Transcript “S” “R” - Blue Valley Schools

(Follow along on page 226 of your textbook.)
First Isolation of DNA
• In 1869, Friedrich Miescher isolated the
first crude preparation of DNA from
bandage pus and named it “nuclein.”
• Along with most scientists, Miescher
believed that proteins were the molecules of
heredity. It would be years before the role
of nucleic acids were recognized.
• However, Miescher laid the groundwork for
the molecular discoveries that followed.
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 “S” and “R” and
used them in a series of four
experiments.
Experiment #1
Laboratory mice were injected with
living “S” cells.
Experiment #1
The mice died.
Blood samples taken from them teemed
with live “S” cells.
(The “S” strain was pathogenic.)
Experiment #2
Laboratory mice were injected with
living “R” cells. The mice did not
develop pneumonia.
Experiment #2
Laboratory mice were injected with
living “R” cells. The mice did not
develop pneumonia.
(The “R” cells were harmless.)
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.
Summary
“S”
“R”
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 #2?
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
Erwin Chargaff, 1947
• Chargaff’s Rule: The number of adenines
approximately equals the number of thymines,
and the number of guanines approximately
equals the number of cytosines.
Alfred Hershey and Martha Chase, 1952
(see Online Activity 11.1)
Watson and Crick, 1953
• James Watson and Francis Crick
published the first accurate model
of the DNA molecule in 1953.
• Crick conjectured that the DNA
strands must be antiparallel.
• Based on Chargaff’s rule, Watson
surmised that the bases must be
paired in a specific way: adenine
with thymine, and cytosine with
guanine.
Rosalind Franklin
• Franklin, an X-ray crystallographer, made the
photo that Watson and Crick used in deducing the
double-helical structure of DNA.
• Franklin died of cancer in 1958, when she was
only 38.
• Her colleague Maurice Wilkins received the Nobel
Prize in 1962 along with Watson and Crick.