Enhancers, accessibility and combinatorial control · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

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Science · Genetics & Evolution · ages 19-21
Name ______________________   Date ____________

  1. Every enhancer requires one exact distance and one orientation relative to its promoter.

    Circle one:   True   False

  2. How can an enhancer thousands of bases away influence a promoter?

    • Its activators must travel along the intervening DNA to reach the promoter
    • It always activates the nearest promoter, regardless of DNA folding
    • DNA folding brings regulatory proteins near promoter machinery

  3. An enhancer is equally accessible in neurons and liver cells. Its required activator is active only in neurons. Which stated difference explains its activity?

    • The required activator's activity
    • The enhancer's accessibility in the two cell types
    • The enhancer's distance from its promoter in each cell type

  4. How can Mediator contribute to enhancer-dependent transcription?

    • By recognising enhancer motifs in place of sequence-specific activators
    • By opening every inaccessible enhancer before an activator can bind
    • By helping regulators communicate with promoter machinery

  5. What does combinatorial transcriptional control mean?

    • Each factor controls a separate gene without interacting inputs
    • Several regulatory inputs jointly affect expression
    • The strongest activator determines expression independently of repressors

  6. Activation above basal output requires both A and B. Access is available and no repressor is present, but only A is supplied. What follows?

    • No activation above basal output
    • Full activation because A can satisfy either required input
    • Zero transcription because basal output also requires this combination

  7. An enhancer becomes inaccessible to its required activators. Its sequence is intact. Without information about other enhancers, which prediction is justified?

    • The gene's total transcription must fall to zero
    • Its unchanged sequence preserves the enhancer's previous contribution
    • This enhancer's contribution can decrease

  8. Activation requires A and B, access and no C. Access, A and B are present, but C is also present. Sam predicts activation from A and B alone. What did he miss?

    • Access cannot support transcription when both activators are present
    • The rule also requires C to be absent
    • C matters only when one required activator is missing
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Answer key

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  1. False · Many enhancers tolerate spacing or orientation changes. This does not make genomic context irrelevant.
  2. DNA folding brings regulatory proteins near promoter machinery · Distance along DNA differs from distance in space. Folding can bring distant regulatory sites near promoter machinery.
  3. The required activator's activity · Accessibility is held equal. The stated difference is whether the required activator is active.
  4. By helping regulators communicate with promoter machinery · Mediator is a coactivator that can help bound regulators influence transcription machinery.
  5. Several regulatory inputs jointly affect expression · Combinatorial control concerns joint regulatory effects, which can change expression levels rather than only switch genes on or off.
  6. No activation above basal output · The activation requirement is incomplete. The rule does not establish zero basal transcription.
  7. This enhancer's contribution can decrease · Loss of access can reduce this enhancer's contribution. Other enhancers and basal initiation remain unknown.
  8. The rule also requires C to be absent · C violates the stated inhibitory condition even though both activators and access are present.
Worksheet · LightMySky