Explain how you would determine the distances between 3 loci

Explain how you would determine the distances between 3 loci (A, B, and C) by P1 co-transduction. What would the results be if the distance between A and B is 60,000 base pairs (bp), and B and C are 35,000 base pairs apart? For this experiment, you have bacteriophage P1, a wild type bacterium, and a mutant bacterium for all 3 loci. P1 packages roughly 2 minutes (80,000 bp) of DNA. Show all work.

Solution

Cotransduction is the transduction of two bacterial genes simultaneously. The frequency of cotransduction can imply the closeness of genes on the chromosome.

P1 phage can be grown first on the wild-type bacterium A⁺B⁺C⁺. The progeny of phages is grown on the mutant bacterium A^-B^-C^-. The bacterial cells are then screened and A⁺ transductants are selected. These transductants are then screened for B⁺ and/or C⁺.

The cotransduction frequency (CF) of two genes is calculated from the experimental data. This frequency is used to map genes by using the formula proposed by Wu:

Cotransduction frequency =1- (distance (kb) between two genes / length of the DNA fragment (kb) packaged in the phage)³

If A and B are 60kb apart, while B and C are 35kb apart and packaged DNA for P1 phage is 80kb, the cotransduction frequency can be obtained as follows:

CF of A and B = 1-(60/80)³ = 0.58

CF of B and C = 1-(35/80)³= 0.92

The results indicate that the gene B is located closer to the gene C than gene A and shows higher cotransduction frequency with gene C.