The first direct evidence showing that the genetic material is DNA rather than RNA or protein was published by O.T. Avery, Macleod and C.M. Mccarty in 1944.
They demonstrated that the component of the cell responsible for the phenomenon of transformation in the bacterium Diplococcus pneumoniae is DNA.
The phenomenon of transformation was first discovered by Frederick Griffith in 1928.
Pneumococci, like all other living organisms, exhibit genetic variability that can be exhibit with different phenotype
The two phenotypic characteristic of importance in Griffith experiment were:
1. presence or absence of a surrounding polysaccharide capsule, and
2. the type of capsule, that is, the specific molecular composition of the polysaccharide present in the capsules.
When grown in appropriate media in petri dishes, pneumococci with capsule form large, smooth colonies and thus designated as Type S.
Such encapsulated pneumococci are quite pathogenic to mammals, so they are virulent
The other type is nonpathogenic (nonvirulent) has no polysaccharide capsule.
Such a non-encapsulated, nonvirulent pneumococci form small, rough-surfaced colonies when grown on medium and are thus designated as Type R.
Colony morphology Reaction with Antiserum
Type Appearance Size Capsule Virulence Type IIS Type IIIS
IIR Rough Small Absent Non-virulent none none
IIS Smooth Large Present Virulent Agglutination none
IIIR Rough Small Absent Non-virulent none none
IIIS Smooth Large Present Virulent none Agglutina
Griffith unexpected discovery was that if he injected heat-killed Type IIIS pneumococci (Virulent when alive) plus live Type IIR pneumococci (nonvirulent) into mice, many of the mice died.
But when mice were injected with heat-killed Type IIIS pneumococci alone none of the mice died.
Thus, the “transformation” of nonvirulent Type IIR cells to virulent Type IIIS cells cannot be explained by mutation, rather some component of dead Type IIIS cells (the “transforming principle”) must convert living Type IIR to Type IIIS.
Subsequent expt. Showed the phenomenon described by Griffith now called “transformation”.