Regulation of genes

1. Regulation of genes

2. The Operon • 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

3. Jacob and Monod • 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

4. The Lac Operon • 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

5. The Lac Operon • 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

6. The Lac Operon • 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

7. The Lac Operon • If allolactose, similar to lactose, is present, it acts as an inducer or allostericeffector • 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

8. Repressed Lac Operon

9. Tryptophan Operon • Repressible; continuously switched on unless turned off by a corepressor • Consists of fivestructural genes that code for the enzymes necessary to synthesize the amino acid tryptophan

10. Tryptophan Operon • 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

11. Tryptophan Operon • 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

12. Tryptophan Operon • Tryptophan acts as an allostericeffector • This is an example of a negative feedback mechanism

14. Prions • 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

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

16. Transposons • 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

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

18. Insertion Sequences • 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

19. Complex Transposons • 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