01Prokaryotic operons
Bacteria group related genes into operons with one promoter and an operator (on/off switch). The lac operon is inducible: a repressor blocks it until lactose (allolactose) binds and removes the repressor. The trp operon is repressible: tryptophan activates the repressor to turn genes off when trp is plentiful.
02Eukaryotic regulation
Chromatin: acetylation of histones loosens DNA (genes ON); methylation of DNA tightens it (genes OFF). These epigenetic changes can be inherited without changing sequence.
Transcription factors bind enhancers and promoters to boost or block transcription. After transcription, microRNAs can block or degrade mRNA.
03Notebook box
Lac operon: no lactose → repressor on operator → OFF. Lactose present → repressor released → ON.
Worked example: A mutant lac repressor can't bind the operator. Effect? → Genes are always on, even without lactose (constitutive).