Lecture 9 Flashcards
How are Hfr strains formed?
Hfr strains are formed by homologous recombination between identical copies of an insertion sequence or transposon present on F and on the chromosome.
What is the structure of the F plasmid, how does this compare with K12 E.coli?
The structure of the F plasmid is:
4 sites of molecular parasites (IS3 x 2, IS2, and Tn10000 (transposon)), this compares to the E.coli K12 chromosome (has 12 copies of IS2, 6 of IS3, providing homology with the F plasmid, giving 18 sites to possibly integrate at and hence several possible Hfr strains, each with characterist origin and direction of transfer).
How are F-prime factors made and what are they helpful for?
F-prime factors involve insertion into the chromosome (making an Hfr chromosome) followed by excision, taking some chromosomal DNA with it. This F prime plasmid will be smaller, but now carries new traits and can be homogenous for mutants with similar DNA (hence making them helpful for complementation tests.
What was the interrupted mating experiment?
The Wollman-Jacob “interrupted Mating” experiment involved isolating HfrH and showed it donated Thr and Leu at high frequencies compared to other markers. Sampling at specific time intervals with a special cross (HfrH was prototrophic and F- lacked Thr and Leu), at specific time intervals and disruption of mating pairs was done to allow for select transfer of Thr+ and Leu+ and analysis of exconjugants for the presence of unselected markers.
The results showed certain traits were more likely to spread, hence showing that conjugation can show how far genes are relative to each other using time of entry, a unit for genetic maps is 1 unit per minute, for E.coli this is 100. By using multiple strains they also showed the chromosome was circular.
