A cross is made between two E. coli strains: Hfr arg+ bio+ leu+ x F- arg- bio- l

Question

A cross is made between two E. coli strains: Hfr arg+ bio+ leu+ x F- arg- bio- leu-. Interrupted mating
studies show that arg enters the recipient last and bio and leu enter first, but they are all so closely linked
that we decided to map them carefully. So, arg recombinants were selected on a minimal medium
containing bio and leu. These recombinants were then tested for the presence of bio+ and leu+. The
following numbers of colonies were found for each genotype:
WT 699
leu- 10
bio- 1
leu- bio- 290

a) Why are there no arg- colonies?
b) How do you test colonies to see if they are bio+ ?
c) Through what does the DNA strand from the Hfr strain leave the bacterium as it moves to the F- bacteria?
d) What are the map distances between the genes, in cM?

Explanation / Answer

Distance between leu and bio genes = 1 x 100 /699 = 0.143 cM

Distance between bio and arg genes = 290 x 100/699 = 41.48 cM

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