OPERONS: BACTERIAL GENE CONTROL

OPERONS Bacterial cells A group of genes that work together Illustrate how genes expression (“on”) and repression (“off”) can be controlled

OPERON COMPONENTS Regulatory gene: Codes for production of the repressor Operator: Binding site for repressor Promoter: RNA poly binding site Repressor: Binds to operator, blocking RNA poly from binding, preventing transcription (turns gene “off”) Inducer: Molecule that changes repressor’s shape Structural genes: Code for enzyme product

THE LAC OPERON: INDUCIBLE Inducible operon: Cluster of genes that turns a metabolic pathway ON lac operon Allows E. coli to digest lactose

Repressor Operator Genes that code for digestive enzymes Regulatory Gene Promoter Repressor RNA Polymerase DNADNA The regulatory gene codes for production of the repressor The repressor binds to the DNA which prevents RNA polymerase from binding to the promoter, which prevents the digestive enzymes from being made.

Lactose When lactose is present in the environment, the lactose binds to the repressors. What do you notice happens to the repressor when lactose binds to it? Repressor It is now an Inactive Repressor

Repressor Operator Genes that code for digestive enzymes Regulatory Gene Promoter Repressor RNA Polymerase

Repressor The new shape of the repressor does not fit into the repressor binding site. What does this mean?

Genes that code for digestive enzymes Regulatory Gene Promoter Operator Repressor RNA Polymerase Digestive Enzyme

Lactose Digestive Enzyme Digestive Enzyme

Once the digestive enzymes have digested all of the lactose, the repressors return to their normal shape, and go back to the repressor binding site. Lactose Digestive Enzyme Repressor

When lactose is not present, the Repressors go back to their original shape and return to the operator on the DNA strand. The digestive enzymes are no longer made. What factor is responsible for the production of digestive enzymes?

LAC OPERON SUMMARY When lactose is present, the lac operon is “ON” Lactose (inducer) binds to repressor, changing its shape so it can’t bind to operator Enzymes to digest lactose are produced When lactose is absent, the lac operon is “OFF” Lactose is not bound to repressor, so repressor can bind to operator, blocking transcription Enzymes to digest lactose are not needed, so not produced Video animation

THE TRP OPERON: REPRESSIBLE Repressible operon: Cluster of genes that turns a metabolic pathway OFF trp operon Allows E. coli to make amino acid tryptophan

The trp operon produces enzymes that synthesize the amino acid tryptophan The trp operon is the opposite of the Lac operon The repressor is active when tryptophan is present When tryptophan builds up within the cell, the cell de- activates the repressor

Inactive Repressor When tryptophan is absent, the repressor is inactive, and the genes on the operon are transcribed and translated

Inactive Represso r When tryptophan is present, the repressor is active, and transcription is repressed This is an example of a repressible enzyme Active Repressor

TRP OPERON SUMMARY When tryptophan is absent, the trp operon is “ON” Repressor is inactive and cannot bind to operator, so transcription occurs Enzymes needed to produce tryptophan are produced When tryptophan is present, the trp operon is “OFF” Tryptophan (co-repressor) binds to repressor, changing its shape so it CAN bind to operator Enzymes needed to produce tryptophan are NOT made since tryptophan is already present in cell

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