Control of Gene Expression If all cells have the same genome, why are they so different from each other?

Two levels of metabolic control Feedback inhibition – quick response to short term fluctuations in amount of required substance Operon model – genes switched on or off by changes in the cell

Jacob and Monod’s Operon Model – cluster of genes under the control of a promoter - genes are either expressed together or not at all

Components of an operon Promoter – sequence recognized by RNA polymerase Operator – segment of DNA to which a regulator binds Structural genes – genes regulated by the operon Regulatory genes – code for regulatory proteins

Operon control Regulates the expression of genes depending on environmental conditions Negative gene regulation repressor binds to operator preventing transcription Repressible transcription usually on but can be inhibited by presence of product trp operon Inducible transcription usually off but can be stimulated by presence of substrate Positive gene regulation activator protein binds to DNA at a site other than the operator allowing transcription to occur

trp operon repressible gene regulation

lac operon inducible gene regulation

lac operon positive gene regulation CRP-cAMP or CAP is an activator

Differential gene expression in eukaryotes Regulation of chromatin structure Regulation of transcription initiation Enhancers (control DNA sequence) Activators Repressors 3. Post-transcriptional regulation RNA processing mRNA degradation Initiation of translation Protein processing and degradation