Why do HK III S + II R when administered together, they become deadly ? (a) III S remain unchanged (b) II R remain avirulent (c) The avirulent II R changed to type III S (d) The HK III S changed to type II R and got virulent

Why do HK III S + II R when administered together, they become deadly ? (a) III S remain unchanged (b) II R re

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Q. Why do HK III S + II R when administered together, they become deadly ?

(a) III S remain unchanged

(b) II R remain avirulent

(d) The HK III S changed to type II R and got virulent

West Bengal WbchseWest Bengal HS (WBCHSE) Board 2026MCQ· 1mImportance★★★★★

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When HK III S and live II R are given together, the transforming principle (DNA) from the heat-killed S enters the living R cells and transforms them into virulent S bacteria — so the mixture becomes deadly.

Heat-killed III S alone is harmless, and live II R alone is harmless. But together, genetic material (the 'transforming principle', later identified as DNA) released from the dead S cells is taken up by the living R cells. This converts (transforms) the avirulent II R into virulent III S bacteria, which multiply, cause pneumonia, and kill the mice — and virulent S can be recovered from the dead mice.

…

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set SEM31 markMCQ
$Q. Read the following passage below and answer next four questions (23 - 26) related to it : In his experiment Griffith isolated two strains of bacteria, one was rough walled with pili on the wall and the other one covered with an extremely slimy coat hence smooth in nature. He injected type II R to mice and the mice could survive. Type III S was injected to mice and it died of Pneumonia. Heat killed (HK) strains of the same (Type III S) when injected to the mice, it was alive or did not die. But when the mice was injected with III S (HK) and II R mixed the mice was found to be dead and virulent III S were recovered from the dead remains of the mice. [Q23] Name the process which is observed in this experiment :$
(a) Bacterial transduction
(b) Bacterial transformation
(c) Bacterial conjugation
(d) Bacterial transfection
›Reveal solutionSolution
In Griffith's 1928 experiment, live avirulent R bacteria mixed with heat-killed virulent S bacteria killed the mice and yielded live S bacteria. The R bacteria were genetically 'transformed' by material from the dead S cells — the process of bacterial transformation.

Griffith worked with Streptococcus (Diplococcus) pneumoniae:

Live S (smooth, virulent) → mice die.

Live R (rough, avirulent) → mice live.

Heat-killed S → mice live.

Heat-killed S + live R → mice die, and live S are recovered. …
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CBSE 2026Set SEM31 markMCQ
$Q. Which bacteria was used in this famous experiment by Griffith ?$
(a) Diplococcus pneumoniae
(b) Bacillus subtilis
(c) Haemophilus influenzae
(d) E. coli
›Reveal solutionSolution

Griffith performed his transformation experiment on the bacterium Streptococcus pneumoniae, historically called Diplococcus pneumoniae, which causes pneumonia in mammals.

The bacterium used by Frederick Griffith (1928) was Streptococcus pneumoniae (older name Diplococcus pneumoniae), the causative agent of pneumonia. It occurs as two strains: the virulent S (smooth, capsulated) type and the avirulent R (rough, non-capsulated) type. The other options — Bacillus subtilis, *Haemoph …

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CBSE 2026Set SEM31 markMCQ
$Q. Which of the following statements is incorrect about HK III (S) ?$
(a) Heat killed III S strain was rough in nature
(b) Avirulent
(c) Heat killed III S strain was smooth in nature
(d) Type III S bacterial strain was heated at high temperature to make the heat killed strain HK III S
›Reveal solutionSolution
The type III S strain is smooth (S = smooth, capsulated); heating only kills it, it does not change its smooth wall to rough. So saying HK III S was 'rough in nature' is incorrect.

Evaluating each statement about heat-killed (HK) type III S:

(a) "Heat killed III S strain was rough in nature" — INCORRECT. The S strain is smooth (has a mucous/polysaccharide capsule); heat kills the cells but does not convert its smooth wall into a rough one.

(b) "Avirulent" — correct; once heat-killed, the S cells cannot cause disease.

(c) "Heat killed III S strain was smooth in nature" — correct; it retains its smooth (capsulated) character. …
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CBSE 2026Set SEM31 markMCQ
$Q. Why do HK III S + II R when administered together, they become deadly ?$
(a) III S remain unchanged
(b) II R remain avirulent
(c) The avirulent II R changed to type III S
(d) The HK III S changed to type II R and got virulent
›Reveal solutionSolution

When HK III S and live II R are given together, the transforming principle (DNA) from the heat-killed S enters the living R cells and transforms them into virulent S bacteria — so the mixture becomes deadly.

Heat-killed III S alone is harmless, and live II R alone is harmless. But together, genetic material (the 'transforming principle', later identified as DNA) released from the dead S cells is taken up by the living R cells. This converts (transforms) the avirulent II R into virulent III S bacteria, which multiply, cause pneumonia, and kill the mice — and virulent S can be recovered from the dead mice.

…

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CBSE 2024Set A11 markMCQ
$Q. In Griffith's experiment, mice infected with the A die from pneumonia infection, but mice infected with B don't develop pneumonia.$
(a) A → S-Strain; B → S-Strain
(b) A → S-Strain; B → R-Strain
(c) A → R-Strain; B → S-Strain
(d) A → R-Strain; B → R-Strain
›Reveal solutionSolution

In Griffith's experiment the virulent S-strain kills mice, while the non-virulent R-strain does not.

Frederick Griffith worked with Streptococcus pneumoniae. The smooth (S) strain has a mucous (polysaccharide) capsule, is virulent, and mice infected with it die of pneumonia. The rough (R) strain lacks the capsule, is non-virulent, and mice inf …
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Q.Why do HK III S + II R when administered together, they become deadly ?

Concept understanding — Griffith's Transformation Experiment

Q.Why do HK III S + II R when administered together, they become deadly ?

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