2. AP Biology D.N.A A student uses restriction enzymes to cut a DNA molecule into fragments. The digested DNA is loaded into the wells of an agarose gel and the gel is subjected to an electric current. Upon completion of the run, the gel is stained. 1. The rate of migration of the DNA fragments through the agarose gel is determined by the (A)ratio of adenine to cytosine in the fragment (B)presence of hydrogen bonds between base pairs (C)length of time the electrophoresis unit is allowed to operate (D) number of nucleotides in the fragment (E)volume of the starting sample 2. The type and density of the gel are important because (A)they influence the rate of migration of the fragments (B)they may cause some DNA molecules to replicate (C)some DNA nucleotides may be lost due to chemical reactions with the gel (D)some DNA molecules may sink to the bottom and not migrate (E)some DNA molecules may cross-link 3. The procedures described can be used to do all of the following EXCEPT (A)isolate and purify certain DNA fragments (B)synthesize novel DNA molecules (C)study the activity of restriction enzymes (D)calculate the size of DNA fragments (E)identify the source of DNA material

3. AP Biology ANNOUNCEMENTS Unit 8 Molecular Genetics Exam is on Thursday, May 8 Review Session will be held on Wednesday from 2:45-3:45 – please come with questions GOAL AVERAGE: 70%

4. AP Biology 2007-2008 Control of Prokaryotic (Bacterial) Genes

5. AP Biology Bacterial metabolism Bacteria need to respond quickly to changes in their environment  if they have enough of a product, need to stop production why? waste of energy to produce more how? stop production of enzymes for synthesis  if they find new food/energy source, need to utilize it quickly why? metabolism, growth, reproduction how? start production of enzymes for digestion STOP GO

6. AP Biology Regulating Metabolism? Feedback inhibition  product acts as an allosteric inhibitor of 1 st enzyme in tryptophan pathway  but this is wasteful production of enzymes = inhibition - -

7. AP Biology Different way to Regulate Metabolism Gene regulation  instead of blocking enzyme function, block transcription of genes for all enzymes in tryptophan pathway saves energy by not wasting it on unnecessary protein synthesis = inhibition - Now, that’s a good idea from a lowly bacterium! - -

8. AP Biology Gene regulation in bacteria Cells vary amount of specific enzymes by regulating gene transcription  turn genes on or turn genes off turn genes OFF example if bacterium has enough tryptophan then it doesn’t need to make enzymes used to build tryptophan turn genes ON example if bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose STOP GO

9. AP Biology Bacteria group genes together Operon  genes grouped together with related functions example: all enzymes in a metabolic pathway  promoter = RNA polymerase binding site single promoter controls transcription of all genes in operon transcribed as one unit & a single mRNA is made  operator = DNA binding site of repressor protein

10. AP Biology So how can these genes be turned off? Repressor protein  binds to DNA at operator site  blocking RNA polymerase  blocks transcription

11. AP Biology operatorpromoter Operon model DNATATA RNA polymerase repressor = repressor protein Operon: operator, promoter & genes they control serve as a model for gene regulation gene1gene2gene3gene4 RNA polymerase Repressor protein turns off gene by blocking RNA polymerase binding site. 1234 mRNA enzyme1enzyme2enzyme3enzyme4

12. AP Biology mRNA enzyme1enzyme2enzyme3enzyme4 operatorpromoter Repressible operon: tryptophan DNATATA RNA polymerase tryptophan repressor repressor protein repressor tryptophan – repressor protein complex Synthesis pathway model When excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNA  blocks (represses) transcription gene1gene2gene3gene4 conformational change in repressor protein! 1234 repressor trp RNA polymerase trp

13. AP Biology mRNA enzyme1enzyme2enzyme3enzyme4 operatorpromoter Inducible operon: lactose DNATATA RNA polymerase repressor repressor protein repressor lactose – repressor protein complex lactose lac repressor gene1gene2gene3gene4 Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA  induces transcription RNA polymerase 1234 lac conformational change in repressor protein! lac

14. AP Biology Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymes Lactose is allosteric regulator of repressor protein

15. AP Biology Jacob & Monod: lac Operon Francois Jacob & Jacques Monod  first to describe operon system  coined the phrase “operon” 1961 | 1965 Francois JacobJacques Monod

16. AP Biology Operon summary Repressible operon  usually functions in anabolic pathways synthesizing end products  when end product is present in excess, cell allocates resources to other uses Inducible operon  usually functions in catabolic pathways, digesting nutrients to simpler molecules  produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do, cell allocates resources to other uses