Question

An Hfr strain is used to map four genes in an interrupted mating experiment. The cross is  Hfr/a+b+c+ rifs ×F−a−b−c− rifr .

(No map order is implied in the listing of the alleles; rifr is  resistance to the antibiotic rifampicin.) The a+gene is required

for  the biosynthesis of  nutrient A, the b+ gene for nutrient B, c+ for nutrient C. The cross is initiated at time = 0,

and  at  various times, the mating mixture is plated on medium. Each plate contains minimal  medium (MM) plus rifampicin.

They then checked to see the presence of specific nutrients in  colonies at the indicated interruption points.

At 5 minutes only nutrient B is seen. At 10 minutes nutrients A and B is seen. At 15 minutes nutrients A, B, and C is present?

What is the order of the gene based on F-factor (origin and terminus)?

A plaque assay is performed where 1 mL of a 10^-8 dilution of excess bacteria were plated. 700 plaques formed at the end of the experiment. What was the original phage concentration?

A recombination experiment with two T4 rII mutants was carried out. The two phages were mixed in B cells. A 10-4 dilution of the lysate was mixed with B cells and plated. 300 plaques appeared the next day. A 10-4 dilution was mixed with K12 cells and plated. 50 plaques appeared the next day.

Answer 1

1. The order of genes from origin to terminus is the ref (rifampicin), b (nutrient B), a (nutrient A) and c (nutrient C).

2. The original page concentration is calculated by plaque-forming units (PFU) per millilitre. in this case, the original phage concentration is 0.7X10¹² PFU/ml.