1. Identifying the Genetic Material

2. Fredrich Miescher Mid 18th Century, isolated nuclei from pus
(white blood cells) on bandages.
From nuclei, discovered unusual acidic material composed of nitrogen and phosphorus.
He and other scientists isolated same material
from other types of cell nuclei
Called substance nuclein in 1871, then nucleic acid
Few understood the significance since at that time inherited disease was thought to be caused by proteins

3. Archibald Garrod What was the connection?
1902, first to link inherited disease with protein
Noted that people who had certain “inborn errors of metabolism” lacked certain enzymes
Other researchers added link between heredity and enzymes in species like fruit flies w/unusual eye colors and mold w/nutritional deficiencies
Both organisms had absent or malfunctioning specific enzymes
What was the connection?

4. Frederick Griffith
1928, first steps in identifying DNA as genetic material
Studying pneumonia in years after the 1918 flu pandemic
Found mice with infection had one of two types of streptococcus pneumonia, only one was virulent
Experimented to determine which one was and found the bacteria could be altered within the cell
Identified phenomenon he called bacterial transformation
Died before he could determine the cause…
The transforming principal: something in the dead virulent bacteria had contributed something to the nonvirulent bacteria and transformed them. But what was it?

5. Avery, MacLeod, and McCarty
Continued Griffith’s experiments
Hypothesized that a nucleic acid was the ‘transforming principle’
By adding enzymes that either broke down either proteins(protease) or DNA(DNase) to bacteria that were broken apart to release their contents, they showed that DNA transforms the bacteria-not the protein
In 1944, their conclusion was that DNA was the hereditary substance that caused bacterial transformation.

6. Hershey and Chase
Furthered DNA research in 1952 when they showed that DNA was the substance of phage (viral) heredity.
They cultured viruses on radioactive media to label the viral protein with sulfur-35 and viral DNA with phosphorus-32.
When they allowed these radioactive viruses to attack normal bacteria, they found that the viruses had injected the phosphorus-32 into the bacteria.
The viruses were putting their DNA into the bacteria to force the bacteria to make more viruses. No protein was found in the cells.
In these so-called “blender experiments” they proved viral DNA, not protein, was the hereditary material.