It is evident that chromosomes are the carriers of genetic material. Chromosomes contain proteins, DNA and RNA. It is universally accepted that DNA is the genetic material in most of the organisms and higher organisms. In most of the plant viruses, RNA is the genetic material. There are many direct evidences for DNA being the genetic material. Here, we will discuss one of the evidences illustrated by Frederick Griffith.
In 1928, the bacteriologist Frederick Griffith conducted an experiment using Diplococcus pneumoniae. He studied two strains of virulent Diplococcus causing pneumonia. The virulent strain synthesized a smooth polysaccharide coat and produces smooth colonies. This strain was called strain-S. Another strain which lacked the proper polysaccharide coat is harmless and produces rough colonies. This strain was called strain-R.
When Griffith injected S-type of cells into the mouse, the mouse died. When R-type cells were injected into the mouse, the mice did not die. He injected heat killed S-type cells into the mouse. The mouse did not die. Griffith killed some smooth strain bacteria and mixed it with live rough strain bacteria. When the mixture of heat killed S-type cells and R-type cells was injected into the mouse, the mouse was dead. The living rough strain of Diplococcus had been transformed into S-type cells. That is the hereditary material of heat killed S-type cells had transformed R-type cells into virulent smooth strains. Thus the phenomenon of changing the character of one strain by transferring the DNA of another strain into the former is called transformation.