Short on time? This page is the recap. For explanations, Figure 1 and worked examples, use the full study guide.
Recap
- Cells keep almost all their genes but express only some, at controlled levels. Most control happens at the start of transcription.
- Regulatory sequences (promoter, operator, control elements) are DNA. Regulatory proteins (repressors, activators, transcription factors) bind them.
- Constitutive genes are always on. Inducible genes switch on when an inducer appears. Repressible genes switch off when a corepressor builds up.
- An operon = promoter + operator + several structural genes, transcribed as one mRNA. The repressor gene lies outside it.
- The operator lies between the promoter and the genes, so a bound repressor blocks RNA polymerase for all the genes at once.
- In eukaryotes, scattered genes that share the same control sequence are switched on together by one transcription factor.
- DNA methylation usually silences; histone acetylation loosens chromatin and usually activates. Both are reversible and can be passed to daughter cells.
- Phenotype = which genes are on and how much product they make. Development uses chains of transcription factors (sequential expression).
Key relationships
| Situation | Repressor state | Operon |
|---|---|---|
| lac, no lactose | active, on operator | off |
| lac, lactose present | allolactose bound, off operator | on (strongly if glucose is scarce: CAP–cAMP helps) |
| trp, little tryptophan | inactive, off operator | on |
| trp, much tryptophan | tryptophan bound, on operator | off |
| Mutation | Likely effect on an inducible operon |
|---|---|
| no working repressor | on all the time (constitutive) |
| operator cannot bind repressor | on all the time (constitutive) |
| promoter cannot bind RNA polymerase | off all the time |
Assumptions
- Fold change compares a test value with a stated reference value; always say which.
- Drug experiments affect many genes, so effects on one gene may be partly indirect.
Mistakes to avoid
- Putting the repressor gene inside the operon.
- Saying lactose (or allolactose) binds the operator.
- Calling an epigenetic change a mutation.
- Mixing up the effects of DNA methylation and histone acetylation.
- Forgetting that the level of expression, not just on/off, shapes phenotype.
Quick self-check
- Why does a repressor bound to the operator stop all three lac genes? (The operator lies upstream of all three genes, in RNA polymerase’s path, and they share one mRNA.)
- A wild-type operon gives 4 units without inducer and 320 with it. Fold induction? (320 ÷ 4 = 80)
- Which change makes a gene easier to transcribe: adding acetyl groups to histones or adding methyl groups to its promoter DNA? (Adding acetyl groups to histones.)
Next: practice questions.