Mutation

1. Mutation • 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

2. Mutation Change in one base Results in an AA change • Base substitution (point mutation) • Missense mutation Figure 8.17a, b

3. Mutation • Results in a nonsense codon • Nonsense mutation Figure 8.17a, c

4. Mutation • Insertion or deletion of one or more nucleotide pairs • Frameshift mutation Figure 8.17a, d

5. Mutation • A mutagen is any agent that can physically or chemically interact with an organisms DNA. • Chemical mutagens include nucleotide analogues. • 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

6. Mutation • UV radiation causes thymine dimers • Light-repair separates thymine dimers Figure 8.20

7. The Frequency of Mutation • Mutation rate = probability that a mutation will occur during cell division. • 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

8. Selection • Positive (direct) selection detects mutant cells because they grow or appear different. (Antibiotic resistance) • Negative (indirect) selection detects mutant cells because they do not grow.

9. Replica Plating Figure 8.21

10. The Ames Test for Chemical Carcinogens Figure 8.22

11. Genetic Recombination • Exchange of genes between two DNA molecules • Crossing over occurs when two chromosomes break and rejoin Figure 8.23

12. Genetic Transfer and Recombination • In bacterial cells, genetic recombination can occur by transformation, conjugation and transduction. • Vertical gene transfer • Horizontal gene transfer • Occurs during reproduction, between generations of cells • Transfer of genes between cells of the same generation

13. Transformation Figure 8.24

14. Recombination Figure 8.25

15. Conjugation Figure 8.27a

16. Conjugation Figure 8.27b

17. Conjugation Figure 8.27c

18. Transduction Phage protein coat Bacterial chromosome Recombinant Phage DNA and proteins are made, and the bacterial chromosome is broken down into pieces. A phage infects the donor bacterial cell. 1 2 Bacterial DNA Donor bacterial DNA Recipient bacterial DNA Phage DNA Recipient cell Recombinant cell Occasionally during phage assembly, pieces of bacterial DNA are packaged in a phage capsid. Then the donor cell lyses and releases phage particles containing bacterial DNA. 3 A phage carrying bacterial DNA infects a new host cell, the recipient cell. 4 Recombinant can occur, producing a recombinant cell with a genotype different from both the donor and recipient cells. 5 Figure 8.28

19. 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

20. Plasmids Figure 8.29

21. Transposons • Segments of DNA that can move from one region of DNA to another • Contain insertion sequences for cutting and resealing DNA (transposase) • Complex transposons carry other genes Figure 8.30a, b