Beginning of Ch.15 Notes

Regulation of Gene Expression

Gene expression in prokaryotes and eukaryotes can adapt to environmental changes.
Multicellular eukaryotes differentiate into multiple cell types so not every gene is expressed in every cell.

Bacteria modify transcription in response to environmental stimuli.
Natural selection favors bacteria that produce necessary gene products when needed.
Gene expression regulation in bacteria is facilitated by the operon model.
- Operon: A unit of genetic regulation consisting of an operator, promoter, and associated genes.
- Example is the trp operon which helps regulate gene expression for tryptophan synthesis.
- Feedback inhibition can also control enzyme production.

Operons allow coordinated control of functionally related genes through a single switch.
- Operator: DNA segment that acts as a regulatory switch.
- Promoter: Site for RNA polymerase binding to initiate transcription.
- Remember the TATA box located at the start of the promoter sequence!
Genes in the operon can be turned off by a repressor protein which blocks RNA polymerase.
- Repressor is produced by a separate regulatory gene.

The trp operon is typically active (on) unless tryptophan is abundant.
When tryptophan is present, it binds as a corepressor to the trp repressor.
- This complex then binds to the operator, switching the operon off and preventing transcription.
- Tryptophan synthesis halts when excess tryptophan is available, ensuring energy efficiency.

Repressible Operons: Normally active, they are turned off by binding of a repressor (e.g., trp operon).
Inducible Operons: Generally inactive, turned on when an inducer molecule inactivates the repressor (e.g., lac operon).
- Inducers are like allolactose for the lac operon that inactivates the repressor to allow transcription.
Key Comparison:
- Inducible (e.g., lac): Repressor is active when gene is off.
- Repressible (e.g., trp): Repressor is inactive when gene is on.

E. coli uses glucose preferentially; in its absence, cAMP receptor protein (CRP) activates transcription.
When glucose is scarce, high cAMP levels activate CRP, which increases RNA polymerase's affinity for the lac operon promoter.
The presence of glucose reduces cAMP levels and lowers transcription rates, affecting overall metabolism depending on available substrates.

In multicellular organisms, precise gene regulation is crucial for cell specialization.
Various stages of expression can be regulated to determine the final phenotype of different cell types.