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 does transformation occur?
Donor cells lyse → DNA in environment
-DNA outside of cell = ‘‘naked’’ DNA
Recipient cells take up nake DNA
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 are the types of transposition?
Conservative
- transposon moved to new DNA molecule
- absent from donor DNA, present in recipient DNA
Replicative
- transposon is replicated AND moved
- present in donor and recipient DNA
