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DNA History and Structure
Pre-AP Biology, Mrs. Krouse
Early ideas about the genetic material of the
cell
Why did scientists originally think protein was the genetic material (aka
genetic molecule of the cell?)
Proteins have a very complex structure
-20 different amino acids can be combined in various orders to
create long polypeptide chains
-Polypeptides can be hundreds of amino acids in length
(Rubisco, an enzyme involved in photosynthesis, is composed of a
polypeptide that is approximately 500 amino acids long)
-Many proteins consist of MULTIPLE polypeptides folded around
each other
The 20
amino acids
Polypeptides and Proteins
A single polypeptide
A protein (contains multiple polypeptides)
Protein folding at the secondary, tertiary, and quaternary levels determines the
protein’s function because in biology, structure (shape)  function
What do we mean when we say “genetic
material?”
• A coded molecule in the cell that provides instructions for creating
cell substances and directing cell activities
• This molecule is passed down through generations of offspring
Review from our Biochemistry Unit (Unit 2)
• What do we call the building blocks of the four macromolecules
(carbohydrates, lipids, proteins, and nucleic acids)? (Hint: Amino
acids are examples of these building blocks)
• What do we call chains of building blocks of the four
macromolecules? (Hint: Polypeptides are an example of these
chains)
Frederick Griffith’s Experiment
• R vs. S strain pneumonia bacteria
Frederick Griffith’s Experiment (ctd)
• Why did the mouse in the fourth trial die?
Heat destroyed the S strain bacteria by breaking apart the bacterial cells, but
it did not destroy the S strain DNA.
Some R strain bacteria were “transformed” by taking in S strain DNA, which
gave them the ability to build the smooth outer capsule and become S strain
bacteria.
The mouse’s immune system was unable to destroy the S strain bacteria (due
to the protective capsule), so it died.
What is transformation?
• Transformation occurs when a cell takes in DNA from its surroundings
(like the R strain bacteria taking in DNA from the heat-killed S strain
bacteria in Trial 4 of Griffith’s experiment)
Did Frederick Griffith know that the genetic
molecule of the bacteria was DNA?
• He did not know that the genetic molecule of the bacteria was DNA.
He knew some molecule had been passed from the heat-killed S
strain to the R strain bacteria to cause them to build a smooth
capsule.
• He called this unknown molecule the “transforming principle.”
Avery, McCarty, and Macleod’s Experiment
• Tried to determine which
molecule was the “genetic
molecule” of the
pneumonia bacteria used
in Griffith’s experiment
Avery, McCarty, and Macleod’s Experiment
(ctd)
• Results = transformation of the R strain bacteria to make them S
strain bacteria…
-did occur when enzymes for breaking down RNA, proteins,
lipids, and carbohydrates were used
-did NOT occur when enzymes for breaking down DNA were used
• Conclusions = DNA must be the genetic molecule of the bacteria cells
because when it is destroyed, the R strain bacteria are not
transformed
Hershey and Chase Experiment
Hershey and Chase Experiment (ctd)
• Trial #1: Radioactively labeling sulfur (35S) found in the protein coat of the
virus
• Results: The radioactive sulfur was only found outside the bacterial host
cell.
• Conclusion: The protein coat of the virus (aka capsid) does not get injected
into the host cell during infection
Hershey and Chase Experiment (ctd)
• Trial #2: Radioactively labeling phosphorus (32P) found in the DNA of the
virus
• Results: The radioactive sulfur was only found inside the bacterial host cell
• Conclusion: The DNA of the virus gets injected into the host cell during
infection
Hershey and Chase Experiment (ctd)
• Overall Conclusion =
DNA is the genetic
molecule of viruses
because it is injected
into host cells during
infection and directs
the host cell to create
baby viruses
Franklin + Wilkins and Watson + Crick
• X-ray crystallography (aka X-ray diffraction)
DNA Structure
• DNA is an example of which type of macromolecule (carbohydrate,
lipid, protein, nucleic acid)?
DNA Structure (ctd)
• Full Name of DNA = deoxyribonucleic acid
• Function of DNA = to store genetic
information
• Monomer of DNA = Nucleotide
• Overall Structure of DNA = two coiled
chains of nucleotides called the double
helix
-Double because there are two chains
-Helix because the chains are twisted
around each other in a spiral fashion
DNA Structure (ctd)
Nucleotide Structure
• Three parts of a nucleotide
-Phosphate group
-Deoxyribose (aka 5-carbon
sugar or pentose sugar)
-Nitrogenous base (aka
nitrogen base)
DNA Double Helix
• Untwisted double helix looks like a
ladder
-Sides of the ladder
(backbone of each chain of
nucleotides) = phosphate
groups bonded to deoxyribose
sugars
-Rungs of the ladder =
nitrogen bases bonded
together by weak hydrogen
bonds
DNA is Antiparallel
• 5 prime (5’) end of a DNA strand
(one chain of nucleotides) = ends
with a phosphate group
• 3 prime (3’) end of a DNA strand =
ends with a sugar
• The strands of the double helix
are antiparallel, which means
they are side-by-side (parallel) but
their 5’ and 3’ ends are opposite
(anti) one another
Nitrogen Bases
• Two types of bases =
-Purines have a double-ring
structure
-Pyrimidines have a single- ring
structure
Memory trick = the name lengths are
opposite the size of the molecule. “Purine”
is a shorter word but has a larger (double
ring) structure. “Pyrimidine” is a longer
word but has a smaller (single ring)
structure
• Purines = adenine and guanine
• Pyrimidines = thymine and cytosine
Hydrogen Bonds between Nitrogen Bases
• Purines only pair with pyrimidines
across the DNA double helix. They
are connected by hydrogen bonds.
• Two hydrogen bonds connect
adenine and thymine.
• Three hydrogen bonds connect
guanine and cytosine.
Why do purines only pair with pyrimidines?
• Pyrimidines cannot pair with
pyrimidines because they are so
small that they don’t get close
enough to each other (given the
width of the double helix) to form
hydrogen bonds with each other
• Purines cannot pair with purines
because they are so large that they
would not fit within the width of
the double helix
Chargaff’s Experiment
• Determined the percentages of each nitrogen base in the DNA double
helix. These frequencies (in humans) were…
- A = 30%
- T = 30%
- C = 20%
- G = 20%
Chargaff’s Experiment (ctd)
• He looked at the
frequencies of nitrogen
bases for the DNA in other
species as well!
Chargaff’s Experiment (ctd)
• Conclusion: Adenine must bond (aka pair) with thymine across the
double helix because they have equal frequencies. This means where
you find an adenine, you will always find a thymine and vice versa.
For the same reason, he concluded that guanine must pair with
cytosine.
• As such, A / T and G / C are said to be complementary to each other.
Their bonding (aka pairing) across the double helix is therefore called
complementary base pairing.
Practice Questions
• If there is 30% adenine in a particular DNA molecule, how much
cytosine is present?
Practice Questions (ctd)
• Write out the nitrogen base sequence of a DNA strand
complementary to the following strand.
T
T
A
G
C
A
T
G
G