Question

Arrange Griffith’s experiment in correct sequence. (i) S strain injected → mice died
(ii) Heat-killed S strain → mice lived
(iii) R strain injected → mice lived
(iv) Heat-killed S + R strain → mice died

• A. (i), (ii), (iii), (iv)
• B. (ii), (i), (iii), (iv)
• C. (ii), (iii), (iv), (i)
• D. (i), (iii), (ii), (iv)

Hint:

Griffith proved that genetic material can transfer between bacteria through transformation.

Answer 1

The Correct Option is D

Concept: Griffith’s experiment (1928) demonstrated bacterial transformation in Streptococcus pneumoniae. He used two strains: virulent S strain (smooth, capsule present) and non-virulent R strain (rough, no capsule).

Step 1: Study R strain behavior.
R strain lacks a polysaccharide capsule, so it is easily destroyed by host immune system. Therefore, mice injected with R strain survive.

Step 2: Study S strain behavior.
S strain has a protective capsule making it virulent. It causes pneumonia and kills mice.

Step 3: Heat-killed S strain experiment.
Heating kills S bacteria, removing virulence, so mice survive.

Step 4: Mixture experiment (key discovery).
When heat-killed S strain is mixed with live R strain, genetic material from dead S strain transforms R strain into virulent form, leading to death of mice. This proves transformation. Final Conclusion: Correct sequence is (i), (iii), (ii), (iv).