Lecture 20: Mutations and Recombination

Quorum Sensing

allows bacteria to communicate with other bacteria in a biofilm

Microbial Genomes

can have many mutations that are an advantage for bacteria

- result of horizontal gene transfer, recombination, and mutagenic repair strategies

Genotype

amino acid sequences

- organism’s genes

Phenotype

physical traits of an organism

- observable characteristics

Wildtype

organism with typical genotype found in nature

Mutant

strain of any cell or virus that differs from the parental strains; DNA level change

- conditional mutant: only see the mutation in certain environmental conditions

Mutations

change in DNA sequence; most are silent, has no effect on organism

- induced, spontaneous, selectable, nonselectable

Induced Mutation

occur due to deliberately manipulating microbes

- ex radiation and chemical agents

Spontaneous Mutations

errors made by DNA polymerase during base pairing; without external intervention

- occurs during DNA replication

Selectable Mutations

give mutant a growth advantage under certain conditions

- ex auxotrophs and antibiotic resistance

Nonselectable Mutations

have neither an advantage nor disadvantage over the parent

- requires examining a large number of colonies

Classes of Mutations

silent, missense, nonsense, frame-shift

- silent, missense, nonsense: substitutions

- frame-shift: insertion/deletion

Silent Mutations

do not change amino acid sequences; have no effect on organism

- 3rd wobble base codon

- substitution; results in same reading frame

Missense Mutation

changes amino acid sequence to another; changes protein function

- substitution; results in same reading frame

Nonsense Mutation

changes amino acid sequence to stop codon

- substitution; results in same reading frame

Frame-Shift Mutation

insertion or deletion of one or two bases; sequence shifts over

Reversion Mutation

reading frame returns to normal phenotype

- genotype remains mutant

- same site: mutation is at same site as original

- second-site: mutation is at different site in DNA

Mutation Rates

as microbes become larger, mutations are less likely to occur

- for microbes with smaller genomes mutations can be beneficial

- RNA viruses: mutations critical for survival; highest mutation rates since RNA polymerase does not have proofreading, ex. the flu

Mutagens

chemical and physical varieties that can cause mutations

Chemical Mutagen

base analogs are incorporated in place of ATCG

- base modifiers modify ATCG via oxidation

Physical Mutagens

electromagnetic radiation

- causes DNA breaks either single stranded or double stranded; must be repaired for DNA to move forward

- UV rays form thymine dimers: thymine pairs with each other instead of adenine

Ames Test

looks for revertant mutants

- uses bacterial strain auxotrophic for histidine

DNA Repair

error-proof repair and error-prone repair

Error-Proof Repair

high fidelity mechanism for non-catastrophic damage

- original sequence is maintained; not causing mutations, outnumber error-prone repair systems

- direct reversal and repair of single-strand damage

Direct Reversal

grabs mutated base and fixes it

Repair of Single-Strand Damage

damaged DNA is remove and repaired using opposite strand as template

Error Prone Repair

low fidelity; original sequence is not maintained, used for catastrophic DNA damage

- repair of double strand damage

Repair of Double Strand Damage

break in the DNA