In an experiment, mating of E. coli Hfr pro+ trp+ ala+ tet s ? F- pro- trp- ala-

Question

In an experiment, mating of E. coli Hfr pro+ trp+ ala+ tet s ? F- pro- trp- ala- tetr is interrupted after several minutes and cells plated on media with tetracycline (tet) and containing the amino acids shown below. The tet marker is known to be transferred very late. Based on the number of colonies observed on plates with different amino acids, what is the order of gene transfer?

proline + alanine 290 colonies
tryptophan + proline 120 colonies
tryptophan + alanine 45 colonies

The answer to this question is : gene orderis trp-ala-pro, I dont understand how they got this. Please explain.

Explanation / Answer

it appears that you are dealing with a Hfr strain that contains two different trasfer sites,

"Hfr1" "Hfr2" on the same plasmid.

thus some of the transformation happens using Hfr1 and some using Hfr2f.

now if we consider:

---------<<Hfr1----pro-------ala---------<<Hfr2-------------------------try--------

being a circular plamid. using Hfr1 pro and ala will have the most frequent trasformation.

also in order to get a try+and ala+ ,transformation should start

from Hfr2 and transfer the whole plasmid which is less likly, thus

giving the least number of colonies.

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