DNA: The Hereditary Material
Is protein or DNA the hereditary
material?

Most scientists thought proteins carried
genetic information because DNA had a
much simpler structure than proteins.
Friedrich Miescher (1869)



He used pus cells to investigate the
chemical composition of DNA.
He discovered that the nuclei of cells
contain large quantities of a substance
that does not act as a protein.
He called this substance nuclein.
Joachim Hammerling (1930)


His experiments suggested that the
hereditary material is located in the
nucleus.
He conducted experiments on a one-celled
green alga-Acetabularia
Frederick Griffith's Experiment
(1920) - The Discovery of
Transformation


He wanted to find a vaccine against
pneumonia caused by Streptococcus
pneumonia
He used two varieties of streptococcus


One variety of bacteria had a capsule (like a
cell wall) – Type S
The other did not have a capsule. – Type R
Frederick Griffith's Experiment
(1920) - The Discovery of
Transformation
Experiment
Result
Injection with live Type S
Mice
contracted
pneumonia and died
Injection with live Type R
Mice lived, immune
system destroyed the
bacteria
Injection with heat killed Type S
Mice remained healthy
Injection with dead Type S and live
Type R
*Note: Neither of these forms caused
disease before, but when placed
together, something occurred to
make the living Type R (naked)
bacteria virulent.
Mice contracted
pneumonia and died
Conclusions



Living bacteria acquired genetic information
from dead bacteria - particularly the instructions
for making capsules, thus transforming the
naked bacteria into encapsulated bacteria.
The transforming agent was discovered to be
DNA. DNA was isolated and added to live naked
bacteria, and they were transformed into the
encapsulated kind
Some scientists still remained unconvinced.
The Hershey-Chase Experiment –
Bacteriophage (1952)
Conclusions
It was the phosphorus-rich DNA, not the protein, which was
the molecule of heredity.
………..By 1920:
It was known that DNA
consisted of three
components:
1. Deoxyribose sugar
2. Phosphate group
(negatively charged)
3. nitrogenous base
Recall, the variation among the
Nitrogen bases: