Short on time? This page is the recap. For explanations, Figures 1 and 2 and worked examples, use the full study guide.
Recap
- Nearly all cells carry the same genome. Differential gene expression gives each cell type its own proteins, structure and function.
- General transcription factors and RNA polymerase assemble at the promoter (often with a TATA box). Alone, they give a low, basal rate.
- Activators bind enhancers. Enhancers can be far upstream or downstream of the start site, even in introns. DNA loops so activators reach the promoter complex through mediator proteins.
- Repressors (negative regulators) bind DNA, for example at silencers, and block transcription.
- Each gene needs a combination of activators. Each cell type has its own set of activators, so it expresses its own set of genes.
- miRNAs (from hairpin RNA) and siRNAs (from double-stranded RNA), about 21–23 nucleotides long, base-pair with mRNA. Result: less translation, mRNA breakdown, or both. DNA unchanged.
Key relationships
| DNA sequence | What binds | Effect on transcription |
|---|---|---|
| promoter | general transcription factors, RNA polymerase | needed to start at all |
| enhancer | activators | raises the rate |
| silencer | repressors | lowers the rate or blocks it |
| Experiment result | Conclusion |
|---|---|
| deleting a region lowers reporter output | region is (or contains) an enhancer |
| deleting a region raises reporter output | region is (or contains) a silencer |
| blocking a miRNA raises protein more than mRNA | the miRNA also blocks translation |
Assumptions
- Reporter genes in cultured cells show what a control region can do, not always what happens on the chromosome.
- Data in % are relative to a stated reference, such as the full-length construct.
Mistakes to avoid
- Saying different cell types have different genes.
- Saying activators bind the promoter, or that enhancers must be upstream.
- Saying miRNAs change DNA or stop transcription.
- Assuming unchanged mRNA means unchanged protein.
Quick self-check
- A full-length construct gives 900 units; with region Q deleted it gives 45. What percentage remains, and what is Q? (45 ÷ 900 × 100 = 5.0%; Q is an enhancer, or contains one.)
- Why can an enhancer 10 000 base pairs away still affect its gene? (The DNA bends into a loop, bringing bound activators into contact with the promoter complex.)
- Cell type 1 has activators A and B; cell type 2 has only A. A gene needs A and B. Which cell expresses it? (Cell type 1 only.)
Next: practice questions.