Comparison of Genetic Material and Replication for Eukaryotes and Prokaryotes

1. Comparison of Genetic Material and Replication for Eukaryotes and Prokaryotes

2. Comparison of Transcription for Eukaryotes and Prokaryotes

3. Regulation of Gene Expression • Enzymes are common feature of biochemical pathways • Constitutive enzymes (60-80%) • Inducible enzymes • Default position off • Repressible enzymes • Default position on

4. Operon model of gene expression • Regulatory gene, operator, promoter and series of structural genes • divided into three regions: • Regulatory gene – codes for regulatory protein • Control region - operator and promoter • Structural genes - genes being transcribed

5. Operator Gene 1 Gene 2 Gene 3 Promoter Operon structure Control region Regulatory gene Regulatory gene – DNA sequence for repressor protein Promoter – Binding site for RNA polymerase Operator – binding site for the repressor protein Structural Genes – DNA sequence for proteins of interest

6. Operon controlled by regulatory region • Protein acts as “on/off” switch • Can act as repressor or inducer • Operon model based on studies of induction of the enzymes of lactose catabolism on E. coli

7. Inducible enzyme Default position is off Enzymes not made until needed

8. Catabolite Repression • glucose represses enzymes for lactose degradation • Low glucose levels corresponds to high cAMP • cAMP binds to catabolite activating protein (CAP) • alarmone • CAP binds to promoter and induces RNA polymerase to bind

11. E.coli grows on either substrate 2-step diauxic growth caused by catabolite repression

12. Repressible enzyme Default position is on Enzymes made until no longer needed

13. Operons rare in eukaryotes • Function differently • Eukaryotes utilize transcription factors or alternate splicing of exons • Expression may be regulated at translation level • Unsure of regulation of expression in archaea • May be more similar to eukaryotes than bacteria

14. Many microbes adapt to changing environments by altering level of gene expression • Global Regulatory Systems • Signal transduction • Transmits information from external environment to inside cell • Allows cell to respond to environmental changes

15. Two-component regulatory systems • Sensors recognize change in environment • Kinase protein in membrane • Response regulators activate or repress gene expression • DNA binding protein

16. Quorum sensing • Based on density of cell population • Activation of genes beneficial only when produced by multiple cells • Vibrio fisheri • Biofilm formation

17. Natural selection • Antigenic variation • Alteration in characteristics of certain surface proteins • Ex. Neisseria gonorrhoeae varies pilin gene at expression locus

18. Regulation may occur at the translation level • Riboswitches • Antisense RNA

19. Bacterial Genetics and Genetic Transfers

20. Genetic Diversity • Eukaryotes - sexual reproduction • Gametes have various genetic combinations • Prokaryotes - asexual reproduction • All offspring are clones of parent cell • No genetic variation

21. Diversity in Bacteria • Bacterial mechanisms for genetic diversity • Mutation • Gene transfer

22. Mutations • Change in genotype • Wild type vs. mutant • May or may not cause phenotypic changes • silent, beneficial, or harmful • Passed vertically to all offspring • Selective pressure can lead to evolution through natural selection

23. Types of Mutations • Change in one base • Results in change of amino acid • Point Mutation (base substitution) • Missense

24. Results in a stop codon • Nonsense

25. Insertion or deletion of one or more bases • Frame-shift mutation

26. Mutagen • Agent that induces mutations • Physical or chemical agents • Spontaneous mutations • Occur in the absence of a mutagen • May be due to error or transposons

27. Transposable Elements (Transposons) • May disrupt proper gene function • Contain insertion sequences (transposase) • Complex (composite) transposons carry other genes

28. Nucleotide excision repair • Endonuclease, DNA ligase & DNA Polymerase • Light repair • Direct repair • Photoactivation of enzymes (photolyase)

29. Induced Mutations • Mutations are essential for understanding genetics • Intentionally produced (induced) to demonstrate function of particular gene or set of genes • Mutations can be induced via • Chemical mutagens • Transposition • Radiation

30. Ames Test • Mutational reversion assay • Tests mutagenicity of compounds • Utilizes a histidine auxotroph

31. Mutations followed by selection may produce microbes with desirable traits • Positive (direct) selection detects mutant cells because they grow or appear different • Ex. Penicillin resistant mutants growing on penicillin containing agar – non mutants will not grow • Eliminates wild type

33. Negative (indirect) selection detects mutant cells because they do not grow • Replica plating to isolate mutants requiring a specific growth factor – auxotroph • Selects for wild type

34. Replica Plating Figure 8.21