DNA and RNA
• Genes are made of DNA, a large, complex molecule. DNA
is composed of individual units called nucleotides. Three of
these units form a code. The order, or sequence, of a code
and the type of code determine the meaning of the
1. On a sheet of paper, write the word cats. List the letters or
units that make up the word cats.
2. Try rearranging the units to form other words. Remember
that each new word can have only three units. Write each
word on your paper, and then add a definition for each word.
3. Did any of the codes you formed have the same meaning?
4. How do you think changing the order of the nucleotides in
the DNA codon changes the codon’s message?
Once it was known that
traits were inherited the
next question was:
What molecule holds the
Discovering the function and
structure of DNA
Frederick Griffith, 1928
Oswald Avery, 1944
Alfred Hershey and Martha Chase, 1952
Rosalind Franklin, 1950’s
James Watson and Francis Crick, 1953
• Rough Pneumococcus are harmless. They
lack a gel capsule that would protect them
from a host organism's immune system
• Smooth Pneumococcus are pathogenic
(they cause disease), and when injected,
give a mouse fatal pneumonia.
• Griffiths injected several combinations of
rough and smooth into hapless mice, and
Results of Experiment
– live rough --> mice okay
– live smooth --> mice pushing up daisies
– killed (boiled) rough --> mice okay
– killed (boiled) smooth --> mice okay
– live smooth + killed rough --> mice kick the
– live rough + killed smooth --> MICE
CROAK! This was a surprise!
• When Griffiths autopsied the mice, he
found LIVE SMOOTH PNEUMOCOCCUS!!
• The heat killed bacteria had passed their
disease-causing ability to the harmless
• He called this transformation:
harmless, live rough bacteria
TRANSFORMED into deadly smooth
bacteria. But how?!
• The factor passed from the heat-killed
smooth bacteria to the live rough
bacteria must contain information to
transform harmless bacteria into
• Perhaps this transforming factor was
• Oswald Avery (1944)
–Repeated Griffith’s experiment in an
attempt to find molecule
responsible for transformation
–Was it a Protein? Lipid?
Carbohydrate? Nucleic acid?
Heat-killed smooth bacteria mixture
Treated with enzymes to
destroy carbs, lipids, Treated with enzymes to
protein and RNA Destroy DNA
Mouse Dies Mouse lives!
DNA STORES AND TRANSMITS GENETIC INFO.
The Hershey and Chase
• Used more advanced science to see if
Avery was right about DNA
• Worked with bacteriophages (Viruses
that attack bacteria)
• Side by side experiments were
performed to see if it was the protein or
the DNA of the virus that infected the
• Conclusion: AVERY WAS RIGHT!
The DNA was passed into the host, not the protein coat.
The components and
structure of DNA
Once Scientists agreed that it was definitely
DNA that contained and passed on the
genetic information the next question
What is DNA?
What does it look like?
What is it made of?
How does it work?
• DNA is composed of small units
• There are four different kinds:
Adenine, Guanine, Cytosine
• Chargaff realized that in DNA
the # of Adenine was always
similar to the number of
Thymine. The # of Cytosine
was always similar to the
number of Guanine...
• A always
bonds to T
• C always
bonds to G
• Franklin and Maurice Wilkins
at King's College, (London,
England) performed X-ray
• Franklin noticed a diffraction
pattern in DNA that showed
that DNA was 2-stranded and
Watson and Crick
• Used Franklin’s X-ray
image to build a
model of the DNA
• The Double helix
shape of DNA
explained how it could
carry information and
how that information
could be copied