BOTANY Higher Secondary Second Year - Textbooks Online

3.7. DNA as a genetic material
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.
R-type cells S-type cells Heat killed S-type cells
Fig. 3.7 Griffith experiments on mouse
Hereditary role of DNA – Bacterial transformation
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
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R-type cells
Heat killed
S-type cells