Mutations

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

Mutation Mutations Transformation Transduction Conjugation Jumping Genes

Mutation Base substitution (point mutation, Neutral mutation) Missense mutation Change in one base Result in possible change in amino acid Figure 8.17a, b

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

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

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

Mutation UV radiation causes thymine dimers Light-repair enzymes separates thymine dimers through photoreactivation SOS repair for extremely damaged DNA (this process is error proned). Figure 8.20

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

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

Induced Mutations caused by chemical or physical agents that damage or alter the chemistry of DNA, or that interfere with DNA repair mechanisms

DNA-damaging agents

Replica Plating Figure 8.21

The Ames Test for Chemical Carcinogens Figure 8.22

Genetic Transfer and Recombination Vertical gene transfer Horizontal gene transfer Occurs during reproduction, between generations of cells Transfer of genes between cells of the same generation

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

Recombination Figure 8.25

Transformation Figure 8.24

Transformation nuclease – nicks and degrades one DNA binding strand protein nuclease – nicks and degrades one strand Mechanism of transformation in S pneumoniae. A long double stranded DNA binds cell surface with the aid of DNA-binding proteins and is nicked by nuclease. One strand is degraded by nuclease. The undegraded strand associates with a competence-specific protein. The single strand enters the cell and is integrated into the host chromosome at a homologous site in the host DNA. competence-specific protein Figure 13.17 Streptococcus pneumoniae

Recipient bacterial DNA Transduction Phage protein coat Bacterial chromosome Recombinant 1 A phage infects the donor bacterial cell. 2 Phage DNA and proteins are made, and the bacterial chromosome is broken down into pieces. Bacterial DNA Donor bacterial DNA Recipient bacterial DNA Phage DNA Recipient cell Recombinant cell 3 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. 4 A phage carrying bacterial DNA infects a new host cell, the recipient cell. 5 Recombinant can occur, producing a recombinant cell with a genotype different from both the donor and recipient cells. Figure 8.28

Transduction Figure 13.20

Specialized Transduction also called restricted transduction carried out only by temperate phages that have established lysogeny only specific portion of bacterial genome is transferred occurs when prophage is incorrectly excised

Specialized transduction Lysogeny Figure 13.20

Conjugation Figure 8.27a

Conjugation Figure 8.27b

Conjugation Figure 8.27c

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

Plasmids Figure 8.29

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