Ques. 1AIPMT 2001
A mutant strain of T4 bacteriophage, R-II, fails to lyse E. coli, but when two strains R-IIX and R-IIY are mixed, they lyse E. coli. What may be the possible reason?
- Both strains have different cistrons
- Both strains have similar cistrons
- It is not mutated
- Bacteriophage transforms into wild type
Answer
(1) Both strains have different cistrons
Solution
As per NCERT, a cistron is a segment of DNA coding for a polypeptide.
∵ The R-II region of T4 has two cistrons (A and B), each coding for a polypeptide needed to lyse the host.
⇒ If R-IIX is mutated in one cistron and R-IIY in the other, then in a mixed infection each strain supplies the functional polypeptide that the other lacks (complementation test of Benzer).
∴ Together they lyse E. coli, which shows that the two strains have mutations in different cistrons.
Answer: (1)