Question

You found five T4 rII- mutants that will not grow on E. coli K. You mixed together all possible combinations of the two mutants (as indicated in the following chart), added the mixtures to E. coli K and scored for the ability of the mixtures to grow and make plaques (indicated as a + in the chart).

1 2 3 4 5

1 - + + - +

2 - - + -

3 - + -

4 - +

5 -

a. How many genes were identified by this analysis?

b. Which mutants belong to the same complementation groups?

Answer 1

	1	2	3	4	5
1	_	+	+	_	+
2		_	_	+	_
3			_	+	_
4				_	+
5					_

If two mutants contain a same mutated gene then they will not form plaques.

If one mutant is having mutation in a particular gene but second mutant contains the wildtype form of that gene, plaque will form. The wildtype form is able to complement the mutated form. This second mutant may be having mutation in a different gene.

When mutant 1 is mixed with 2,3 and 5 then plaques are formed. This suggest that mutation in particular gene ( say gene A) of mutant 1 is being complemented by a wild type form gene A  in mutants 2,3 and 5. This implies that 2,3 and 5 are having mutations in different genes and not on gene A as in mutant 1.

When Mutant 2 is mixed with 4 then plaque is formed. This implies there is a mutation in a different gene ( lets call it gene B ) of mutant 2. This mutant gene is complemented by active gene in mutant 4.

Mixture of mutant 3 and 4 gives plaques, this implies mutation in the same gene B as in mutant 2 . Because both mutations in 2 and 3 are compensated by 4 to make plaques and also mixture of 2 and 3 are not making plaques. Mixture of mutant will not plaques when they carry mutation in the same gene.

mixture of 4 and 5 is able to make plaques . 4 is also able to make plaque when mixed with 2 and 3 also meaning it does not have a mutated gene B. Mutant 4 contains a mutated gene which be complemented by wild type gene in 5. This gene is same as mutated gene A in mutant 1 as mixture of 1 and 4 is unable to form plaque implying that they contain mutation in the same gene A.

Therefore in total 2 genes are identified to be mutated in this analysis.

b) Mutants having mutation in the same gene come under same complementation group. They are unable to complement each other for example: mutant 1 and 4 having mutation in same gene A are unable to form plaque/ complement each other.

mutants 1 and 4 belong to the same complementation group

mutants 2, 3 and 5 belong to the same complementation group