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

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

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

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

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

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

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

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

Replica Plating Figure 8.20

The Ames Test for Chemical Carcinogens Figure 8.21

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

Transformation Figure 8.23

Recombination Figure 8.24

Conjugation Figure 8.26a

Conjugation Figure 8.26b

Conjugation PLAY Animation: Bacterial Conjugation Figure 8.26c

Transduction Figure 8.27

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