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.