Lecture 18 - Genetics Of Homologous Repair Flashcards
Consequences of homologous recombination?
New genetic materials
The life cycle of the lamba bacteriophage?
Attaché to e.coli and injects DNA into bacterium and forces the bacteria to synthesis phage proteins and phage DNA. These then destroy the cell and are released to infect more
What does a lambda infection results in?
The formation of plaques
What are the characteristics of the lambda plaques?
They are turbid or clear, small or large
What are some ways the recombination of genes in e.coli can happen?
Breaking a parental strand
Presence of heteroduplex DNA at the site of crossing over
What leads to the observation of negative interference?
Crosses and the measurement o recombination frequencies
Early models for the mechanism of recombination - break join
Both double strands are broken and have religased with the wrong strand so the opposite strand binds with another one.
There is no DNA synthesis
Early models for the mechanism of recombination - copy-choice
Both DNA molecules are being replicated and at some point the replication templates are swapped over which creates a totally newly synthesised product.
Early models for the mechanism of recombination - break copy
One of the double strands breaks and uses the other as a template which leads to part of the DNA being newly synthesised.
The Meselson and Weigle experiment - They investigated the question: does recombination occur by breaking parental DNA? How did they do this - identifying regions of the genome?
They worked out the genome of the bacteriophage lambda and identified the region which coded for whether it was turbid or clear and the size of the molecules. When + the plaques are turbid c is clear. + = large, mi = small.
The Meselson and Weigle experiment - the two types of phages used in this experiment?
They took two populations of the lambda bacteriophage. One a wild type for the ++ genes = turbid large plaques and the second a population of mutants (clear, small plaques).
Wild type had heavy DNA due to it being cultivated in isotopes and the mutant had light DNA due to it being cultivated normally.
The Meselson and Weigle experiment - what did they do with the two phages?
They coinfected the same e.coli population. The E.coli had been cultivated normally making the DNA light.
The Meselson and Weigle experiment - what happened after the coinfection?
They collected the population and centrifuged them in caesium chloride which separates different phages by density. The bottom fraction is the heaviest and the DNA in these phages had to be wild type.
Medium phages had one strand wild type one strand mutant.
The Meselson and Weigle experiment - what did they do with the fractions?
Isolated them and infected a new population of E.coli to compare the phenotype of the newly obtained lysis plaques to the weight of the DNA in the phage
What did the results of the Meselson and Weigle experiment show?
That to get a turbid and a small plaque means that recombination must have happened. This experiment was not good enough to determine if there was no newly synthesised DNA or if there was only a little.
