Chapter 14, Problem 23P

Starting with an F^- strain that was prototrophic (that is, had no auxotrophic mutations) and Str^s, several independent Hfr strains were isolated. These Hfr strains were mated to an F^- strain that was Str^r Arg^- Cys^- His^- Ilv^- Lys^- Met^- Nic^- Pab^- Pyr^- Trp^-. Interrupted-mating experiments showed that the Hfr strains transferred the wild-type alleles in the order listed in the following table as a function of time. The time of entry for the markers within parentheses could not be distinguished from one another.

 

a.

From these data, derive a map of the relative position of these markers on the bacterial chromosome. Indicate with labeled arrows the position and orientation of the integrated F plasmid for each Hfr strain.

b.

To determine the relative order of the trp, pyr, and cys markers and the distances between them, HfrB was mated with the F- strain long enough to allow transfer of the nic marker, after which Trp+ recombinants were selected. The unselected markers pyr and cys were then scored in the Trp+ recombinants were selected. The unselected markers pyr and cys were then scored in the Trp+ recombinants, yielding the following results:

 

Draw a map of the trp, pyr, and cys markers relative to each other. (Note that you cannot determine the order relative to the nic or his genes using these data.) Express map distances between adjacent genes as the frequency of crossing-over between them.