Lecture 9 Bacterial Genetics Flashcards
How do new Genotypes occur in microbes?
Through
Mutation
- heritable change in DNA
- SOURCE of new genotypes
Gene Transfer
- DNA exchange between cells
- SPREAD of new genotypes
What is wildtype and mutant?
Wild type = normal
Mutant=changed
What is a phenotype mutation?
Change in observable properties of the organism
What are the different types of mutations?
Point Mutations
- single nucleotide change
- substituion
- same length
Frame Shift Mutations
- Insertion OR Deletion of nucleotides
- Also known as Indel
- Most Dangerous
- lengthened or shortened
What are the types of point mutations?
Transition
- Purine to Purine A⇔G
- Pyrimidine to Pyrimidine C⇔T
Transversion
- Purine to Pyrimidine A/G⇔C/T
- Pyrimidine to Purine C/T⇔AG
Reversion
-change from mutant to wild type
What are the types of effects of mutations?
Silent
-mutation results in same amino acid
Nonsense
-mutation inserts premature stop codon
Missense
-Mutation causes different amino acid
What are the types of Frame Shift Mutations?
Insertions
-Adding nucleotides
Deletions
-Removing nucleotides
Both shift the reading frame by adding/removing nucleotides
What are the sources of mutations?
Spontaneous Mutations
- due to errors in DNA replication
- DNA Polymerase not 100% accurate
Induced Mutations
-Due to external agents, ie mutagens
How often do spontaneous mutations occur?
10-9 /bp
Lethal mutations balanced by adaptive mutations
Vriruses have a higher frequency of mutations
What are the types of mutagens?
Physical, Chemical, and Biological
What are the types of physical mutagens?
UV
-pyrmidine dimers
Ionizing
-breaks DNA strand
What are the types of chemical mutagens?
Nucleotide base analogs
- resemble nucleotide but not, used by DNA polymerase by mistake
- causes point mutation
Chemical modifiers
- change structure of nucleotides
- cause point mutation
Intercalating agents
- chemical inserted between DNA base pairs
- cause frameshift mutation
Sometimes mutagenisis causes carcinogenesis. How do we test for chemicals that mutagenic?
The Ames Test
- start with bacteria we’ve mutated with single substitution
- place in low nutrient media
- add mutagen
- if mutagenic then we will see growth b/c some of the mutations caused by mutagen will be reversions and the mutated bacteria will go back to wild type
When using the Ames test, how can you tell if you have spontaneous or induced mutations from the potential mutagen?
Spontaneous Mutations
-Few colonies b/c non-mutagen
Induced Mutations
-Many colonies b/c mutagen
How does gene transfer occur and what are the possible outcomes?
Horizontal Gene transfer between source and receiver
- Rejected - degraded by recipient cell
- Accepted - replicates independently, eg plasmid
- Accepted - inserted in recipient chromosome
How is DNA inserted into the chromosome?
Homologous recombination between Donor DNA and Recipient DNA
- Homologous = similar DNA sequences
- crossing over like in meiosis
What are the different types of gene transfer?
Transformation
-naked DNA uptake from the environment
Transduction
-virus-mediated, virus injects bacterial DNA into host cell
Conjugation
-direct passage of DNA from one cell to antoher
How does transformation occur?
Donor cells lyse → DNA in environment
-DNA outside of cell = ‘‘naked’’ DNA
Recipient cells take up nake DNA
What kinds of cells can undergo transformation gene transfer?
Competent cells able to take up naked DNA fragments
-not all bacterial cells are competent
How does bacteria choose the right naked DNA to uptake from environment for transformation gene transfer?
Induced competency
-quorum sensing causes bacteria to take up DNA
Binding sequence
- must be present to uptake DNA
- recognition sequence in DNA must be present to be taken up
What is transduction?
Form of gene transfer where virus transfers DNA between cells
- viruses that infect bacteria=bacteriophage
- bacteriophages inject their genome into host cells
- normal phage destroys host cells
- in transduction the bacteriophage injects it’s genome that contains some of last bacterial host into new host
- New host cell intact w/ new DNA
What is conjugation?
Form of gene transfer that involves cell-to-cell contact
- direct gene transfer
- bacterial sex
- plasmids and chromosomes are exchanged
What is machinery is involved in conjugation?
F Plasmid
-genes for replication and transfer functions
Sex Pilus
- contacts recipient cell
- retracts to bring cells together
What does the F plasmid do?
single strand of plasmid enters recipient cell and synthesizes complementary stran
- gives recipient cell ability to conduct horizontal gene transfer
- F plasmid integrates into chromosome via recombination
