAA-3’, 5’-CTCTTGAG-3’, 5’-TCCTTGGA-3’, 5’-TTCTTAAA-3’, 5’-CCCTTAAA-3’, what is the consensus sequence?

Accepted Answer

5’-CCCTTGAA-3’

Question 3

How does the similarity of a promoter sequence to the consensus sequence affect the rate of transcription initiation in E. coli?

Accepted Answer

Promoters more similar to the consensus sequence bind RNA polymerase more efficiently, increasing the rate of transcription initiation.

Question 4

Which of the following is the most economical rate-limiting step in the central dogma for controlling gene expression in E. coli?

Accepted Answer

Transcription initiation

Question 5

A mutation in the trp operator prevents the trp repressor from binding. What is the expected effect on trp operon expression?

Accepted Answer

The trp operon will be constitutively expressed, even when tryptophan is present.

Question 6

Which of the following best explains why the lac operon is only highly expressed (activated) when lactose is present and glucose is absent?

Accepted Answer

cAMP levels are high when glucose is absent, allowing CRP to bind DNA and activate transcription, while allolactose inactivates the repressor when lactose is present.

Question 7

If a mutation prevents the lac repressor from binding allolactose, what is the most likely outcome for lac operon expression?

Accepted Answer

The operon will remain repressed even when lactose is present.

Question 8

Which of the following is a cis-element in the lac operon?

Accepted Answer

Lac operator

Question 9

What is the effect of a mutation in the CRP protein that prevents it from binding cAMP on lac operon expression when lactose is present and glucose is absent?

Accepted Answer

The operon will be expressed at basal levels, but not activated.

Question 10

Which of the following best describes the default state of the trp operon and the lac operon, respectively?

Accepted Answer

Trp operon: on; Lac operon: off

Question 11

A gene has a promoter sequence that differs from the consensus sequence by three nucleotides. What is the expected effect on the basal rate of transcription for this gene?

Accepted Answer

The basal rate of transcription will be lower than for a gene with a consensus promoter.

Question 12

Which of the following scenarios would result in the highest level of lac operon expression?

Accepted Answer

Lactose present, glucose absent

Question 13

If the trp repressor protein is mutated so that it cannot bind tryptophan, what will happen to trp operon expression in high tryptophan conditions?

Accepted Answer

The operon will be expressed (on) even in high tryptophan.

Question 14

Which of the following best explains why different genes in E. coli have different basal levels of expression?

Accepted Answer

They have different promoter sequences, affecting RNA polymerase binding.

Question 15

Which of the following is a correct match between a cis-element and its trans-acting factor?

Accepted Answer

Lac operator – Lac repressor

Question 16

Which of the following best describes the role of allosteric regulation in the lac operon?

Accepted Answer

Allolactose binds to the lac repressor, causing a conformational change that releases it from the operator.

Question 17

Which of the following is true about the anatomy of a typical prokaryotic operon with three genes?

Accepted Answer

It has one promoter, one transcription stop site, three translation start sites, and three translation stop sites.

Question 18

Which of the following best describes the effect of cAMP binding to CRP in the lac operon system?

Accepted Answer

cAMP binding activates CRP, allowing it to bind to its cis-element and enhance transcription.

Question 19

A strain of E. coli cannot break down lactose, even when lactose is the only sugar available. Which component is most likely mutated?

Accepted Answer

The lac repressor (super-repressor that never releases the operator)

Regulation of Gene Expression in Prokaryotic Operons: The lac and trp Systems in E. coli

Guida di studio - Domande di pratica

Study guide - Flashcard