Which combination most likely silences gene expression?

Answer: DNA methylation at CpG islands plus histone deacetylation Methylation of CpG islands and removal of acetyl groups from histones tighten chromatin (heterochromatin), reducing transcription factor access and silencing transcription.

How does tryptophan act as a corepressor in this system?

Answer: Tryptophan binds the inactive repressor and converts it to an active form that binds the operator. The trp operon is repressible: tryptophan acts as a corepressor by binding the otherwise inactive repressor, enabling it to bind the operator and shut off trp biosynthesis when amino acid levels are high.

What functional role do enhancer sequences play even when located far from the gene's promoter?

Answer: They serve as binding sites for activator proteins; DNA looping brings them to the promoter to stimulate transcription. Enhancers are cis-regulatory elements bound by activators. DNA looping physically associates these activators with the basal transcription machinery at the promoter, increasing transcription frequency.

AP Biology · Topic 6.5

Regulation of Gene Expression Practice

Part of Gene Expression and Regulation.(IST-2.A)

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$Which combination most likely silences gene expression?$
- A
  $Loss of methyl groups from histones$
- B
  $Open chromatin plus enhancer activation$
- C
  $DNA methylation at CpG islands plus histone deacetylation$
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- D
  $Histone acetylation plus DNA demethylation$
Why
$Methylation of CpG islands and removal of acetyl groups from histones tighten chromatin (heterochromatin), reducing transcription factor access and silencing transcription.$
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Sample 2difficulty 2/5
$How does tryptophan act as a corepressor in this system?$
- A
  $Tryptophan binds the inactive repressor and converts it to an active form that binds the operator.$
  check_circle
- B
  $Tryptophan inactivates the operator by methylation.$
- C
  $Tryptophan binds the operator directly.$
- D
  $Tryptophan increases RNA polymerase processivity.$
Why
$The trp operon is repressible: tryptophan acts as a corepressor by binding the otherwise inactive repressor, enabling it to bind the operator and shut off trp biosynthesis when amino acid levels are high.$
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Sample 3difficulty 2/5
$A eukaryotic gene has an enhancer located thousands of base pairs upstream of its promoter. A transcription factor (TF) bound at the enhancer associates with RNA polymerase II at the promoter via DNA looping.$
$What functional role do enhancer sequences play even when located far from the gene's promoter?$
- A
  $They terminate transcription at long distances.$
- B
  $They serve as binding sites for activator proteins; DNA looping brings them to the promoter to stimulate transcription.$
  check_circle
- C
  $They prevent splicing of the resulting pre-mRNA.$
- D
  $They are translated by ribosomes to produce activator proteins.$
Why
$Enhancers are cis-regulatory elements bound by activators. DNA looping physically associates these activators with the basal transcription machinery at the promoter, increasing transcription frequency.$
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Sample 4difficulty 2/5
$Which generalization correctly distinguishes inducible from repressible operons?$
- A
  $Inducible operons exist only in eukaryotes; repressible operons only in prokaryotes.$
- B
  $Inducible operons lack a repressor; repressible operons have an activator.$
- C
  $Inducible operons are normally OFF and turn ON in response to a small molecule; repressible operons are normally ON and turn OFF when a corepressor accumulates.$
  check_circle
- D
  $Inducible operons cannot be regulated allosterically.$
Why
$The lac operon is an inducible system (default OFF; allolactose induces) typical of catabolic pathways. The trp operon is repressible (default ON; tryptophan represses), typical of biosynthetic pathways.$
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Sample 5difficulty 2/5
$An operon (in prokaryotes) is$
- A
  $A cluster of co-regulated genes transcribed together as one mRNA from a single promoter$
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- B
  $A single gene whose expression is regulated by multiple transcription factors$
- C
  $A regulatory protein that activates or represses transcription at multiple genes$
- D
  $A cluster of unrelated genes each transcribed from its own separate promoter$
Why
$Operons enable coordinate regulation of related genes. They're rare in eukaryotes (which use individual gene regulation).$
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