lac Operon

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  • Created by: Megan
  • Created on: 07-03-14 20:06

Enzyme Induction

Escerichia Coli bacteria can synthesis 3000 polypeptides but there is varioation in the number of polupeptides within the cell - there may be 10,000 ribosomal polypeptides and just 10 molecules of some regulatory proteins. Enzymes involved in basic cellular function are synthesised at a fairly constant rate. 

Inducible enzymes are synthesised as varying rates according to the cell's circumstances. Bacteria produce enzymes to metabolise certain nutrients only when those nutrients are present. E. Coli normally respire glucose but can use lactose as a respiratory substrate

E. Coli grown in a cultuire medium with no lasctose are placed ina  medium with lactos. At first they cannot metabolise lactose as three are only tiny anouts of the two enzymes needed to metabolise it - Beta-Galatosidase - catalyses hydrolysis of lactose into glucose and glatactose. - Lactose permease - transports lactose into the cell.

After a few minutes, E. Coli bacteria increase the  rate of synthesis fo these enzymes by aobut 1000 times - Lactose triggers the production of the two enzymes and so is an inducer

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lac system genes form an operon

The structural genes Z & Y - Z codes for the enzyme beta-galactosidase, Y codes for lactose permease. Each consists of a sequence of base pairs that can be transcribed into a length of mRNA

The Operator Region - a length of DNA that switches strucutral genes on or off

The Promote region - a length of DNA to which the enzyme RMA polymerase binds tom to begin the transcription of structural genes Z and Y

The Regulator Gene - not part fo the operon and is some distrance from the lac Operon

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How lac Operon works without lactose being present

1) Regulatory gene transcribed and translated - Repressor protein synthesis (this has two binging sites, 1 for binding to lactose and 1 for binding to the operator region)

2) Repressor Protein binds to the operator region - covers part of the Promoter region where RNA polymerase should attach

3) As a result, the genes are not translated and the enzymes beta-glactosidase and lactose permease cannot be synthesised

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How the lac Operon works with lactose present

1) Lactose (inducer) molecules bind to the other site on the repressor protein so the protein changes shape -> the other binding site cannot bind to the operator region so the repressor protein dissociates from the operator region

2) The P region is unblocked. RNA polymerase can now bind to it and can initiate the transcription of mRNA for Z and Y genes

The operator-repressor inducer system acts as a molecular switch, It allows transcription and translation of structural genes into beta-galactosidase and lactose permease

E. Coli bacteria can use the lactose permase enzyme to take up lactose fromt he medium into their cells. They can convert the lactose to glucose and galactose using beta-galactosidase enzymes. These sugars can then be used for respiration thus gaining energy from lactose

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