 Discovered in the bacterium, E. coli  Used as a model for gene regulation  An operon is a set of genes and the switches that control the expression of those genes

 Discovered the operon in the 1940’s  Found two types: inducible (Lac) operon and repressible (tryptophan) operon  The Lac operon is switched off until it is induced to turn on  The tryptophan operon is always in the on position until it is no longer needed

 Lactose is not available to bacteria as an energy source  So, the genes necessary to utilize lactose are not transcribed  In order for E. coli to utilize lactose, three structural genes must be transcribed in order to produce the enzymes necessary for the breakdown of lactose into glucose and galactose

 The three enzymes necessary are b- galactosidase, permease and transacetylase  These enzymes are coded for by thee structural genes in the Lac operon  In order for this transcription to occur, RNA polymerase must bind to DNA at the promoter

 If a repressor binds to the operator, RNA polymerase is prevented from binding to the promoter and transcription of the structural genes is prevented or blocked  The relationship between RNA polymerase and the repressor is an example of noncompetitive inhibition  Both substances are competing for two active sites, one of which blocks the other

 If allolactose, similar to lactose, is present, it acts as an inducer or allosteric effector  It binds to the repressor, causing the repressor to change shape or conformation  Now, the repressor can no longer bind to the operator and RNA polymerase is free to bind to the promoter  Structural genes are now transcribed and lactose is utilized

 Repressible; continuously switched on unless turned off by a corepressor  Consists of five structural genes that code for the enzymes necessary to synthesize the amino acid tryptophan

 The repressor molecule encoded by the regulator gene is initially inactive  RNA polymerase is free to bind to the promoter and transcribe the structural genes, resulting in tryptophan production

 When the inactive repressor combines with a specific corepressor molecule ( tryptophan ), it changes shape and binds to the operator  This prevents RNA polymerase from binding to the promoter and blocks the further production tryptophan  If tryptophan levels are high, no more is needed, so no more is made

 Tryptophan acts as an allosteric effector  This is an example of a negative feedback mechanism

 Prions are not cells and are not viruses  Misfolded versions of a protein normally found in the brain  If prions enter a normal brain, they cause all of the normal versions of the protein to misfold in the same way

 Prions are infectious and cause several brain diseases  Scrapie in sheep  Mad cow disease in cattle  Creutzfeldt-Jakob disease in humans

 Transferring genetic elements sometimes called jumping genes  Discovered by Barbara McClintock  Some transposons jump in a cut-and-paste fashion from one part of the genome to another

 Others make copies of themselves that move to another region of the genome, leaving the original behind  Two types: insertion sequences and complex transposons

 Consist of one gene that codes for transposase, an enzyme that moves the sequence from one place to another  Causes a mutation if it lands within a DNA region that regulates gene expression

 Longer than insertion sequences and include extra genes  Antibiotic resistance or seed color  McClintock hypothesized the existence of transposons when she saw patterns in corn color that made sense only if some genes were mobile