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Microbial Genetics

7. Microbial Genetics. Stages of Translation. Three stages Initiation Elongation Termination All stages require additional protein factors Initiation and elongation require energy (GTP). Animation: Translation. PLAY. Initiation. Figure 7.15. Elongation. Figure 7.16.1. Elongation.

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Microbial Genetics

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  1. 7 Microbial Genetics

  2. Stages of Translation • Three stages • Initiation • Elongation • Termination • All stages require additional protein factors • Initiation and elongation require energy (GTP) Animation: Translation PLAY

  3. Initiation Figure 7.15

  4. Elongation Figure 7.16.1

  5. Elongation Figure 7.16.2

  6. Elongation Figure 7.16.3

  7. Elongation Figure 7.16.4

  8. Elongation Figure 7.16.5

  9. Elongation Figure 7.16.6

  10. Polyribosome Figure 7.17a

  11. Termination • Release factors somehow recognize stop codons and modify ribosome to activate ribozymes which sever polypeptide from final tRNA • Ribosome dissociates into subunits • Polypeptides released at termination may function alone or together

  12. Translation in Eukaryotes • Initiation occurs when ribosomal subunit binds to 5′ guanine cap • First amino acid is methionine

  13. Regulation of Genetic Expression • 75% of genes are expressed at all times • Other genes are regulated so they are only transcribed and translated when cell needs them • Allows cell to conserve energy • Regulation of protein synthesis • Typically halt transcription • Can stop translation directly

  14. Control of Translation • Regulation of genetic expression can be at the level of translation • Riboswitch • mRNA molecule that blocks ribosomes and translation of the polypeptide they encode • Short interference RNA (siRNA) • RNA molecule complementary to a portion of mRNA, tRNA, or a gene that binds and renders the target inactive

  15. The Operon Figure 7.18

  16. Operons • Inducible operons – must be activated by inducers • Lactose Operon • Repressible operons – transcribed continually until deactivated by repressors • Tryptophan Operon Animation: Operons PLAY

  17. The Lactose Operon Figure 7.19a

  18. The Lactose Operon Figure 7.19b

  19. The Tryptophan Operon Figure 7.20a

  20. The Tryptophan Operon Figure 7.20b

  21. Mutations of Genes • Mutation – change in the nucleotide base sequence of a genome; rare • Almost always deleterious • Rarely lead to a protein having a novel property that improves ability of organism and its descendents to survive and reproduce Animation: Mutations and DNA Repair PLAY

  22. Mutations of Genes • Types • Point mutations (most common) – one base pair is affected • Insertions, deletions, and substitutions • Frameshift mutations – nucleotide triplets after the mutation displaced • Insertions and deletions

  23. Effects of Mutation Figure 7.21a-c

  24. Effects of Mutation Figure 7.21d-e

  25. Mutagens • Radiation • Ionizing radiation – induces breaks in chromosomes • Nonionizing radiation – induces pyrimidine dimers • Chemical mutagens • Nucleotide analogs – disrupt DNA and RNA replication and cause point mutations • Nucleotide-altering chemicals – result in base-pair substitution mutations and missense mutations • Frameshift mutagens – result in nonsense mutations

  26. DNA Repair Figure 7.25a-b

  27. DNA Repair Figure 7.25c-d

  28. Identifying Mutants, Mutagens, and Carcinogens • Mutants – descendents of cell that does not successfully repair a mutation • Wild types – cells normally found in nature • Methods to recognize mutants • Positive selection • Negative (indirect) selection • Ames test

  29. Genetic Recombination and Transfer • Exchange of nucleotide sequences often mediated by DNA segments composed of homologous sequences • Recombinants – cells with DNA molecules that contain new nucleotide sequences • Vertical gene transfer – organisms replicate their genomes and provide copies to descendants

  30. Genetic Recombination and Transfer • Horizontal gene transfer – donor contributes part of genome to recipient; three types • Transformation • Transduction • Bacterial Conjugation Animation: Horizontal Gene Transfer PLAY

  31. Griffith’s Experiments Figure 7.30

  32. Transformation • Transforming agent was DNA; one of conclusive pieces of proof that DNA is genetic material • Cells that take up DNA are competent; results from alterations in cell wall and cytoplasmic membrane that allow DNA to enter cell

  33. Transduction Figure 7.31

  34. Transduction • Generalized transduction – transducing phage carries random DNA segment from donor to recipient • Specialized transduction – only certain donor DNA sequences are transferred

  35. Bacterial Conjugation Animation: Bacterial Conjugation PLAY Figure 7.32a

  36. Bacterial Conjugation Figure 7.32b

  37. Bacterial Conjugation Figure 7.33

  38. Transposons and Transposition • Transposons – segments of DNA that move from one location to another in the same or different molecule • Result is a kind of frameshift insertion (transpositions) • Transposons all contain palindromic sequences at each end

  39. Transposons and Transposition • Simplest transposons are insertion sequences which have no more than two inverted repeats and gene for transposase • Complex transposons contain one or more genes not connected with transposition (e.g. antibiotic resistance) Animation: Transposons PLAY

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