Control of Gene Expression The Operon Model Two

Control of Gene Expression The Operon Model

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

Operon – cluster of genes under the control of a promoter m. RNA sequence - genes are either expressed together or not at all

Components of an operon 1. 2. 3. 4. 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 1. Repressible • transcription usually on but can be inhibited • trp operon 2. Inducible • transcription usually off but can be stimulated • 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 negative and positive regulation

Differential gene expression in eukaryotes 1. Regulation of chromatin structure 2. Regulation of transcription initiation – – – Enhancers Activators Repressors 3. Post-transcriptional regulation – – RNA processing m. RNA degradation Initiation of translation Protein processing and degradation