biology : class11 Coactivators and Corepressors : Receptors, Second Messengers, Gene Regulation : Hormone Action MCQ Questions and Answers
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Coactivators and Corepressors
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MCQ Questions
1. A ligand-bound nuclear receptor activates transcription even though it cannot efficiently bind the basal transcription machinery by itself. Which recruited factor most directly explains this activation?
A.
A coactivator that links the receptor to Mediator and general transcription factors
B.
A corepressor that removes acetyl groups from histones
C.
A membrane receptor that generates cyclic AMP
D.
A ribosomal protein that initiates translation
Correct Answer:
A coactivator that links the receptor to Mediator and general transcription factors
Explanation:
Coactivators do not usually bind DNA at the response element independently; they are recruited by activated transcription factors and help connect them with Mediator and the basal transcriptional machinery.
2. A hormone-dependent transcriptional activator recruits a protein complex possessing histone acetyltransferase activity. What is the most likely immediate consequence at the regulated gene?
A.
Histone acetylation that generally loosens chromatin and facilitates transcription
B.
Histone deacetylation that generally produces a more compact chromatin state
C.
Direct degradation of the hormone receptor
D.
Removal of all nucleosomes from the entire chromosome
Correct Answer:
Histone acetylation that generally loosens chromatin and facilitates transcription
Explanation:
Many coactivator complexes contain histone acetyltransferases. Acetylation of histone tails commonly reduces chromatin compaction and makes regulatory DNA more accessible to transcriptional machinery.
3. A nuclear receptor represses a target gene in the absence of its ligand by recruiting a complex containing histone deacetylase. Which change best accounts for this repression?
A.
Increased histone acetylation and enhanced coactivator recruitment
B.
Reduced histone acetylation and formation of a less accessible chromatin structure
C.
Conversion of the receptor into a membrane ion channel
D.
Immediate translation of the target gene's mRNA
Correct Answer:
Reduced histone acetylation and formation of a less accessible chromatin structure
Explanation:
Unliganded receptors can recruit corepressors containing histone deacetylases. Deacetylation generally promotes tighter chromatin packing, thereby reducing access of transcriptional machinery to the gene.
4. Which observation most strongly indicates that a protein is functioning as a coactivator rather than as a DNA-binding transcription factor?
A.
It binds a hormone response element directly and activates transcription alone
B.
It is recruited by a DNA-bound receptor and enhances transcription without recognizing the response element independently
C.
It forms the peptide hormone before receptor binding
D.
It transports the hormone through the bloodstream
Correct Answer:
It is recruited by a DNA-bound receptor and enhances transcription without recognizing the response element independently
Explanation:
A defining feature of many coactivators is that they are recruited by DNA-bound transcription factors and support activation without directly binding the regulatory DNA sequence themselves.
5. A mutation prevents a ligand-bound receptor from interacting with its coactivator but does not affect receptor binding to DNA. What is the most likely result?
A.
Normal DNA occupancy but impaired activation of transcription
B.
Loss of all receptor synthesis but normal transcription
C.
Permanent activation of every gene in the nucleus
D.
Conversion of the receptor into a cytoplasmic second messenger
Correct Answer:
Normal DNA occupancy but impaired activation of transcription
Explanation:
DNA binding and transcriptional activation are separable receptor functions. If coactivator recruitment is defective, the receptor may remain attached to its response element but fail to assemble an effective activating complex.
6. A receptor mutation causes it to recruit a corepressor even after hormone binding. Which transcriptional outcome is most likely?
A.
Persistent repression of the receptor's target gene despite the presence of hormone
B.
Unrestricted transcription caused by constitutive coactivator recruitment
C.
Failure of the hormone to enter the bloodstream
D.
Direct cleavage of the target gene's DNA
Correct Answer:
Persistent repression of the receptor's target gene despite the presence of hormone
Explanation:
Hormone binding normally shifts many nuclear receptors toward coactivator recruitment. If a corepressor remains associated after hormone binding, chromatin-repressing activity can persist and transcription remains low.
7. Why can the same nuclear receptor activate one gene but repress another gene in different cellular contexts?
A.
Different response-element arrangements and available cofactor complexes can determine whether coactivators or corepressors are recruited
B.
The receptor changes into a different type of hormone molecule at each gene
C.
Every gene contains identical regulatory sequences but different ribosomes
D.
Coregulators act only during translation and cannot affect transcription
Correct Answer:
Different response-element arrangements and available cofactor complexes can determine whether coactivators or corepressors are recruited
Explanation:
Receptor output depends on the regulatory DNA context and the cell's complement of coregulators. These factors influence whether receptor binding favors activating coactivator complexes or repressing corepressor complexes.
8. A coactivator complex recruits a chromatin-remodeling activity in addition to a histone acetyltransferase. What is the combined functional advantage of these activities?
A.
They can increase DNA accessibility both by altering histone modifications and by repositioning or restructuring nucleosomes
B.
They prevent the receptor from binding its response element
C.
They replace the promoter with a ribosomal RNA gene
D.
They terminate transcription by degrading all nuclear RNA
Correct Answer:
They can increase DNA accessibility both by altering histone modifications and by repositioning or restructuring nucleosomes
Explanation:
Coactivators can promote transcription through multiple coordinated mechanisms. Histone acetylation changes chromatin interactions, while remodeling complexes alter nucleosome positioning or structure to improve access to regulatory DNA.
9. Which experimental result would provide the strongest evidence that a suspected corepressor acts through histone deacetylation?
A.
Corepressor recruitment is accompanied by reduced histone acetylation, and a histone deacetylase inhibitor restores transcription
B.
Corepressor recruitment increases histone acetylation and strengthens transcription
C.
The suspected corepressor is found only in extracellular fluid
D.
The target gene is translated more rapidly when the corepressor is present
Correct Answer:
Corepressor recruitment is accompanied by reduced histone acetylation, and a histone deacetylase inhibitor restores transcription
Explanation:
This result links the corepressor to deacetylase-dependent repression: recruitment reduces histone acetylation, whereas blocking deacetylase activity reverses the transcriptional effect.
10. A synthetic molecule stabilizes the interaction between a nuclear receptor and a coactivator, but it does not alter receptor DNA-binding affinity. Which effect is most directly predicted?
A.
Greater transcription of genes regulated by receptor response elements
B.
Reduced recruitment of the basal transcription machinery
C.
Conversion of the receptor into a corepressor-containing complex
D.
Inhibition of all histone acetylation throughout the cell
Correct Answer:
Greater transcription of genes regulated by receptor response elements
Explanation:
Stabilizing receptor–coactivator interaction should enhance assembly of the activating transcriptional complex. Because DNA binding is unchanged, the direct effect is stronger activation at receptor-regulated genes.
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