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Griffith's Experiment and the Transforming Principle

Biology · Molecular Basis of Inheritance · NEET

In 1928, Frederick Griffith worked with Streptococcus pneumoniae (the pneumonia bacterium). He mixed live harmless R bacteria with heat-killed virulent S bacteria and injected them into mice. The mice died, and live S bacteria were found inside them. So something (a "transforming principle") passed from the dead S bacteria and changed R into S. Memory hook: "R came Rough, but got a Smooth coat from a dead friend." Griffith proved transformation happens but did NOT tell us it was DNA.
Griffith's Transformation Experiment (1928)SLive S(virulent)Mice DIERLive R(harmless)Mice LIVESHeat-killed S(dead)Mice LIVESRHeat-killed S+ Live RMice DIER was transformed into S by the "transforming principle" (later shown to be DNA)
The four injections: live S kills, live R and heat-killed S are safe alone, but heat-killed S mixed with live R kills the mice. R turned into virulent S, proving transformation.

Your doubts, answered

What exactly did Griffith's experiment prove?

It proved that TRANSFORMATION happens. A living R strain (harmless) can be changed into an S strain (virulent) by some material coming from heat-killed S bacteria. Griffith called this material the 'transforming principle'. Important for NEET: he proved transformation, NOT that DNA is the genetic material. He did not know the chemical nature of the transforming principle.

Why did the mice die when live R and heat-killed S were mixed, even though each one alone was safe?

Heat-killed S alone is dead, so it cannot cause disease. Live R alone has no polysaccharide coat, so it is not virulent. But when mixed, the transforming principle from the dead S bacteria entered the live R bacteria. This made the R bacteria produce a smooth polysaccharide coat and become virulent S. These new live S bacteria then killed the mice. Griffith even recovered living S bacteria from the dead mice.

Did Griffith prove that DNA is the genetic material?

No. This is the most common NEET trap. Griffith only showed transformation happens and that genetic material transfers. He did not identify the chemical nature. Avery, MacLeod and McCarty later showed the transforming principle was DNA. Hershey and Chase gave the UNEQUIVOCAL (final) proof that DNA is the genetic material.

What is the difference between the S strain and the R strain?

S strain makes Smooth, shiny colonies because it has a mucous polysaccharide (capsule) coat, and it is virulent (kills mice). R strain makes Rough colonies, has NO polysaccharide coat, and is non-virulent (does not cause pneumonia). The coat protects the bacteria from the host's immune system, so the coat is the reason S is deadly.

Which bacterium did Griffith use and why is it named in NEET matching questions?

He used Streptococcus pneumoniae (pneumococcus), the bacterium that causes pneumonia. NEET 2025 directly matched 'Frederick Griffith' with 'Streptococcus pneumoniae'. Remember this pairing: Griffith = Streptococcus pneumoniae = Transformation.

⚠️ The NEET trap
✗ Griffith's experiment proved that DNA is the genetic material.
✓ Griffith only proved that transformation occurs. The transforming principle was later identified as DNA by Avery, MacLeod and McCarty, and Hershey-Chase gave the final proof.
🧠 NEET repeatedly asks 'who gave the FINAL/unequivocal proof' (answer: Hershey-Chase) and matches Griffith only with 'Transformation'. Never attach the word 'proof of DNA' to Griffith.

Real NEET questions

NEET 2024

Match List I with List II. A. Frederick Griffith B. Francois Jacob & Jacque Monod C. Har Gobind Khorana D. Meselson & Stahl | I. Genetic code II. Semi-conservative mode of DNA replication III. Transformation IV. Lac operon

A · A-III, B-IV, C-I, D-II ✓
B · A-II, B-III, C-IV, D-I
C · A-IV, B-I, C-II, D-III
D · A-III, B-II, C-I, D-IV
Solution: Griffith discovered TRANSFORMATION (III). Jacob & Monod = Lac operon (IV). Khorana = Genetic code (I). Meselson & Stahl = Semi-conservative replication (II). So A-III, B-IV, C-I, D-II. Note Griffith links only to transformation, not to DNA proof.
NEET 2025

Match List-I with List-II. A. Alfred Hershey and Martha Chase B. Euchromatin C. Frederick Griffith D. Heterochromatin | I. Streptococcus pneumoniae II. Densely packed and dark-stained III. Loosely packed and light-stained IV. DNA as genetic material confirmation

A · A-IV, B-III, C-I, D-II ✓
B · A-III, B-II, C-IV, D-I
C · A-II, B-IV, C-I, D-III
D · A-IV, B-II, C-I, D-III
Solution: Hershey & Chase = confirmation of DNA as genetic material (IV). Euchromatin = loosely packed, light-stained (III). Griffith = Streptococcus pneumoniae (I). Heterochromatin = densely packed, dark-stained (II). So A-IV, B-III, C-I, D-II. Remember: Griffith is paired with Streptococcus pneumoniae.

Solved Molecular Basis of Inheritance NEET PYQs

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Frequently asked

In which year did Griffith perform his experiment?

1928. He used Streptococcus pneumoniae and discovered the transforming principle.

What is meant by the 'transforming principle'?

It is the unknown material that passed from heat-killed S bacteria into live R bacteria and changed them into virulent S bacteria. Griffith did not know it was DNA; that came later.

Why does heat-killed S alone not kill the mice?

Because the bacteria are dead. They cannot grow, make a coat, or cause infection on their own. Only when their transforming principle enters live R bacteria does virulence appear.

What limitation did Griffith's experiment have?

It did not reveal the biochemical (chemical) nature of the genetic material. It showed transformation happens but not that DNA is responsible. That question was answered by Avery, MacLeod and McCarty.

How is Griffith's work asked in NEET?

Mostly as matching questions: Griffith with 'Transformation' and with 'Streptococcus pneumoniae'. A common trap option wrongly credits him with proving DNA is the genetic material.

Related concepts:
The Search for Genetic Material: Timeline of Key ExperimentsAvery, MacLeod and McCarty: DNA is the Transforming PrincipleHershey-Chase Experiment: DNA is the Genetic MaterialWhat is DNA? Molecular Basis of Inheritance Explained SimplyProperties a Genetic Material Must Have (DNA vs RNA)
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Molecular Basis of Inheritance PYQs