Lecture 6 Flashcards
What is recombination, how does this typically occur in bacteria?
Recombination is a new combination of alleles of genes, it can occur in bacteria (who typically only have one set of chromosomes) when multiple bacteriophages infect them.
What must recombination involve?
due to exchange of genetic material between corresponding chromosomes recombination must involve breakage and rejoining of DNA, the molecular mechanisms are best understood in microorganisms like fungi.
What does the Holliday model involve?
The Holliday Model involves: alignment of DNA sequences, this is followed by breakage of single DNA strands on each chromosome, strand exchange and rejoining of the DNA occurs, the cross over branch (Holliday junction) can move, giving a segment of hybrid (heteroduplex) DNA where each strand is derived from a different parent molecule.
The strands can then be cut either vertically or horizontally (this is purely relative, there is no intrinsic direction), 50% will be heteroduplexes and recombinants, 50% will be heteroduplexes but no recombinants (not all were changed, only two).
What is clearly right about the original Holliday model? What are some issues?
The Holliday structure is a key intermediate and has been observed experimentally using electron microscopy. Though the model has been refined e.g after a double-strand breaks some DNA is degraded, forming single stranded DNA, this can then base-pair with DNA in a partner chromosome. DNA synthesis and ligation can lead to a hollday juntion.
What are the modern steps in the Holliday model?
The steps in the Holliday model now are:
- double strand of one chromosome breaks.
- erosion (degradation of DNA strands to create single strand regions)
- invasion and displacement occurss, whereby the homologous chromosome base pairs with the single strand parts.
- polymerization occurs (DNA synthesis), forming a heteroduplex region.
- resolution to crossover via nicks (4).
How does recombination in E.coli work?
Recombination at the molecular level is best understood in E.coli, it involves a number of enzymes. The RecBCD protein nicks and unwinds the DNA to generate single-stranded DNA, the RecA protein then coats the single-stranded DNA and then catalyses base-pairing of the DNA with the “target” double-stranded molecule. Proteins (RuvA and RuvB) cause branch migration and RuvC protein resolves Holliday junctions via DNA cleavage.
What can recombination also allow for?
Recombination can also allow repair of double-strand DNA breaks in eukaryotic cells.
