Bio H – Molecular biology
Experiments to Leading to DNA Discoveries
Edited Notes by Leigh Eisenman and Kevan Higgins
Griffith (1928)
Identified transforming principle. He worked with two types of the bacteria that cause pneumonia: smooth
(virulent – causes disease) and rough (avirulent – doesn’t cause disease)
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injected rats with smooth bacteria -> they died
injected rats with rough bacteria -> they lived
heated smooth bacteria to kill them then injected into rats -> rats lived
heated smooth and then mixed with living rough bacteria and injected -> rats died
Griffith observed that something was transferred from the heat-killed smooth cells to the rough cell to change
the rough cell into a killer. He called this unknown material that was transferred a transforming principle,
meaning some molecule that carries information from one cell to another. In this case, this molecule carried
information on how to cause illness and “transformed” an avirulent cell into a virulent cell. At the time, no one
knew what this powerful molecule was, but suspected it was either DNA or protein.
Strain S
(Virulent)
Strain R
(Avirulent)
Heat killed
Strain S
Heat killed
Strain S
and Strain R
Avery, MacLeod, McCarthy (1944)
They were able to identify DNA as Griffiths transforming principle through the following experiment.
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took a sample from heated smooth bacteria and treated it with DNAase (an enzyme that digests DNA) then mixed digested extract with rough bacteria and injected into rats -> the rats lived suggesting that the
transforming molecule wasn’t passed to the living cells successfully.
Took a sample from heated smooth bacteria, and treated it with protease (an enzyme that digests
proteins) -then mixed with rough bacteria and injected into rats -> rat died suggesting that the
transforming molecule was still intact and therefore NOT protein.
Bio H – Molecular biology
This showed that DNA, not protein, has ability to transform cells (for posterity's sake, they were actually mice,
not rats)
Heat killed Strain S with
digested DNA and Strain R
Heat killed Strain S with
digested Protein and Strain R
Hershey and Chase (1950)
Proved DNA was hereditary material of cell. They worked with a bacterial virus (phage) comprised of protein
coat and a DNA core. They wanted to find out which part of the phage (DNA or protein), produced new phages.
Protein coat (proteins contain Sulphur (S))
DNA core (DNA contains Phosphorous (P))
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Batch 1: give viruses CHONS and radioactive P (called P32 ) to build themselves. This way any
molecule or part of the virus that requires P, will be radioactive and can be detected. Since DNA
contains P and proteins do not, only the DNA core of the viruses becomes radioactive in this batch.
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Batch 2: give viruses CHONP and radioactive S (called S35 ) to build themselves. This way any
molecule or part of the virus that requires S, will be radioactive and can be detected. Since Protein
contains S and DNA does not, only the Protein coat of the viruses will be radioactive in this batch.
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allowed each batch of phages to infect a separate set of bacterial cells, then disrupted cells in a blender
to produce a mixture in which the bacteria remained intact but the protein shells of the phage would be
sheered off the bacteria and would be free in the solution (see diagram below)
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mixture was then placed in centrifuge to separate the bacterial cells from the phage protein coats.
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In the first batch (radioactive DNA), portion of the mixture containing the whole bacteria was
radioactive, which meant that the radioactive DNA from the virus was now inside the bacterial cell.
In the second batch (radioactive protein), the portion of the mixture containing the whole bacteria was
not radioactive, but the portion containing just the virus coats was suggesting that the protein coat is not
transferred to the bacteria during infeciton.
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Bio H – Molecular biology
Watson and Crick (1953)
Watson and Crick discovered the "double helix" structure of DNA, a discovery which, along with Hershey and
Chase, marked the beginning of molecular biology. Technically the majority of the work was completed by a
Graduate Student named Rosalind Franklin. The final interpretations of the work that were accepted were done
by Watson and Crick and therefore they are credited with the discovery.
Directions: Use the information attached AND your textbook to answer the following Questions in YOUR
OWN WORDS.
1. What were the two major classes of compounds suspected of passing on genetic information before
these scientists did their work?
2. In your own words what is the Transforming principle:
3. Explain why the mixture of the heat killed S strain (virulent) and the live R (avirulent) strain resulted in
dead rats.
Bio H – Molecular biology
4. How did Avery/McLeod change the Griffith experiment to specifically identify the transforming
principle?
5. List all the elements that make up DNA (look in your book if you don’t remember):
6. List all the elements that make up Protein (look in your book if you don’t remember):
7. Get two different colored pencils/pens. Draw two pictures of a bacteriophage. Then color code the part
of the phage that will be radioactive if grown in S35 OR P32. (Think about it: the coat is made of
protein and the genetic material is DNA. So if the S is radioactive, which part will be radioactive: the
protein coat or the DNA? Likewise think about what would be labeled with radioactive phosphorous. )
S35 labeled:
P32 labeled: