Mechanisms of Genetic Variation and Bacteriophage and Genetic Exchange Flashcards
Mutations occur as a consequence of (3):
- Errors in the replication process.
- Direct damage to the genetic material by light or chemical agents.
- Errors made by the mutation repair systems.
DNA repair mechanisms (4):
- Photoreactivation
- Excision repair
- Recombinational repair
- Dimer bypass/Error prone repair
Photoreactivation:
A single light-inducible enzyme, photolyase, recognizes pyrimidine dimers and splits the cyclobutane ring formed between pyrimidines.
- Error free.
Excision repair:
uvrABC recognizes mispaired basses in the helix and nicks both sides. DNA pol I fills in the empty space. DNA ligase seals the nick.
- SOS response.
Recombination repair:
Chromosome is replicated into two chromosomes, one will have a single stranded region with DNA damage, the other is normal.
A section of the normal DNA is used to repair the damaged daughter strand by recombination (patching).
- Error proof.
Dimer bypass/Error prone repair:
DNA Pol III inserts any bases into the daughter strand across from a dimer or other damaged DNA.
- umuC and umuD are believed to modify the fidelity of DNA Pol III
Induction of DNA repair systems:
- RecA protease is activated by damage to DNA.
- RecA protease cleaves and inactivates lexA repressor protein.
- This cleavage results in the induction of all the genes in SOS regulation.
Reciprocal recombination:
Recombination in which no genetic information is lost in the products.
Gene conversion:
Crossover events that result in gain/loss of genetic information as a consequence of the resolution of the heteroduplex region.
Two requirements for generalized recombination:
- Homologous sequences.
2. recA, recBCD and other rec gene products
Insertion element:
Transposase gene is flanked by short inverted repeats.
Composite transposon:
Resistance gene(s) flanked by insertion elements.
TnA family:
(Transposase, resolvase + drug resistance) flanked by inverted repeats.
Host restriction:
Degradation of foreign DNA by the host cell.
Bacteriophage host specificity:
- Host range is dependent on presence of receptors on host cell wall and presence of specific components of the host replication and transcription apparatus.
Virulent phage infection:
Phage infection invariably leads to lysis of host organism.
Temperate phage infection:
Phage infection may lead to ether host cell lysis or lysogeny.
Lysogeny:
State of dormancy for the phage.
Concatamers:
Many linear unit length genomes that are covalently linked together.
Lysogenic conversion:
Prophage introduces new genes that are expressed in the host and impart novel phenotypes.
Transformation:
The uptake of naked DNA into a cell followed either by recombination with the host genome or self-replication
Transfection:
The uptake of naked bacteriophage DNA by cells, followed by the production of phage particles or lysogeny.
Transduction:
The process of transferring host cell DNA sequences by bacteriophage to a recipient cell.
- Generalized
- Specialized
Generalized transduction:
- Bacterial DNA is degraded and pieces are mistakenly packaged into phage heads.
- Phage injects DNA into host bacteria; DNA can recombine with host genome by generalized recombination.
Generalized transducing phage:
Phage that can package random segments of the bacterial chromosome and introduce these segments into the host organism.
Specialized transduction:
When a prophage excises improperly and an adjacent region of the host genome is attached to the phage chromosome.
- Specialized transducing phage.
Conjugation:
The transfer of DNA from a bacterial cell of one mating type (male) to cell of the other mating type (female).
F factor can exist in 3 different states inside the cell:
- F+ plasmid
- Hfr chromosome
- F’ plasmid
Self-replicating plasmid (F+):
Circular DNA molecule that replicates independently while the bacterial chromosome is replicating.
Hfr chromosome:
F plasmid integration into host chromosome.
F’ plasmid formation:
F plasmid can excise imprecisely from an Hfr chromosome and take bacterial genes with it.
Type I excision:
Results in a portion of the F plasmid being left behind in the bacterial chromosome and a portion of the bacterial chromosome being attached to F.
Type II excision:
Recombination between flanking bacterial sequences such that no F sequences are left behind and that bacterial sequences from both sides of the integration site are now on the F’ plasmid.
