Griffith’s experiment to identify the genetic material
Frederick Griffith conducted
experiments in 1928 with Streptococcus
pneumoniae, which was the milestone for discovery of DNA as the genetic
material. There are two different strains of Streptococcus pneumoniae, one with smooth shiny colonies called the
S strain, while the other with rough colonies (R). The difference is due to the
presence of mucous coat in S strain bacteria, whereas the R strain bacteria
lacked them. Because of the presence of the mucous coat, S strain is lethal,
while R strain is not lethal.
Griffith injected both S and R
strains to two different groups of mice separately. The group which was
infected with the S strain developed pneumonia and died while that infected
with the R strain stayed alive.
Next, Griffith heat-killed the S
strain bacteria and injected into mice, but the mice stayed alive, which
suggests that heat killed S- strain are avirulent (inefficient to cause the
disease). Then, he mixed the heat-killed S and live R strains and injected into
the mice. The mice died. In addition, he found living S strain bacteria in dead
mice (Fig. 7.9). This suggests that some transforming principle is present in
the S form that has mediated the transformation of R form to S form. Later, the
experiments conducted by Avery, Macleod and Mc Carty in1944 showed that the
transforming principle is DNA, which carries the genetic information from one
generation to another.
· DNA is the genetic material of all living organisms except for
RNA viruses like HIV. It is the greatest super chip and enormous data can be
stored in high density.
· DNA inside the zygote contains all the information required for
developing into an individual organism.
DNA inherited from
their parents is responsible for the characteristic features of an individual,
i.e. shape of the eyes, ears, nose, colour of the skin, height, longevity,
ability to withstand stress, certain congenital diseases like diabetes and
DNA is the source of information for the synthesis of all cellular
proteins. The segment of DNA that contains information for a protein is known