Microbiology Chapter 16

What are mutations

stable, heritable changes in sequences of bases in DNA

What are point mutations

mutations that include a single nucleotide
from alteration
from addition or deletion

What are larger mutations

mutations that include multiple nucleotides

Types of larger mutations

Insertions, deletions, inversions, duplication, and translocation

Insertion

insertion of nucleotides
lyosgenic conversion

What mutations include lysogenic conversions

insertions and deletions

Deletion

removal of nucleotides
includes lysogenic conversion

Inversion

whole part of genome is flipped (UGA to AGU)

Example of inversion mutation

salmonella can change protein to prevent recognition by immune system

Duplication

nucleotide sigement is copied and repeated

Translocation

nucleotide segment is moved to a new location

Spontaneous mutations

changes in DNA that occur without external influence

What are the causes of spontaneous mutations

errors in DNA replication (by DNA polymerase) and mobile genetic elements (Transposons)

Examples of errors in DNA replication spontaneous mutations

tautomerization - alters base pairing (C pairing with A), transition: purine to purine, transversion: purine to pyrimidine
insertion or deletions - generally on A/T repetitive patterns

Examples of mobile genetic elements spontaneous mutations

insertion sequences - simple small transposons
large complex transposons - insertion sequence flanking a resistance gene (Tn10 = IS10 + tetR + IS10)

What are mutations subject to

selective pressure (induced mutations)

What are induced mutations caused by

agents that directly damage DNA (mutagens)

Types of induced mutations

chemical and physical

Examples of chemically induced mutations

base analogs - 5-bromouracil
DNA-modifying agents - nitrogen mustard
intercalating agents - ethidium bromide (EtBr) and Actinomycin D

Actinomycin D

peptide antibiotic that inhibits transcription elongation by RNA polymerase in bacteria and eukaryotes

Examples of physical induced mutations

UV radiation and x-ray

Mutations effect on protein coding genes

leads to observable phenotypes especially in prokaryotes

Wild type

original version

Kinds of Mutations

Forward, Reversion, and Suppresor

Forward mutations

silent, missense, nonsense, and frameshift

Silent mutation

changes in nucleotide does not change the amino acid

Missense mutation

change in nucleotide does change the amino acid

Nonsense mutation

change in nucleotide turns the codon into a stop codon

Frameshift mutation

insertion/deletion of nucleotide, not in multiples of 3, that causes the amino acids downstream to be off by one nucleotide (mutated)

Reversion mutation

a second mutation that changes a forward mutation (base substitution) back into the original phenotype

Suppressor mutation

a second mutation that compensates for the effects of a previous mutation

Intragenic suppressor mutations

both mutations are in the same gene

Extragenic suppressor mutations

mutations are in different genes
ex. a mutation on the codon and a mutation on the anticodon cancel each other out

What fixes mutations

DNA polymerase

What can increase mutations

mutagens (UV light or actinomycin D)

How to find mutants

use sensitive detection methods and/or methods to increase frequency

What is used to study cellular mechanisms

mutations

What is an auxotroph

a conditional mutant

What is a phototroph

wild type strain

What is the ames test used for

test whether a chemical is a mutagen

What is the ames test

starts with an auxotroph and then add a chemical compound to see if it causes mutations
if there’s a mutation then it may revert back to the phototroph

If a mutagen is identified then…

further study is needed to see if it is a carcinogen

What does proofreading

DNA polymerase

Why is DNA polymerase not allowed to make mistakes

since it would affect multiple proteins

Why is RNA polymerase allowed to make more mistakes

because it will only affect one protein

Base excision repair

removes only the damaged base

Nucleotide excision repair

removes a short stretch of nucleotides

What allows it to differentiate between old and new DNA

hemimethylation

What does the parent strand of DNA have

methylation

When is the SOS response activated

to repair extensive DNA damage

When are the SOS genes transcribed/translated

under stress

SOS response proteins

LexA and RecA

LexA

a repressor that inhibits SOS genes by binding to the promoter

What does the LexA protein do

prevents RNA polymerase form binding to the promoter inhibiting transcription

What activates RecA

DNA damage

What does RecA do

destroy LexA so that SOS genes can be activated and it initiates recombination repair

What do phages recognize

RecA so that it can remove itself (lysogenic to lytic)

What kind of sexual reproduction do eukaryotes do

vertical gene transfer

What kind of asexual reproduction do prokaryotes do

horizontal gene transfer (HGT)

Types of HGT

transformation, transduction, and conjugation

Bacterial transformation

requires bacteria to be competent
similar to T4P and T2SS
single stranded DNA
only keeps necessary information

Bacterial transduction

requres a phage
results from mistakes in packaging host DNA
phages may accidentally carry host DNA instead of its own
continuation of phage life cycle depends on whether it gained mostly host or phage information

Bacterial conjugation

involves a plasmid and sex pili
requires cell to cell contact

Bacterial Plasmids

small, autonomously replicating DNA molecules that are independent from the host chromosome

What is an episome

a plasmid that can integrate reversibly into the host chromosome

Conjugative plasmids

plays a role in conjugation between the same species

What encodes the genes to form sex pilus and T4SS

F factor

What allows for conjugation in plasmids

F factor

What bacteria have an F plasmid

E. coli

What does an insertion sequence do

allow for insertion, addition of the plasmid into the chromosome

What is the end resulr of F+ and F- mating

recipient becomes F+

How does the F+ transfer a copy of the plasmid

via rolling circle replication

What is transferred in F+ and F- mating

one strand of DNA
NOT donor genes

What is a Hfr cell

a bacterial cell with the F factor integrated into the chromosome

What is transferred during Hfr Conjugation

Donor genes
allows it to make sex pilus and T4SS
Plasmid is incorporated

Hfr conjugation result

end is F- because it only has a part of the plasmid

During Hfr conjugation

a complete copy of the F factor is not transferred because it is in the middle

Where can resistance genes be found on

bacterial chromosome mutation, R plasmids, composite transposons, and other mobile genetic elements