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THE lac OPERON © 2007 Paul Billiet ODWSODWS
The control of gene expression Each cell in the human contains all the genetic material for the growth and development of a human Some of these genes will be need to be expressed all the time These are the genes that are involved in of vital biochemical processes such as respiration Other genes are not expressed all the time They are switched on an off at need © 2007 Paul Billiet ODWSODWS
Operons An operon is a group of genes that are transcribed at the same time. They usually control an important biochemical process. They are only found in prokaryotes. © NobelPrize.org Jacob, Monod & Lwoff © 2007 Paul Billiet ODWSODWS
The lac Operon The lac operon consists of three genes each involved in processing the sugar lactose One of them is the gene for the enzyme β- galactosidase This enzyme hydrolyses lactose into glucose and galactose © 2007 Paul Billiet ODWSODWS
Adapting to the environment E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide However, lactose needs to be hydrolysed (digested) first So the bacterium prefers to use glucose when it can © 2007 Paul Billiet ODWSODWS
Four situations are possible 1. When glucose is present and lactose is absent the E. coli does not produce β-galactosidase. 2. When glucose is present and lactose is present the E. coli does not produce β-galactosidase. 3. When glucose is absent and lactose is absent the E. coli does not produce β-galactosidase. 4. When glucose is absent and lactose is present the E. coli does produce β-galactosidase © 2007 Paul Billiet ODWSODWS
The control of the lac operon © 2007 Paul Billiet ODWSODWS
1. When lactose is absent A repressor protein is continuously synthesised. It sits on a sequence of DNA just in front of the lac operon, the Operator site The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing Regulator gene lac operon Operator site zya DNA I O Repressor protein RNA polymerase Blocked © 2007 Paul Billiet ODWSODWS
2. When lactose is present A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site) This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site zya DNA IO © 2007 Paul Billiet ODWSODWS
2. When lactose is present A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site) This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site Promotor site zya DNA I O © 2007 Paul Billiet ODWSODWS
3. When both glucose and lactose are present This explains how the lac operon is transcribed only when lactose is present. BUT….. this does not explain why the operon is not transcribed when both glucose and lactose are present. © 2007 Paul Billiet ODWSODWS
When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstable and it keeps falling off Promotor site zya DNA IO Repressor protein removed RNA polymerase
4. When glucose is absent and lactose is present Another protein is needed, an activator protein. This stabilises RNA polymerase. The activator protein only works when glucose is absent In this way E. coli only makes enzymes to metabolise other sugars in the absence of glucose Promotor site zya DNA IO Transcription Activator protein steadies the RNA polymerase © 2007 Paul Billiet ODWSODWS
Summary CarbohydratesActivator protein Repressor protein RNA polymerase lac Operon + GLUCOSE + LACTOSE Not bound to DNA Lifted off operator site Keeps falling off promoter site No transcription + GLUCOSE - LACTOSE Not bound to DNA Bound to operator site Blocked by the repressor No transcription - GLUCOSE - LACTOSE Bound to DNA Bound to operator site Blocked by the repressor No transcription - GLUCOSE + LACTOSE Bound to DNA Lifted off operator site Sits on the promoter site Transcription © 2007 Paul Billiet ODWSODWS
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