Griffith Transforming Principle
கிரிஃபித்தின் உருமாற்றக் கொள்கை
Biology is not magic; it is chemistry in motion. Griffith’s experiment is the foundation of molecular genetics, proving that genetic information is transferable. If you confuse the S-strain and R-strain characteristics, you have already lost the mark.
The idea
Frederick Griffith used Streptococcus pneumoniae. S-strain bacteria are virulent due to a mucous polysaccharide coat, while R-strain are non-virulent. He injected heat-killed S-strain with live R-strain into mice, resulting in the death of the mice. He recovered live S-strain bacteria from the dead mice, proving that the R-strain was transformed into virulent S-strain by a 'transforming principle'.
Why it matters
This experiment provided the first evidence that genetic material could be transferred between organisms, laying the foundation for molecular genetics.
NEET lens
NEET asks specifically about the mouse mortality results. Remember: Heat-killed S + Live R = Death. Do not confuse this with the Avery, MacLeod, and McCarty experiment which identified the biochemical nature of the principle. Memorize the strain characteristics precisely.
Clinical significance
This principle explains horizontal gene transfer, the mechanism behind antibiotic resistance in clinical pathogens. When bacteria exchange genetic material, they acquire resistance genes, rendering standard treatments ineffective. Understanding this is critical for managing hospital-acquired infections.
Introduction
Frederick Griffith (1928) discovered transformation: some 'transforming principle' from dead bacteria could change the nature of live bacteria.
Core concept
Experiment with Streptococcus pneumoniae: S strain (smooth, has a polysaccharide mucous coat, virulent) and R strain (rough, no coat, non-virulent). (1) S strain injected → mice die. (2) R strain → mice live. (3) Heat-killed S strain → mice live. (4) Heat-killed S + live R → mice die, and living S bacteria were recovered from the dead mice. Conclusion: the R strain had been transformed by a transforming principle from the heat-killed S strain. Later, Avery, MacLeod and McCarty (1933-44) showed the transforming principle was DNA: DNase destroyed it, while proteases and RNases did not.
Go deeper
Griffith could not identify the biochemical nature of the transforming principle; that was done by Avery, MacLeod and McCarty, and the final proof that DNA is the genetic material came from Hershey and Chase.
From Class 10
Even dead bacteria can leave behind 'instructions' that living bacteria pick up and use — that is transformation.