Griffith’s experiments showed that hereditary material can
pass from one bacterial cell to another.
 In 1928 Griffith was studying a bacterium called
Streptococcus pneumoniae. He was trying to develop a
vaccine against a virulent strain of the bacterium.
 Virulent – disease causing
 Vaccine – a substance that is prepared from killed or
weakened microorganisms & introduced into the body
to protect it from future infection by that
microorganism.
 S bacteria is virulent because it has a capsule. The
capsule protects it from attacks by the human immune
system. It is able to survive long enough in the human
body to replicate and cause disease.
 R bacteria is NOT virulent because it lacks a capsule.
Without the capsule it is left defenseless against the
human immune system. Therefore it is destroyed in
the human body before it is able to cause disease.
 Injected mouse with live R bacteria (w/o capsule).
 DID NOT kill mice
 Injected mouse with live S bacteria (w/ capsule).
 Killed mice
 Injected mouse with heat killed S bacteria.
 DID NOT kill mice
 Injected mouse with heat killed S bacteria mixed with
live R bacteria.
 Killed mice
 The R bacteria took up the DNA found in the heat
killed S bacteria and became virulent!
 The transfer of genetic material from one cell to
another cell or from one organism to another organism
is called transformation.
 Discovered that Adenine / Thymine and Cytosine /
Guanine occurred in equal percentages in DNA.
 For Example: If you had 22 Adenine and 5 Cytosine
then you would have 22 Thymine and 5 Guanine.
 Led to base pairing discovery
•
Discovered that Adenine / Thymine and Cytosine /
Guanine occurred in equal percentages in DNA.
• For Example: If you had 22 Adenine and 5 Cytosine then
you would have 22 Thymine and 5 Guanine.
• Led to base pairing discovery
Hershey and Chase confirmed that DNA, and not protein, is
the hereditary material.
 In 1952, two researchers, Martha Chase and Alfred
Hershey, set out to test whether DNA or protein was
the hereditary material viruses transfer when viruses
enter bacteria.
 Bacteriophage - is a virus that infects bacteria.
 Grew bacteriophage (with radioactive sulfur ) in dish
of e.coli (sulfur is only found in proteins)
 Grew bacteriophage (with radioactive phosphorous )
in dish of e.coli (phosphorous is only found in DNA)
 They traced the radioactive elements that had
entered the e.coli …to see which one was injected into
the cell.
 The DNA of the virus and not the proteins was what
was in the e.coli causing it to produce more viral DNA.
 Hershey and Chase confirmed that DNA, and not
protein, is the hereditary material.
Watson and Crick used the information gathered by others to
determine one of the most important discoveries in human
history – the Double Helix.
 Until Hershey and Chase’s experiment, most people
believed that protein was the hereditary material
(because protein was involved in basically everything
to do with cells and because it was believed DNA’s
structure was too simple to encode the secret to life).
 After the Hershey and Chase experiment in 1952
proved DNA was in fact the hereditary material, the
race was on to discover it’s structure to gain a better
understanding of it.
 Watson and Crick discovered DNA shape (double
helix) in 1953 using information gained from other
scientists, mainly Wilkins / Franklin.
 Crick actually studied mostly proteins and Watson
studied neither before 1952.
 Watson and Crick went to lectures from other
scientists concerning DNA and compiled information
they gained from them.
 Watson “borrowed” X-rays done by Wilkins / Franklin
showing a vague picture DNA’s shape (you could tell
that it was the same thickness all the way through).
 Watson, Crick, and Wilkins received a Nobel prize
from it while Franklin (who did most of the work) died
from cancer due to her exposure to X-rays.