Download Chapter 9 DNA: The Genetic Material

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 “heatkilled” 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.