Transcript The Discovery of DNA

(Follow along on page 226 of your textbook.)
The year was 1928…
Frederick Griffith, an army
medical officer, was attempting to
develop a vaccine against
Streptococcus pneumoniae.
Griffith never did develop a
vaccine. But his work
unexpectedly opened a door to
the molecular world of heredity.
Griffith’s Bacteria
Griffith isolated and cultured two
different strains of the bacterium. He
noticed that colonies of one strain had a
rough surface appearance, but those of
the other strain appeared smooth. He
designated the strains “R” and “S” and
used them in a series of four
experiments.
Experiment #1
Laboratory mice were injected with
living “R” cells. The mice did not
develop pneumonia.
Experiment #1
Laboratory mice were injected with
living “R” cells. The mice did not
develop pneumonia.
(The “R” cells were harmless.)
Experiment #2
Laboratory mice were injected with
living “S” cells.
Experiment #2
The mice died.
Blood samples taken from them teemed
with live “S” cells.
(The “S” strain was pathogenic.)
Experiment #3
“S” cells were killed by exposure to
high temperatures. Mice injected with
these cells did not die.
Experiment #3
“S” cells were killed by exposure to
high temperatures. Mice injected with
these cells did not die.
(The heat-killed “S” cells were harmless.)
Experiment #4
Live “R” cells were mixed with
heat-killed “S” cells and injected into
mice.
Experiment #4
The mice died.
Blood samples taken from them teemed
with live “S” cells.
What Happened?
Maybe the heat-killed “S” cells in
the mixture weren’t really killed. But
what if that were true? What should
have happened to the mice in
Experiment #3?
What Else Might Have Happened?
Maybe the harmless “R” cells in the
mixture mutated into a killer form. But
what if that were true? What should
have happened to the mice in
Experiment #1?
The Simplest Explanation:
Heat did kill the “S” cells “R”
but did not destroy their
hereditary material, including
the part that specified “how to
cause infection.” Somehow,
that material had been
transferred from dead “S”
cells to living “R” cells where
it was put to use.
“S”
The Importance of Griffith’s Work
Essentially, what Griffith had done
was similar to putting a harmless live
kitten and a stuffed toy tiger cub in a
box and suddenly finding yourself with
a live roaring tiger!!!!
The Importance of Griffith’s Work
Griffith called the process he had
observed “transformation.” He had
shown that something had been
transferred from the heat-killed diseasecausing bacteria to the live harmless
bacteria. Scientists soon began
searching for this unknown material.
Today, we know this material is:
Oswald Avery, 1944
Hershey and Chase
(see Online Activity 11.1)