Microbiology chapter 8: Mutations and HGT

Addition and deletion mutations

-1 or 2 nucleotides: frameshift, -3 nucleotides: adds or deletes one amino acid

SOS repair

-A global, last-ditch repair mechanism following extensive DNA damage, -Recombination and an error-prone DNA polymerase "something is better than stalling," -Increases mutation rate itself

Haploid

-Bacteria are this, -only one copy of gene, no backup

Transduction

-Bacterial DNA transfer by bacteriophages, -Generalized: any genes of donor cell, -Specialized: only genes adjacent to integrated phage DNA are transferred

Chemical mutagens

-Can modify or resemble DNA bases, often results in base pairing mistakes, -base substitutions more common in aerobic environments because Reactive Oxygen Species can damage DNA, -Alkylating agents: increases the chance that an incorrect nucleotide will be incorporated during DNA replication -nitrous acid: -base analogs: resemble nucleobases but have different hydrogen-bonding properties. The analogs can be mistakenly used in place of the nucleobases when nucleotides are made

Conjugation

-Direct DNA transfer between bacterial cells, -Requires contact between donor and recipient cells, -Conjugative plasmids direct their own transfer, -plasmid transfer and chromosome transfer

Specialized transduction

-Due to mistakes during excision of a temperate phage, -Some flanking bacterial DNA is taken along with phage DNA, -Infect new host, -New DNA may integrate via homologous recombination

Generalized transduction

-Due to packaging error during phage assembly, -Host DNA fragments are mistakenly packaged into some phage heads, -Transducing particle: has bacterial DNA, -Can infect new host, -New DNA may integrate via homologous recombination

Plasmid transfer in E.coli

-F. plasmid is a conjugative plasmid, it encodes proteins that promote transfer. 1. F Pilus contacts recipient cell 2. F Pilus retracts and pulls cells together 3. Strand of F plasmid transferred and complement is synthesized

Transposons: jumping genes

-Jumping genes, -Move from one location in the genome to another, -Can contain additional genes (ex. antibiotic resistance or toxins), -Often will inactivate gene into which it inserts -Cannot replicate independently, -Encode transposase that "moves" the DNA to a new location: "cut and paste"

DNA repair mechanisms

-Mismatch repair, -Photoreactivation, -Excision repair, -Glycosylase, -SOS repair

DNA repair

-Mistakes often repaired before progeny, -1st line of defense: Polymerase proofreading, -2nd: Repair mechanisms

Spontaneous mutationd

-Occur during normal cell processes, -Random and infrequent, but at characteristic rates, -Mutation passed to daughter cells, -Occasionally changed back with reversion, -Environment provides selective pressure and only some cells are able to survive and reproduce

Excision repair

-System that cuts out defective pieces of DNA strand, including mutations, and replaces it with a new corrected piece of DNA, -Repairs distorted DNA, -Gycosylase finds and removes oxidized guanine

Mismatch repair

-The cellular process that uses specific enzymes to remove and replace incorrectly paired nucleotides. -Repairs Base substitution

Radiation as a mutation

-UV: induces thymine dimer, -X rays cause breaks in DNA backbone and can alter bases

Plasmid

-circular, double stranded replicon, -Found in most Bacteria, Archaea, and some Eucarya, -Generally nonessential; cells can be "cured," -Few to 1000s of genes, -Narrow or broad host range

Transformation

-naked DNA uptake by bacteria, -Recipient cell must be competent or capable of DNA uptake -Competence is a tightly regulated process, -Most bacterial cells take up DNA regardless of origin however some accept DNA only from closely related bacteria, -Naked DNA must recombine with the host chromosome or plasmid with OoR to be maintained

2 types of mutation

1. Base substitution mutations, 2. Addition/Deletion: frameshift mutation

Griffith's experiment: transformation of pneumococci

1. Capsule needed for pathogenesis 2. Only living cells are pathogenic 3. Deal encapsulated cells transformed the living non-encapsulated cells in pathogenic encapsulated cells

Conjugative chromosome transfer

1. Hfr cell produces F pilus, 2. Part of chromosome is transferred to recipient cell (single strand), 3. Cells separate, DNA transfer is interrupted, chromosome breaks, 4. DNA is integrated via homologous recombination. Recipient cell remains F- due to incomplete transfer

Sources of genetic change in bacteria

1. Mutation, 2. Horizontal gene transfer

Transformation process

1. dsDNA binds the cell, 2. Single strand enters, other strand degraded, 3. Strand integrates by homologous recombination, strand it replaces degrades, 4. DNA replicates, modified DNA survives

Intercalating Agents

Chemical mutagen that inserts into the DNA strand, can result in frameshifting

Nonsense mutation

Codes for a stop codon

Missense mutation

Codes for different amino acid

Silent mutation

Codes for same (wild type) amino acid

Replicon

DNA that will be replicated

Knockout/null mutation

Gene is inactivated

Base substitution mutation

Incorrect nucleotide/s incorporated during DNA synthesis, -Silent, missense, or nonsense

Homologous recombination

Integration of ssDNA into the chromosome at sites of homology

Maintaining transferred DNA

Must have its own origin or replication or by integrates into the chromosome

Indirect selection

Mutant can not grow in selective media conditions

Direct selection

Mutant grows in selective media conditions

Mutation rate

Probability of a mutation with each cell division, typically between 10^-4 and 10^-12 for a given gene

Point mutation

Single base pair change

Photoreactivation

Using the energy of light to break the covalent bonds joining thymine dimers, thereby restoring the DNA to its original state

Induced mutation

When an agents, or mutagen, damages or changes DNA increasing the mutation rate