What is an operon?
A cluster of genes controlled by one promoter and expressed together.
Types of operons?
Constitutive (always ON), Inducible (OFF→ON), Repressible (ON→OFF).
Why is the lac operon inducible?
It requires allolactose to remove the repressor and start transcription.
What activates high lac transcription?
Low glucose → high cAMP → CAP activates transcription.
When is lac operon highest?
When lactose is present and glucose is absent.
Why is the trp operon repressible?
Tryptophan acts as a corepressor, activating the repressor to stop transcription.
What happens when tryptophan is low?
The repressor is inactive and transcription continues.
What is attenuation?
Early termination of transcription based on tryptophan levels.
What is a riboswitch?
A 5’UTR RNA element that regulates transcription by binding a ligand.
Ligand effect on riboswitch?
Ligand present = terminator loop forms and transcription stops.
Major levels of eukaryotic regulation?
Chromatin, transcription, RNA processing, mRNA stability, translation.
Euchromatin vs heterochromatin?
Euchromatin = open/active, Heterochromatin = closed/silent.
Effect of histone acetylation?
Opens chromatin and activates transcription.
Effect of histone deacetylation?
Closes chromatin and represses transcription.
What binds the promoter first?
TFIID binds the TATA box first.
Role of enhancers?
Bind activators to increase transcription.
Role of silencers?
Bind repressors to decrease transcription.
What is alternative splicing?
One gene produces multiple mRNAs and proteins.
What decreases mRNA stability?
Short poly-A tail, decapping, endonuclease cleavage.
What is NMD?
Nonsense-mediated decay of mRNAs with premature stop codons.
Epigenetics definition?
Heritable gene expression changes without altering DNA sequence.
Main epigenetic mechanisms?
DNA methylation, histone modification, non-coding RNAs.
Where does DNA methylation occur?
CpG islands near promoters.
Effect of DNA methylation?
Silences transcription.
