Control of Gene Expression in Eukaryotes
High-Yield Summary
- Unlike prokaryotic operon-level control, eukaryotic regulation spans multiple levels: chromatin accessibility, transcription initiation, and gene amplification.
- Euchromatin: loosely packed, open, transcriptionally active, lighter under microscope. Heterochromatin: densely packed, closed, transcriptionally silent, darker.
- Histone acetyltransferases (HATs) add acetyl groups → loosen chromatin → increase transcription. Histone deacetylases (HDACs) remove acetyl groups → tighten chromatin → decrease transcription.
- DNA methylation (by DNA methyltransferases) typically causes long-term gene silencing — blocks transcription factor binding or recruits chromatin-compacting proteins.
- Transcription factors have a DNA-binding domain (finds promoters, within ~25bp of start site, and enhancers, farther away — often hundreds to thousands of bp) and an activation domain (interacts with RNA polymerase/coactivators). Can act as activators or repressors. DNA looping brings distant enhancers physically close to the promoter.
- Gene amplification (extra gene copies) and gene duplication (copied gene, extra version expressed) both increase gene product output beyond a single copy.
Key Terms
- Histone acetyltransferase (HAT)
- Adds acetyl groups to histones, loosening chromatin and increasing transcription.
- Histone deacetylase (HDAC)
- Removes acetyl groups from histones, tightening chromatin and decreasing transcription.
- DNA methylation
- Addition of methyl groups to DNA by DNA methyltransferases, typically causing long-term gene silencing.
- Enhancer
- Regulatory DNA sequence located far (often hundreds–thousands of bp) from the gene; transcription factors bind here to boost transcription.
- Response element
- Specific DNA sequence near or far from a gene that a transcription factor's DNA-binding domain recognizes.
- DNA looping
- Physical bending of DNA that brings a distant enhancer-bound transcription factor into contact with the promoter complex.
Euchromatin vs. Heterochromatin
| Feature | Euchromatin vs. Heterochromatin |
|---|---|
| Packing | Loosely packed / Densely packed |
| Transcriptional state | Active / Silent |
| Accessibility | Open / Closed |
| Microscope appearance | Lighter / Darker |
Histone Modification Enzymes
| Enzyme | Effect |
|---|---|
| HAT (adds acetyl groups) | Loosens chromatin → increases transcription |
| HDAC (removes acetyl groups) | Tightens chromatin → decreases transcription |
Common MCAT Trap
- Acetylation loosens chromatin (more transcription); methylation of DNA silences genes (less transcription) — two different chemical modifications with roughly opposite regulatory directions. Don't conflate histone acetylation with DNA methylation.
- Enhancers can sit far from the gene they regulate (often much farther than the ~25bp minimum) — DNA looping, not proximity, is what makes them functional. A distant DNA sequence affecting a gene doesn't mean it's non-functional or unrelated.
- Eukaryotic regulation is multi-level (chromatin + transcription factors + amplification) — unlike the single operon-level control point in prokaryotes. A question contrasting the two systems is testing this structural difference.
Quick Recall
What does a histone acetyltransferase do to chromatin and transcription?
What is the typical long-term effect of DNA methylation?
What two functional domains does a transcription factor have?
How does DNA looping help a distant enhancer function?