Explain the bacterial transformation experiments of Avery and his colleagues and the results they obtained.

Frederick Griffith performed a different experiment on bacterium streptococcus pneumonia. There are 2 strains of streptococcus i. e. virulent strain i. e. bacteria have a smooth capsule, and the avirulent strain, the bacteria have a rough capsule. Though the smooth live bacteria have virulent strain kills the mouse when it is injected into it. But the rough bacteria did not affect the mouse. When he heat-killed the smoothe bacteria and mix with the rough bacteria it kills the mouse. The single heat-killed smooth bacteria did not kill the mouse. Griffith concluded that there is some transforming principle responsible for making the avirulent bacteria into virulent and this process is called the transformation of bacteria. He failed to show which element transforms from the heat-killed bacteria to rough bacteria.

This experiment was further proceeded by Avery, Macleod, Mccarty. They cultured large quantities of smooth pneumonia cells. The cells are harvested from the culture and heat-killed. Then the heat-killed bacterial cells are homogenized and cell extracts were treated with cell lysates separately. The cell lysates are - DNAse, RNAse, protease, trypsin, lipase.

Then he injected the cell extracts with rough bacteria and put this in Petri dish to form the bacterial colony. They observed each colony show a smooth bacterial colony except the DNAse treated cell. DNAse degrades the DNA and the smooth bacterial colony was not formed. This results, DNA is the element which transforms from the smooth bacteria to rough one and makes it virulent.

Avery and his colleagues finally show DNA is the genetic material.