Prokaryotic Gene Control: Operons

Prokaryotic Gene ControlAP Biology Chapter 18Learning Target 1I can describe how genetic variation occurs in bacteria.

Recall: Bacterial StructureProkaryotic cellsLack nucleiLack membrane-bound organelles (have ribosomes for protein synthesis)May have flagella for movementMay have capsule around cell wallVarious shapes, arrangements

Bacterial GeneticsSingle, circular bacterial chromosome in nucleoid regionOften contain one or multiple plasmidsBonus genes independent of bacterial chromosomeCode for traits such as antibiotic resistance, fertility factors

Bacterial VariationEven though bacteria are asexual, they are not clones!Variation introduced by:MutationsChromosome and/or plasmid replicationTransformationTransductionConjugationTransposable elements/transposons

5MutationsChanges in DNA during replicationGrow and divide FAST, so errors happen and get passed on quicklyEx: drug resistant bacteria

TransformationAlteration of a bacterial cells genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environmentEx: harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells

7TransductionPhages carry bacterial genes from one host cell to anotherGeneralized transduction random genes are transferredSpecialized transduction genes adjacent to prophage site in temperate phages

8ConjugationThe direct transfer of genetic material between cellsBacterial sex

9TransposonsTransposable genetic elementsjumping genesPieces of DNA that move around within the genome

Learning Target 2I can describe how bacterial cells use the operon system to regulate gene expression.

Prokaryotic MetabolismProkaryotes must use substances and synthesize macromolecules fast enough to meet their needsIf enzymes arent needed, genes for their production must be offConservation of resources

OperonsGenes grouped together operating together in prokaryotic cellsCan be inducible Normally off but able to turn on when neededCan be repressible Normally on but able to turn off when not neededCoordinated control of genes required for metabolismLearning Target 3I can identify and explain the functions of the following structures with regard to the operon system.

Important Terms to KnowPromoterRegion of DNA that signals for RNA polymerase to bindOperatorRegion of chromosome to which the repressor binds when the operon is turned offRepressorA protein that suppresses the transcription of a geneRegulatory geneCodes for production of the repressor proteinStructural geneCodes for enzyme productInducerMolecule that binds to and changes repressors shape

Learning Target 4I can use the trp operon as an example to describe a repressible operon and explain how this system works.

Example: trp OperonRepressibleGenes are normally on and produce tryptophan for the cellIf tryptophan is present, genes are turned off and tryptophan in the environment is utilized

Normal ON PositionCell requires tryptophan for metabolismTypically has to make it for itselfTo do this:Repressor is inactive and not bound to the operatorPromoter is available to RNA polymeraseStructural gene is transcribedmRNA is translated into enzymesEnzymes produce tryptophan

ON PositionNo tryptophan present in environmentCell makes its own tryptophan-producing enzymesIf trp is Abundant: Turning OFFIf the cell has adequate supply of tryptophan, it turns the operon OFF and uses what is availableSave resources dont make it if its already there!To do this:Tryptophan binds to the repressor, activating itActivated repressor binds to the operatorThis blocks RNA polymerase from transcribing mRNANo genes are transcribed no proteins produced

OFF PositionTryptophan is present in environmentNo need to produce tryptophan producing enzymes

VideoMcGraw HillExample: lac OperonInducibleGenes are normally off in absence of lactose If lactose is present, genes are turned on and enzymes to digest it are produced

Normal OFF PositionIf no lactose if present in the cell, no lactose-digesting enzymes are producedSave resources dont make it if its not needed!To do this:Active repressor binds to operatorThis blocks RNA polymerase from transcribing mRNANo genes are transcribed no proteins produced

OFF PositionNo lactose in environmentNo need to produce lactose-digesting enzymes

If lac is Present: Turning ONLactose is not normally presentIf it is, bacteria will digest and use itTo do this:Allolactose inducer binds to repressor, inactivating itInactivated repressor cannot bind to operatorRNA polymerase can bind to promoterGenes coding for lactose-digesting enzymes transcribedmRNA is translated into enzymes that digest lactoseWhen lactose supply is depleted, inducer separates from repressor, allowing it to bind to operator again and block transcription

ON PositionLactose is present in environmentEnzymes to digest it are produced

VideoMcGraw Hill