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Nucleic Acids– “Informational Polymers”: Code for all of the
proteins in an organism
– Polymer: Nucleic Acid• Monomers: Nucleotides
–Each Nucleotide is made up of:1) Phosphate Group2) Pentose 5-C Sugar
» Ribose or deoxyribose
3) Nitrogenous bases» Adenine (A), Cytosine(C), Thymine (T), Guanine(G), and
Uracil (U)
Nucleic Acids• Polymers– DNA – RNA (tRNA, mRNA,
rRNA)• The nitrogenous bases (A,
C, G, T) are what makes up the DNA “code”
• RNA codes are transcribed or made from DNA codes
• Proteins are then translated or made from RNA codes
How do we know that all of our genetic information
comes from DNA? (instead of protein)
• What type of experiment would you design to determine that DNA is the source of all genetic information?
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
• Frederick Griffiths was a bacteriologist studying pneumonia
• He discovered two types of bacteria:– Smooth colonies– Rough colonies
CONCLUSION:
The smooth colonies must carry
the disease!
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
• When heat was applied to the deadly smooth type…
• And injected into a mouse…
• The mouse lived!
• Griffith injected the heat-killed type and the non-deadly rough type of bacteria.
• The bacteria “transformed” itself from the heated non-deadly type to the deadly type.
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
Griffith’s Experiment did not prove that DNA was responsible for
transformationHow would you design an
experiment to prove that DNA was responsible for
transformation?
Avery, McCarty, and MacLeodRepeated Griffith’s Experiment
Oswald Avery Maclyn McCarty Colin MacLeod
They then added Heat-Killed Smooth to the non-deadly Rough Type of
Bacteria
Carbohydrates Lipids
Proteins
RNA DNA
To the Heat-Killed Smooth Type, they added enzymes that
destroyed…
S-Type Carbohydrates
Destroyed
S-Type Lipids
Destroyed
S-Type Proteins Destroye
d
S-Type RNA
Destroyed
S-Type DNA
Destroyed
Conclusion:
DNA was the transforming factor!
The Hershey-Chase Experiment
Alfred Hershey & Martha
Chase worked with a
bacteriophage:
A virus that invades
bacteria. It consists of a
DNA core and a protein
coat
DNA
Protein coat
movie
Protein coats of bacteriophages labeled with Sulfur-35
DNA of bacteriophages labeled with Phosphorus-32
Bacterium
Bacterium
Phage
Phage1. Hershey and Chase
mixed the radioactively-labeled viruses with the bacteria
The viruses infect the bacterial cells.
Protein coats of bacteriophages labeled with Sulfur-35
DNA of bacteriophages labeled with Phosphorus-32
2. Separated the viruses from the bacteria by agitating the virus-bacteria mixture in a blender
Protein coats of bacteriophages labeled with Sulfur-35
DNA of bacteriophages labeled with Phosphorus-32
3. Centrifuged the mixture so that the bacteria would form a pellet at the bottom of the test tube
4. Measured the radioactivity in the pellet and in the liquid
The Hershey-Chase results reinforced the Avery,
McCarty, and MacLeod conclusion:
DNA carries the genetic code!
However, there were still important details to
uncover…
But first, Review -- What are the
monomers of DNA?
• Composed of repeating nucleotides containing:– Deoxyribose 5-Carbon sugar– Phosphate group– Nitrogen base: (4 kinds)
Adenine (A) Thymine (T)Cytosine (C) Guanine (G)
The Race to Discover DNA’s Structure
In the 1940’s, Linus Pauling discovered the alpha-helical structure of proteins.
The Race to Discover DNA’s Structure
1950
Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine
Erwin Chargaff
Why do you think the bases match up
this way?
Adenine + Guanine = Too wide
Thymine + Cytosine = Too Narrow
Adenine + Thymine Perfect Fit Cytosine + Guanine
The Race to Discover DNA’s Structure
Maurice Wilkins
Rosalind Franklin
X-Ray diffraction image of DNA taken by Franklin in
1951
The Race to Discover DNA’s Structure
James Watson Francis Crick
1953
Compiled data from previous scientists to build a double-helical model of DNA
The Race to Discover DNA’s Structure was Over
• DNA is made up of :– Four nitrogenous bases: Adenine, Thymine,
Guanine and Cytosine– These follow the rules of base-pairing
• Purines bond with Pyrimidines:– Adenine bonds with Thymine– Guanine bonds with Cytosine
– A sugar-phosphate backbone• A sugar-phosphate backbone plus
one nitrogenous base is called a nucleotide.
• DNA is arranged in a double-helix, made up of nucleotide monomers.
Purines Pyrimidines
DNA OverviewCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P
SS
S
S
P
S
P
S
P
P
S
P
S
P
S
S
S
S
a. Double helix c. One pair of bases
P
S
P
P
P
P
P
OC
CC
purine basepyrimidine basephosphate3' end5' end
3' end5' end
5'1'
3' 2'
5'
4' 1'
1'
3' 2'
4'
2' 3'
5'
4'
deoxyribose
b. Ladder structure
3' end 5' end
C C