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Microbial Genetics

8. Microbial Genetics. 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. Base substitution ( point mutation ) - Change in one base

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Microbial Genetics

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  1. 8 Microbial Genetics

  2. 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

  3. Base substitution (point mutation) - Change in one base Can be a missense or nonsense mutation. Missense mutation - Result in change in amino acid Mutation Figure 8.16a–b

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

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

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

  7. Mutation • UV radiation causes thymine dimers. • Light-repair enzymes separate thymine dimers. Figure 8.19

  8. The Frequency of Mutation • Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes • Mutagens increase to 10–5 to 10–3 per replicated gene. • 10 to 1000 more times likely to occur

  9. Selection • Positive (direct) selection detects mutant cells because they grow or appear different. • Ex. Looking for penicillin resistance on penicillin containing agar • Negative (indirect) selection detects mutant cells because they do not grow. • Use of replica plating can be used for ID PLAY Animation: Mutations and DNA Repair

  10. Replica Plating Figure 8.20

  11. The Ames Test for Chemical Carcinogens Figure 8.21

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

  13. Transformation Figure 8.23

  14. Recombination Figure 8.24

  15. Conjugation Figure 8.26a

  16. Conjugation Figure 8.26b

  17. Conjugation PLAY Animation: Bacterial Conjugation Figure 8.26c

  18. Transduction Figure 8.27

  19. Plasmids • Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid (F factor plasmid in E. coli) • Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds, sometimes toxins • R (resistance) factors: Encode antibiotic resistance

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