Overview
This lecture explains the molecular mechanisms and regulatory strategies cells use to control gene expression, highlighting both prokaryotic and eukaryotic systems.
Central Dogma and Gene Expression
- Gene expression involves transcription (DNA to mRNA) and translation (mRNA to protein).
- mRNA undergoes modifications like 5' capping, poly-A tail addition, and intron splicing before leaving the nucleus.
- Proteins may be further modified after translation before becoming functional.
Regulation in Prokaryotes: Operons
- Cells express only the genes needed at any time to conserve energy.
- The operon consists of a promoter, operator, and structural genes, functioning as a single control unit.
- The operator acts as an on/off switch for transcription.
- A repressor protein binds the operator to block RNA polymerase, inhibiting gene expression.
- Tryptophan operon: repressor activated by tryptophan to inhibit synthesis (feedback inhibition).
- Lactose operon: allolactose deactivates the repressor, allowing transcription of genes for lactose metabolism.
- Negative gene regulation involves repressors that inhibit transcription.
- Positive gene regulation involves activators (e.g., cAMP-CRP complex) that enhance transcription.
Eukaryotic Gene Regulation
- All somatic cells contain the same DNA but selectively express genes for specialized functions.
- DNA wraps around histones to form nucleosomes, making genes inaccessible unless histones are chemically modified.
- Enzymatic modifications like acetylation, methylation, or phosphorylation of histones can activate or silence genes.
- Transcription factors bind promoter regions (often the TATA box) to help initiate transcription.
- Enhancers and activators interact at distant DNA regions to enhance transcription rates.
- Formation of the transcription initiation complex is required for RNA polymerase to begin transcription.
- Regulation can also occur post-transcriptionally or by blocking translation.
Key Terms & Definitions
- Transcription — the process of copying DNA to mRNA.
- Translation — the synthesis of a polypeptide from an mRNA template.
- Operon — a cluster of genes under the control of a single promoter and operator.
- Operator — DNA sequence acting as a switch for gene transcription.
- Repressor — protein that binds the operator to inhibit transcription.
- Feedback inhibition — end-product inhibits its own synthesis pathway.
- Transcription factor — protein that assists RNA polymerase in initiating transcription.
- Enhancer — DNA region enhancing transcription from a distance.
- Histone modification — chemical changes to histone proteins affecting gene accessibility.
Action Items / Next Steps
- Review biochemistry tutorials on DNA replication, transcription, and translation.
- Prepare to study more complex gene regulatory systems in future lessons.