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Microbial Genetics

8. Microbial Genetics. Translation. mRNA is translated in codons (three nucleotides) Translation of mRNA begins at the start codon: AUG Translation ends at a stop codon: UAA, UAG, UGA. PLAY. Animation: Translation. Figure 8.2. Translation. Figure 8.8. Translation. Figure 8.10.

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Microbial Genetics

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  1. 8 Microbial Genetics

  2. Translation • mRNA is translated in codons (three nucleotides) • Translation of mRNA begins at the start codon: AUG • Translation ends at a stop codon: UAA, UAG, UGA PLAY Animation: Translation Figure 8.2

  3. Translation Figure 8.8

  4. Translation Figure 8.10

  5. Regulation of Bacterial Gene Expression • Constitutive enzymes are expressed at a fixed rate. • Other enzymes are expressed only as needed. • Repressible enzymes • Inducible enzymes

  6. Operon PLAY Animation: Operons Figure 8.12, step 1

  7. Regulation of Gene Expression Figure 8.13

  8. Mutation • A change in the genetic material • Mutations may be neutral, beneficial, or harmful. • Mutagen: Agent that causes mutations • Spontaneous mutations: Occur in the absence of a mutagen

  9. Base substitution (point mutation) Missense mutation Change in one base Result in change in amino acid Mutation Figure 8.16a–b

  10. Nonsense mutation Results in a nonsense codon Mutation Figure 8.16a, c

  11. Frameshift mutation Insertion or deletion of one or more nucleotide pairs Mutation Figure 8.16a, d

  12. Mutation • Ionizing radiation (X rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose-phosphate backbone. • Nucleotide excision repairs mutations.

  13. Mutation • UV radiation causes thymine dimers. • Light-repair separates thymine dimers. Figure 8.19

  14. The Frequency of Mutation • Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes • Mutagens increase to 10–5 or 10–3 per replicated gene.

  15. Selection • Positive (direct) selection detects mutant cells because they grow or appear different. • Negative (indirect) selection detects mutant cells because they do not grow. PLAY Animation: Mutations and DNA Repair

  16. Genetic Transfer and Recombination • Vertical gene transfer: Occurs during reproduction between generations of cells. • Horizontal gene transfer: The transfer of genes between cells of the same generation. PLAY Animation: Horizontal Gene Transfer

  17. Transformation Figure 8.23

  18. Recombination Figure 8.24

  19. Conjugation Figure 8.26a

  20. Conjugation Figure 8.26b

  21. Conjugation PLAY Animation: Bacterial Conjugation Figure 8.26c

  22. Transduction Figure 8.27

  23. Plasmids • Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid • Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds • R factors: Encode antibiotic resistance

  24. Plasmids Figure 8.28

  25. Sensitive to DNase? a. b. c.

  26. You have isolated a strain of E. coli that is resistant to penicillin, streptomycin, chloramphenicol, and tetracycline. You also observe that when you mix this strain with cells of E. coli that are sensitive to the four antibiotics, they become resistant to streptomycin, penicillin and chloramphenicol, but remain sensitive to tetracycline. Explain what is going on.

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