Transcriptional Control and Gene Regulatory Networks
Core transcription machinery
Promoters and polymerases
Transcription begins when sequence-specific transcription factors and general transcription factors assemble at promoters and recruit RNA polymerase. Promoter architecture, enhancer repertoire, and chromatin accessibility jointly determine the rate and pattern of transcription. The output of a locus is often better described by burst frequency and burst size than by a single steady-state value.
Which DNA element can regulate a gene from a large distance and often in an orientation-independent manner?
Enhancers act at long range through chromatin looping and cofactor recruitment.
Correct answer: Enhancer
What is transcriptional bursting?
Bursting explains cell-to-cell variability in expression even among genetically identical cells.
Correct answer: Intermittent stochastic switching of a gene between active and inactive transcriptional states.
Regulatory logic
Network principles
Gene regulatory networks are composed of interacting regulators that encode feedforward loops, feedback loops, and combinatorial logic. These structures can create ultrasensitivity, robustness, lineage commitment, and oscillatory behavior. Network inference integrates perturbation, time-series, and single-cell data to recover causality.
Advanced perspective
A regulator can be activating in one cell type and repressive in another because its function depends on cofactors, chromatin context, and post-translational state.
Network motifs
| Motif | Behavior | Example |
|---|---|---|
| Negative feedback | Stabilization and noise reduction | Protein autorepression |
| Positive feedback | Bistability | Cell fate commitment |
| Feedforward loop | Delay filtering or pulse generation | Developmental patterning |
| Mutual inhibition | Alternative fate selection | Lineage branching |
Which network motif is most associated with bistable cell-fate decisions?
Positive feedback can reinforce one of two stable expression states.
Correct answer: Positive feedback
Why does chromatin accessibility matter for enhancer function?
Accessible chromatin is a prerequisite for most enhancer-mediated regulation.
Correct answer: Because enhancers require factor binding and physical communication with promoters, both of which depend on accessible DNA.