Frederick Griffith’s experiment showed that the R (rough, non-virulent) strain transformed into the S (smooth, virulent) strain when mixed with heat-killed S strain, revealing the transforming principle (later identified as DNA). Option (A) is correct.

Option Analysis

(A) R strain became S strain when mixed with heat killed S strain

This observation was key: live R bacteria gained virulence after exposure to heat-killed S bacteria extracts, proving a heat-stable “transforming principle” transferred genetic traits, suggesting DNA as the genetic material.

(B) R strain remained R strain when mixed with heat killed S strain

This did not occur; R strain alone with heat-killed S stayed non-virulent, showing no spontaneous change without the mixture effect that revealed transformation.

(C) S strain became R strain when mixed with heat killed R strain

Impossible outcome; heat-killed R (already non-virulent) could not confer rough traits to virulent S, contradicting Griffith’s findings where transformation went from non-virulent to virulent.​

(D) R strain became S strain when mixed with live S strain

Live S would kill mice directly without transformation evidence; the key was heat-killed S enabling safe R-to-S change, isolating the genetic transfer mechanism.​​

Frederick Griffith R Strain Heat Killed S Strain Discovery

Frederick Griffith’s 1928 experiment with Streptococcus pneumoniae demonstrated R strain became S strain when mixed with heat killed S strain, proving genetic transformation exists. This breakthrough identified DNA as the likely genetic material through bacterial transformation.

Griffith Experiment Key Observation Explained

Mice injected with heat-killed S + live R died unexpectedly, yielding live S bacteria from blood—showing R strain transformed by S strain’s “transforming principle.” Controls confirmed: live S killed, live R harmless, heat-killed S harmless alone.