1)(9 pts) A derivative of a Salmonella strain with a Pyr- mutation (which result

Question

1)(9 pts) A derivative of a Salmonella strain with a Pyr- mutation (which results in auxotrophy for uracil)was isolated. Other than this phenotype, the mutant was wild-type. To determine where themutation mapped, it was mated with five different Hfr donor strains that are each auxotrophic forcysteine due to a cysGmutation. The map position and orientation of oriTin the Hfr donors are shown in the figure below.

a. How could you counter select against the recipient cells in this experiment?

b. How could you counterselect against the donor cells in this experiment?

c. The results are shown in the following table. Based upon these results, what is the most likely

map position of the pyrmutation? [Indicate the map position relative to the Hfr origins.]

Donor strain                            Pyr+ colonies

Hfr 1                                       490

Hfr 2                                           0

Hfr 3                                       320

Hfr 4                                           0

Hfr 5                                         50

2)   (9 pts) We have an Hfr strain that is gal+ met+arg+ampS and we wish to map the relative position of the met and arg genes. We cross this strain with an F- strain having the genotype gal- met-arg-ampR. We know from an interrupted mating experiment that gal enters last. Note that gal = a carbon source (galactose) while arg and met are amino acids. Amp = an antibiotic.

If you plate on the following media:

Media A)   minimal media lacking arginine but containing methionine

Media B)   minimal media lacking methionine but containing arginine

Media C)   minimal media lacking arginine and methionine

Media D)   minimal media containing arginine and methionine

You obtain the following results:

Result 1)   5% of the recombinants are arg- met-

Result 2) 10% of the recombinants are arg+ met-

Result 3)   0% of the recombinants are arg- met+

Result 4) 85% of the recombinants are arg+ met+

Match the media with the correct phenotype.

What other chemical was in all the media? (why?)

Which gene is first?

How would you check to see if the galactose gene has moved?

Explanation / Answer

a> For counter selection against recipient cells, the growth of the strain should be grown on minimal medium but without uracil.

b> For the selection against donor cells, the growth of the strain should be grown on minimal medium without cysteine.

c> The map position relative to the Hfr origins according to the pyr mutation should be positioned between oriT of Hfr 1 and oriT of Hfr3.

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