Question

3. (4) Suppose that you use a phage P1 grown on a strain of E. coli which is cys+ ton trp- to infect a recipient strain of E. coli which is cys- tons trp+. These bacteria are then plated on complete media with phage T1 (tons sensitive to phage T1). You obtain 50 ton? transductants which you go on to test on minimal media for the other markers. cyst trp+ cyst trp- cys- trpt cys- trp- 10 24 13 a. Based on these results, what is the co-transduction frequency between cys and tont? b. What is the distance between ton and trp? c. Which of the markers is closest to ton? d. What is the correct gene order?

Answer 1

Solution:-

a.) Co-transduction frequency = (number of double transductants)/(total number of transductants)

Double transductants = cys+ton^R

All the given transductants are ton^R since they exhibited growth on T1 phage media

Thus, number of cys+ton^R = 3+10 = 13

Co-transduction frequency = 13/50 = 0.26

b.) co-transduction frequency, C = 1 - (d/L)³

where, d = distance between genes and L = length of transduced fragment

For ton and trp, C = (10+13)/50 = 0.46

L = 100 kB (known for phage P1)

substituting in the given equation, 0.46 = 1 - (d/100)³

$d = 100*3\sqrt0.54$ = 81.4 kB

c.) Higher the cotransduction frequency, closer are the genes

Since cotransduction frequency of cys&ton is 0.26, whereas cotransduction frequency of trp&ton is 0.46, thus, trp is closer to ton.

d.) Recombinant with lowest frequency represents double crossover, with middle gene exhibiting the crossover in relation to parent (donor) genotype

Thus cys+trp+ is the double crossover, with trp as the middle gene

Hence, gene order is cys trp ton.