Chapter 12 DNA: The Genetic Material Identification of the Genetic Material (DNA) In 1928, an experiment unrelated to genetics led to the discovery of DNA. Frederick Griffith, a bacteriologist, was trying to make a vaccine against the bacterium, Streptococcus pneumonia.

Griffith worked with two strains of S. pneumoniae. The “smooth” bacteria, had a protective coat around it that prevented the body’s immune system from killing it, was virulent (able to cause disease). The “rough” bacteria, did not have the protective capsule and was not virulent.

Griffith knew that mice infected with the smooth bacteria grew sick and died, while mice infected with the rough bacteria did not. He thought initially that the capsule on the smooth bacteria was causing the disease.

To determine if S bacteria produced a poison, Griffith “heat- killed” the S bacteria and injected the mice. The mice still lived. Griffith concluded that the cause of pneumonia was not a poison released by the S bacteria.

He then mixed the harmless live R bacteria with the harmless heat-killed S bacteria. The mice died. Griffith examined the blood of the dead mice and found that the live R bacteria had made the smooth, protective capsule and became virulent. This discovery is called transformation, a change in the bacterial cells by taking up foreign DNA.

Avery’s Experiments In 1944, Oswald Avery demonstrated that DNA is the material responsible for transformation. DNA had the instructions for the making of the capsule in the S strain of S. pneumoniae.