Regulation of Gene Expression Questions and Answers

Q: Catabolite repression in bacteria is regulated by the concentration of

Catabolite repression is regulated by the concentration of glucose.When glucose is plentiful, bacteria such as E. coli use it first and repress the genes for using other sugars. In E. coli, glucose transport through the phosphotransferase system lowers adenylate cyclase activity, so cAMP falls; without cAMP the CRP (CAP) activator cannot bind its sites, and operons such as lac are transcribed poorly. Glucose uptake also causes inducer exclusion.Lactose is the inducer of the lac operon, not the regulator of catabolite repression. Amino acid levels regulate attenuation and the stringent response, and mRNA concentration is an outcome of regulation, not its signal.

Q: A drug was developed to inhibit histone acetyl transferase in eukaryotic cells. Cells treated with this drug have more compact chromatin and show reduction in transcription of several genes. Which one of the following statements best explains why the drug has this effect?

The best explanation is that histone acetylation neutralizes positive charge on histones, thereby loosening DNA-histone interactions and promoting transcription.Histone tails are rich in lysine, whose positively charged side chains hold the negatively charged DNA tightly. Histone acetyl transferases add acetyl groups to these lysines, removing their charge, so the DNA is held less tightly and the chromatin opens for transcription factors and RNA polymerase. Blocking the enzyme leaves the lysines charged, so the chromatin compacts and transcription of many genes falls, exactly what the drug does.Acetylation removes positive charge rather than adding it. H1 is the linker histone outside the nucleosome core, and its acetylation does not directly dismantle nucleosomes. Euchromatin is the decondensed, active form, so the last option has it backwards.

Q: Which one of the following σ (sigma) factors is responsible for the transcriptional regulation of nitrogen assimilation?

Nitrogen assimilation genes are transcribed with σ 54.In E. coli, σ54 (RpoN, also called σN) directs RNA polymerase to promoters of nitrogen-regulated genes such as glnA, which encodes glutamine synthetase. Unusually, the σ54 holoenzyme cannot open the promoter on its own; it needs an ATP-hydrolysing enhancer-binding activator, which for nitrogen genes is NtrC, switched on when nitrogen is scarce.σ70 is the housekeeping factor for most genes in growing cells. σ28 (FliA) transcribes flagellar and chemotaxis genes. σ24 (σE) responds to stress in the cell envelope.

Q: Mechanism of gene regulation does not include :

Gene regulation does not include the rate of replication.Gene expression can be controlled at every step from DNA to protein: the rate at which a gene is transcribed (promoters, enhancers, transcription factors, chromatin), the processing of the primary transcript (capping, splicing, alternative splicing, polyadenylation), the stability and lifetime of the mRNA, and translation and protein modification after that.Replication copies the whole genome once per cell cycle, and how fast it runs does not set how much a particular gene is expressed. That is why the rate of replication is the odd one out, while the other three options are standard levels of gene regulation.

Q: Inducible genes are transcribed because ..................... .

Inducible genes are transcribed because the inducer inactivates the repressor protein.In an inducible operon such as lac, the repressor normally binds the operator and blocks transcription. When the inducer (allolactose, or IPTG) is present, it binds the repressor and changes its shape so that it can no longer bind the operator, and RNA polymerase can transcribe the genes.Repressor bound to the operator is the switched-off state, so option C describes why the genes are not transcribed. The repressor does not act at the promoter, and the repressor-inducer complex leaves the operator rather than binding it.

Q: Mutation in gene rel A in bacteria results in .................... .

Mutation in relA gives relaxed mutants.The RelA protein makes the alarmone ppGpp when ribosomes stall for lack of charged tRNA during amino acid starvation. ppGpp shuts down rRNA and tRNA synthesis, the stringent response. In relA mutants no ppGpp is made, so stable RNA synthesis continues despite starvation; such mutants are called relaxed.Silent mutations change no amino acid, constitutive mutants express genes regardless of conditions, and dominance describes allele interaction. None describes the loss of the stringent response.

Q: Lac repressor is an example of :

The lac repressor is an example of the helix turn helix motif.Like many bacterial regulators (the lambda and trp repressors, CAP), the lac repressor binds DNA through two alpha-helices joined by a short turn; the second, recognition helix lies in the major groove of the operator.The helix loop helix motif belongs to eukaryotic factors such as MyoD and Myc, where it mainly mediates dimerisation. Zinc fingers, C2H2 (as in TFIIIA) and C2C2 (as in the steroid receptors), are also eukaryotic DNA-binding domains and do not occur in the lac repressor.

Q: Stringent response in bacteria leads to accumulation of these unusual nucleotides :

The stringent response accumulates ppGpp and pppGpp.When bacteria are starved of amino acids, uncharged tRNA in the ribosome triggers the RelA protein to make guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), the 'magic spot' nucleotides. They shut down rRNA and tRNA synthesis and redirect the cell's resources.pGpp, pGppp, pppGp and ppGp are not the alarmones of this response, and cAMP and cGMP are cyclic second messengers: cAMP signals carbon starvation through CAP, not amino-acid starvation.

Q: The heat shock response in E.coli increases the level of a regulator molecule that binds to promoter for RNA polymerase to increase the production of heat shock protein :

The regulator is the \(\sigma^{H}/\sigma^{32}\) factor.In E. coli a rise in temperature rapidly increases the level of sigma 32, encoded by rpoH: its mRNA is translated more efficiently at high temperature, and the protein is stabilised because the chaperones that normally target it for degradation are busy with unfolded proteins. Sigma 32 binds RNA polymerase and directs it to heat shock promoters, increasing production of chaperones such as DnaK and GroEL and of proteases.ppGpp and pppGpp are alarmones of the stringent response to amino acid starvation. Diadenosine polyphosphates such as AppppA accumulate under heat and oxidative stress, but they are small signalling nucleotides, not the factor that binds promoters together with RNA polymerase, and the last option names no such factor either.

Catabolite repression in bacteria is regulated by the concentration of

A.amino acids
B.glucose
C.messenger RNA
D.lactose

Exam Year(s)

GATE Life Sciences - 2023,

Answer: B

Catabolite repression is regulated by the concentration of glucose.

When glucose is plentiful, bacteria such as E. coli use it first and repress the genes for using other sugars. In E. coli, glucose transport through the phosphotransferase system lowers adenylate cyclase activity, so cAMP falls; without cAMP the CRP (CAP) activator cannot bind its sites, and operons such as lac are transcribed poorly. Glucose uptake also causes inducer exclusion.

Lactose is the inducer of the lac operon, not the regulator of catabolite repression. Amino acid levels regulate attenuation and the stringent response, and mRNA concentration is an outcome of regulation, not its signal.

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Which one of the following describes the role of enhancers in eukaryotic gene regulation?

A.Enhancers always lie immediately upstream of the promoter and function only in a fixed orientation
B.Enhancers can act at variable distances from the promoters and may function upstream, downstream, or within introns of genes
C.Enhancers directly bind to RNA polymerase II to initiate transcription
D.Enhancers overlap with the RNA polymerase II binding site

Exam Year(s)

GATE Life Sciences - 2026,

Answer: B

Enhancers can act at variable distances from the promoters and may function upstream, downstream, or within introns of genes.

Enhancers are DNA elements bound by activator proteins. They can sit many kilobases from the gene they control, on either side of it or inside an intron, and they work in either orientation. DNA looping brings the bound activators close to the promoter, where they recruit coactivators such as Mediator, which in turn help RNA polymerase II assemble there.

So enhancers do not need to lie immediately upstream or in a fixed orientation. They act through bound proteins rather than by binding RNA polymerase II themselves, and they are separate from the core promoter where the polymerase binds.

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A drug was developed to inhibit histone acetyl transferase in eukaryotic cells. Cells treated with this drug have more compact chromatin and show reduction in transcription of several genes. Which one of the following statements best explains why the drug has this effect?

A.Histone acetylation increases positive charge on histones, thereby tightening DNA binding and repressing transcription
B.Histone H1 directly binds the nucleosome core and its acetylation directly causes nucleosome disassembly
C.Histone acetylation neutralizes positive charge on histones, thereby loosening DNA-histone interactions and promoting transcription
D.Euchromatin is normally condensed, so reduced acetylation of histones converts it into a transcriptionally active state

Exam Year(s)

GATE Life Sciences - 2026,

Answer: C

The best explanation is that histone acetylation neutralizes positive charge on histones, thereby loosening DNA-histone interactions and promoting transcription.

Histone tails are rich in lysine, whose positively charged side chains hold the negatively charged DNA tightly. Histone acetyl transferases add acetyl groups to these lysines, removing their charge, so the DNA is held less tightly and the chromatin opens for transcription factors and RNA polymerase. Blocking the enzyme leaves the lysines charged, so the chromatin compacts and transcription of many genes falls, exactly what the drug does.

Acetylation removes positive charge rather than adding it. H1 is the linker histone outside the nucleosome core, and its acetylation does not directly dismantle nucleosomes. Euchromatin is the decondensed, active form, so the last option has it backwards.

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Which one of the following σ (sigma) factors is responsible for the transcriptional regulation of nitrogen assimilation?

A.σ 70
B.σ 54
C.σ 28
D.σ 24

Exam Year(s)

IIT JAM - 2026,

Answer: B

Nitrogen assimilation genes are transcribed with σ 54.

In E. coli, σ⁵⁴ (RpoN, also called σ^N) directs RNA polymerase to promoters of nitrogen-regulated genes such as glnA, which encodes glutamine synthetase. Unusually, the σ⁵⁴ holoenzyme cannot open the promoter on its own; it needs an ATP-hydrolysing enhancer-binding activator, which for nitrogen genes is NtrC, switched on when nitrogen is scarce.

σ⁷⁰ is the housekeeping factor for most genes in growing cells. σ²⁸ (FliA) transcribes flagellar and chemotaxis genes. σ²⁴ (σ^E) responds to stress in the cell envelope.

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Mechanism of gene regulation does not include :

A.rate of transcription
B.processing of RNA transcript
C.stability of RNA transcript
D.rate of replication

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: D

Gene regulation does not include the rate of replication.

Gene expression can be controlled at every step from DNA to protein: the rate at which a gene is transcribed (promoters, enhancers, transcription factors, chromatin), the processing of the primary transcript (capping, splicing, alternative splicing, polyadenylation), the stability and lifetime of the mRNA, and translation and protein modification after that.

Replication copies the whole genome once per cell cycle, and how fast it runs does not set how much a particular gene is expressed. That is why the rate of replication is the odd one out, while the other three options are standard levels of gene regulation.

Discuss in Forum

Inducible genes are transcribed because ..................... .

A.The inducer inactivates the repressor protein
B.Repressor binds to the promoter
C.Repressor binds to the operator
D.Repressor-inducer bind to the operator

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: A

Inducible genes are transcribed because the inducer inactivates the repressor protein.

In an inducible operon such as lac, the repressor normally binds the operator and blocks transcription. When the inducer (allolactose, or IPTG) is present, it binds the repressor and changes its shape so that it can no longer bind the operator, and RNA polymerase can transcribe the genes.

Repressor bound to the operator is the switched-off state, so option C describes why the genes are not transcribed. The repressor does not act at the promoter, and the repressor-inducer complex leaves the operator rather than binding it.

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Mutation in gene rel A in bacteria results in .................... .

A.Relaxed mutants
B.Silent mutants
C.Constitutive mutants
D.Dominant mutants

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: A

Mutation in relA gives relaxed mutants.

The RelA protein makes the alarmone ppGpp when ribosomes stall for lack of charged tRNA during amino acid starvation. ppGpp shuts down rRNA and tRNA synthesis, the stringent response. In relA mutants no ppGpp is made, so stable RNA synthesis continues despite starvation; such mutants are called relaxed.

Silent mutations change no amino acid, constitutive mutants express genes regardless of conditions, and dominance describes allele interaction. None describes the loss of the stringent response.

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Lac repressor is an example of :

A.Helix loop helix
B.Helix turn helix
C.C2C2 zinc finger
D.C2H2 zinc finger

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The lac repressor is an example of the helix turn helix motif.

Like many bacterial regulators (the lambda and trp repressors, CAP), the lac repressor binds DNA through two alpha-helices joined by a short turn; the second, recognition helix lies in the major groove of the operator.

The helix loop helix motif belongs to eukaryotic factors such as MyoD and Myc, where it mainly mediates dimerisation. Zinc fingers, C2H2 (as in TFIIIA) and C2C2 (as in the steroid receptors), are also eukaryotic DNA-binding domains and do not occur in the lac repressor.

Discuss in Forum

Stringent response in bacteria leads to accumulation of these unusual nucleotides :

A.pGpp and pGppp
B.ppGpp and pppGpp
C.pppGp and ppGp
D.cGMP and cAMP

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The stringent response accumulates ppGpp and pppGpp.

When bacteria are starved of amino acids, uncharged tRNA in the ribosome triggers the RelA protein to make guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), the 'magic spot' nucleotides. They shut down rRNA and tRNA synthesis and redirect the cell's resources.

pGpp, pGppp, pppGp and ppGp are not the alarmones of this response, and cAMP and cGMP are cyclic second messengers: cAMP signals carbon starvation through CAP, not amino-acid starvation.

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10.

The heat shock response in E.coli increases the level of a regulator molecule that binds to promoter for RNA polymerase to increase the production of heat shock protein :

A.\(_{ppp}G_{ppp}\)
B.\(\sigma^{H}/\sigma^{32}\) factor
C.AppppA
D.Diadeosine-3'

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

The regulator is the \(\sigma^{H}/\sigma^{32}\) factor.

In E. coli a rise in temperature rapidly increases the level of sigma 32, encoded by rpoH: its mRNA is translated more efficiently at high temperature, and the protein is stabilised because the chaperones that normally target it for degradation are busy with unfolded proteins. Sigma 32 binds RNA polymerase and directs it to heat shock promoters, increasing production of chaperones such as DnaK and GroEL and of proteases.

ppGpp and pppGpp are alarmones of the stringent response to amino acid starvation. Diadenosine polyphosphates such as AppppA accumulate under heat and oxidative stress, but they are small signalling nucleotides, not the factor that binds promoters together with RNA polymerase, and the last option names no such factor either.

Discuss in Forum

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