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GENE REGULATION

The process by which information from a gene isused in the synthesis of a functional gene product.These products are often proteins, but in non-protein coding genes such as rRNA genesor tRNA genes, the product is a functional RNA.Gene regulation gives the cell control overstructure and function, and is the basis for cellular

differentiation, morphogenesis and the versatility and adaptability of any organism.Gene regulation may also serve as a substrate forevolutionary change.

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REGULATION OF GENE EXPRESSIONInduction, or turning genes on, and repression, or

turning genes off - can be accomplished by both positivecontrol mechanism and negative control mechanism.Regulator genes - genes encoding products thatregulate the expression of other genes.

Positive control mechanism - the product of theregulator gene is required to turn on the expression of one or more structural genes.Negative control mechanism - the product of theregulator genes is necessary to shut off the expression of structural genes.Positive and negative regulation are illustrated for bothinducible and repressible systems.

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Contd The product of the regulator gene acts by binding to a sitecalled the regulator protein binding site (RPBS) adjacent to thepromoter of the structural genes. When this happened, transcription of the structural genes isturned on in a positive control system or turned on in anegative control system.

Regulator gene product called activators because they activategene expression positive control system, And repressors because they repress genes expression negative control system. Whether or not a regulator protein can bind to the RPBSdepends on the presence or absence of effector molecules inthe cells.Effector molecules :

- inducers induction of gene expression- co-repressors repression of gene expression

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Prokaryotic Regulation

Bacteria do not require the same enzymes all the timeEnzymes are produced as needed.Operon is a group of structural and regulatory genesthat function as a single unit.

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Prokaryotic Regulation: The Operon Model

Operon consist of three componentsPromoter

DNA sequence where RNA polymerase first attachesShort segment of DNA

OperatorDNA sequence where active repressor bindsShort segment of DNA

Structural GenesOne to several genes coding for enzymes of a metabolic pathway Translated simultaneously as a blockLong segment of DNA

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Repressible Operons: The trp OperonThe regulator codes for a repressor

If tryptophan (an amino acid) is absent:Repressor is unable to attach to the operator (expression is normally on)

RNA polymerase binds to the promoterEnzymes for synthesis of tryptophan are produced

If tryptophan is present:

Combines with repressor as corepressorRepressor becomes functional

Blocks synthesis of enzymes and tryptophan

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INDUCIBLE SYSTEM NEGATIVE CONTROL

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INDUCIBLE SYSTEM POSITIVE CONTROL

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REPRESSIBLE SYSTEM-NEGATIVE CONTROL

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REPRESSIBLE SYSTEM POSITIVE CONTROL

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