Mechanisms of Genetic Variation Flashcards
What is the medical relevance of the fact that mutations occur prior to exposure to the selective agent?
Drug resistance occurs spontaneously
Use of antibiotics in animal feed selects and enriches for drug resistance organisms like Campylobacter
What is photoreactivation?
A single light-inducible enzyme called photolyase recognizes pyrimidine dimers and splits the cyclobutane ring formed ring formed between pyrimidines
Considered error-free pathway because original bases are restored
What is excision repair?
uvrABC exinuclease recognizes mispaired bases and nicks both sides of the damaged strand
DNA pol I fills the empty space and DNA ligase seals the nicks
error-free
What is recombination repair?
Postreplication
One daughter will have a damaged strand and normal strand, the normal strand is used to repair the damaged strand via recombination
Error-proof
What is error-prone repair?
Enzymes involved are induces as part of SOS response (DNA damage)
DNA pol III inserts any bases into the daughter strand across from a dimer or other damage
Advantage is daughter cells receive covalently closed circular chromosomes, albeit carrying a number of mutations
What is RecA?
Protease that is activated by damage to DNA
What is LexA?
Repressor of the SOS regulon
How are the DNA repair systems induced?
RecA is activated via DNA damage, which then cleaves and inactivates the lexA repressor protein
This results in the induction of all the genes in SOS regulon including the recA gene and DNA repair systems
What is homologous (generalized) recombination?
Exchange of genetic information between two genomes with identical or nearly identical sequences
Requires recA and recBCD proteins
What is site-specific recombination?
recA-independent exchange of information between genomes with limited sequence homology, ususally requires specialized recombination enzymes
What is reciprocal recombination?
A crossover event that usually results in two parental and two recombinant type ascospores
No genetic information is lost
What is gene conversion?
Crossover events that result in gain/loss of genetic information as a consequence of the resolution of the heteroduplex region
What is site-specific recombination?
Recombination events that are dependent on transposons and the use of specialized recombination enzymes
E.g. phage lambda integration
What are the consequences of transposition?
Transposons can inactivate genes by inserting into the coding portion or activate quiescent genes by providing a constituitive promoter
What is the difference between a simple transposon and complex transposon?
Simple transposons encode only a transposase
Composite transposons also contain genes for antibiotic resistance
How are complex transposons implicated in antibiotic resistance spread?
These complex transposons can jump from a chromosomal location to a plasmid which can be transferred to other strains by conjugation
What is phase variation?
Antigen switching caused by site-specific inversion of a promoter than controls transcription of the two antigens
E.g. H1/H2 switching in salmonella
What is the host range of a bacteriophage dependent on?
Presence of receptors on host cell wall and presence of specific components of the host replication and transcription apparatus
What is lysogeny?
State of dormancy for the phage
Best equated with a state of dormancy and immunity to superinfection by identical or related phage, not integration of phage into the genome.
What are concatamers?
Many linear unit length genomes that are covalently linked together
Generated during the lytic cycle of a phage to fill new phage capsids
Describe the lysogenic mode of phage infection?
Circular DNA integrates into bacterial chromosome via site-specific recombination
Dormant prophage resides in bacterial chromosome until mutagenesis of the lysogenic bacteria
Breaking the lysogenic state allows the phage to escape compromised host
Why is temperance (lysogeny) significant?
Allows the phage to safely spread as a passenger and escape deleterious agents in the process
What is lysogenic conversion?
Prophage introduces new genes that are expressed in the host and impart novel phenotypes
What are merodiploids?
Partial diploid, required for recombination
Formed after all three types of genetic exchange in bacteria
What is transformation?
The uptake of naked DNA into a cell followed either by recombination with the host genome or self-replication
What is transfection?
Uptake of naked bacteriorphage DNA by cells, followed by the production of phage particles or lysogeny
What is transduction?
Process of tranferring host ccell DNA sequences by bacteriophage to a recipient cell
Two types: generalized and specialized
What is generalized transduction?
During the lytic process, DNA from the first bacteria genome is mistakenly packaged into the phage
The phage then goes on to infect another bacterium, transferring DNA from the first bacterium via generalized recombination
What is specialized transduction?
A prophage excises improperly and an adjacent region of the host genome is attached to the phage chromosome
The phage then transfers this hybrid chromosome to a new host
Only one or a few specific genes can be transferred to the new host
What is conjugation?
Transfer of DNA from a bacterial cell of one mating type to cell of another mating type
What designates the donor or recipient bactieria?
Donor is determined by the presence of a transmissible plasmid called the F factor
Donors are F+ and recipients are F-
What are the functions of the F factor?
Contains for the synthesis of F pilus
Encodes genes that enable its transfer to F- cells, Tra genes
Use host replication machinery
What are the three different forms of F factor that can exist inside the cell?
F+ plasmid - circular plasmid that replicated independently
Hfr chromosome - F plasmid is integrated into the chromosome
F’ plasmid - F plasmid excises imprecisely from Hfr chromosome and takes bacterial genes with it
What are R plasmids?
Closely related to F plasmids and carry genes that encode resistance to antibiotics
