Chapter 16 – Regulation of Gene Expression

Why do prokaryotes regulate gene expression?

To conserve energy by making proteins only when needed and quickly respond to environmental changes.

What is the role of promoters in prokaryotes?

DNA site where RNA polymerase binds to initiate transcription; selective activity controls gene expression.

Define repressor and activator proteins.

Repressor blocks transcription (negative regulation); activator stimulates transcription (positive regulation).

Components of the lac operon?

Promoter, operator, structural genes (β-galactosidase, permease, transacetylase).

How does negative regulation of the lac operon work?

Repressor binds operator → blocks transcription. Allolactose (inducer) binds repressor → releases operator → transcription starts.

How does the trp operon (repressible) work?

Normally on; tryptophan binds repressor → repressor binds operator → transcription stops.

Positive regulation of lac operon?

Low glucose → ↑cAMP → binds CRP → CRP binds promoter → helps RNA polymerase bind → more transcription.

What are sigma factors?

Proteins guiding RNA polymerase to specific promoters; e.g., Sigma-70 → housekeeping, Sigma-38 → stress response.

What is the basal transcription apparatus?

Minimal complex of general transcription factors (GTFs) at promoter to initiate transcription.

Role of specific transcription factors (TFs)?

Bind enhancers/silencers to positively/negatively regulate specific genes or cell types.

What is a master regulator TF?

TF controlling key developmental pathways; e.g., MyoD for muscle differentiation.

Two main viral reproductive cycles?

Lytic: immediate reproduction → host lysis. Lysogenic: viral DNA integrates into host genome → replicated silently until triggered.

HIV life cycle steps relevant to gene regulation?

Membrane fusion → capsid release → reverse transcription → integrase inserts DNA → provirus latent → transcription blocked → Tat protein activates full-length RNA → viral proteins produced.

Define epigenetics.

Study of heritable changes in gene expression not caused by DNA sequence changes.

DNA methylation effects?

Adds methyl to cytosines → usually silences transcription; reversible; involved in development and disease (cancer).

Histone modifications?

HATs → acetylation → open chromatin → activate transcription. HDACs → remove acetyl → repress transcription.

How is gene expression regulated after transcription?

RNA splicing/alternative splicing, small RNAs (miRNA/siRNA), translational control, protein longevity.

Alternative splicing significance?

Produces multiple proteins from one gene; controlled by RNA elements/structures; contributes to complexity.

miRNA vs siRNA?

miRNA: endogenous, inhibits translation, targets multiple mRNAs. siRNA: usually exogenous, targets specific RNA for degradation.