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Control of gene expression: Transcritpion Regulation of gene expression
-some genes are expressed in all cells at constant levels- house keeping genes
-other genes vary in their expression- the control is by Bacteria
-Chromosome of E coli : single circular DNA
-Expression of gene is regulated according to availability of food
-E.g 5 E.coli genes code for enzymes that synthesise amino acid tryptophan-genes are arranged as a single operon: Operon is a group of genes with related function which are under the control of a single promoter. Operons are only found in prokaryotes.
-genes are adjacent to one another and are transcribed from a single promoter as one long Mrna
-Promoter- specific DNA sequence that directs RNA polymerase to bind to DNA, open DNA double helix, begin synthesising RNA
-When tryptophan is present in the growth medium and enters the cells, the promoter is switched off. Within the promoter is a regulatr element called an operator-regulatory DNA that is recognised by repressor protein-binding of the protein prevents RNA polymerase binding to promoter region
-Gene regulatory proteins that switch genes off are transcriptional repressors
-Transcriptional activators: bind to a nearby site on DNA, contact the RNA polymerase that results in an increase in the probability that transcript will be initiated. 2) DNA looping: gene regulation
-Lac operon codes for proteins required to transport lactose into cell and break it down
-Operon is under both negative and positive transcriptional controls,Lac repressor protein and CAP (Catbolite activator protein)-
expression of the lac operon allows bacteria to use lactose as an energy source. In the absence of lactose, the lac repressor protein binds to the operator and blocks expression when an inducer molecule is present, in this case- allolactose (the inducer molecule of lactose)- it binds to the repressor protein and changes its shape so it
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