DNA
The Genetic Material
AP Biology
2006-2007
Scientific History
 The march to understanding that DNA is
the genetic material
T.H. Morgan (1908)
 Frederick Griffith (1928)
 Avery, McCarty & MacLeod (1944)
 Erwin Chargaff (1947)
 Hershey & Chase (1952)
 Watson & Crick (1953)
 Meselson & Stahl (1958)

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1908 | 1933
Chromosomes related to phenotype
 T.H. Morgan

working with Drosophila
 fruit flies

associated phenotype with
specific chromosome
 white-eyed male had specific
X chromosome
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1908 | 1933
Genes are on chromosomes
 Morgan’s conclusions
genes are on chromosomes
 but is it the protein or the
DNA of the chromosomes
that are the genes?

 initially proteins were thought
to be genetic material…
Why?
What’s so impressive
about proteins?!
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The “Transforming Principle”
 Frederick Griffith

Streptococcus pneumonia bacteria
 was working to find cure for pneumonia
harmless live bacteria (“rough”)
mixed with heat-killed pathogenic
bacteria (“smooth”) causes fatal
disease in mice
 a substance passed from dead
bacteria to live bacteria to change
their phenotype


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“Transforming Principle”
1928
The “Transforming Principle” mix heat-killed
live pathogenic
strain of bacteria
A.
mice die
live non-pathogenic heat-killed
strain of bacteria
pathogenic bacteria
B.
C.
mice live
mice live
pathogenic &
non-pathogenic
bacteria
D.
mice die
Transformation = change in phenotype
something in heat-killed bacteria could still transmit
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disease-causing properties
1944
DNA is the “Transforming Principle”
 Avery, McCarty & MacLeod

purified both DNA & proteins separately from
Streptococcus pneumonia bacteria
 which will transform non-pathogenic bacteria?

injected protein into bacteria
 no effect

injected DNA into bacteria
 transformed harmless bacteria into
virulent bacteria
mice die
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What’s the
conclusion?
1944 | ??!!
Avery, McCarty & MacLeod
 Conclusion

first experimental evidence that DNA was the
genetic material
Oswald Avery
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Maclyn McCarty
Colin MacLeod
1952 | 1969
Confirmation of DNA
 Hershey & Chase
classic “blender” experiment
 worked with bacteriophage

 viruses that infect bacteria

Why use
Sulfur
vs.
Phosphorus?

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grew phage viruses in 2 media,
radioactively labeled with either

35S
in their proteins
 32P in their DNA
infected bacteria with
labeled phages
Hershey
Protein coat labeled
with 35S
Hershey
& Chase
DNA labeled with 32P
T2 bacteriophages
are labeled with
radioactive isotopes
S vs. P
bacteriophages infect
bacterial cells
bacterial cells are agitated
to remove viral protein coats
Which
radioactive
marker is found
inside the cell?
Which molecule
carries viral
genetic
info?
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35S
radioactivity
found in the medium
32P
radioactivity found
in the bacterial cells
Blender experiment
 Radioactive phage & bacteria in blender

35S
phage
 radioactive proteins stayed in supernatant
 therefore viral protein did NOT enter bacteria
 32
P phage
 radioactive DNA stayed in pellet
 therefore viral DNA did enter bacteria

Confirmed DNA is “transforming factor”
Taaa-Daaa!
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1952 | 1969
Hershey
Hershey & Chase
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Martha Chase
Alfred Hershey
By using paper chromatography,
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By using paper chromatography,
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By using paper chromatography,
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Chargaff
 DNA composition: “Chargaff’s rules”
varies from species to species
 all 4 bases not in equal quantity
 bases present in characteristic ratio

 humans:
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
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That’s interesting!
What do you notice?
Rules
A = T
C = G
1947
1953 | 1962
Structure of DNA
 Watson & Crick

developed double helix model of DNA
 other leading scientists working on question:
 Rosalind Franklin: X-ray crystallography
 Maurice Wilkins
 Linus Pauling
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Franklin
Wilkins
Pauling
1953 article in Nature
Watson and Crick
Watson
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Crick
Rosalind Franklin (1920-1958)
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But how is DNA copied?
 Replication of DNA

base pairing suggests
that it will allow each
side to serve as a
template for a new
strand
“It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic
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material.”
— Watson & Crick
Models of DNA Replication
 Alternative models

become experimental predictions
conservative
P
1
2
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Can you design
a nifty experiment
to verify?
semiconservative
dispersive
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Old Strand
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Old Strand
New Strand
Old Strand
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Old Strand
New Strand
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THE
SEMICONSERVATIVE
MODEL
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One Strand of DNA
 The backbone
of the molecule
is alternating
phosphate and
deoxyribose, a
sugar, parts.
 The teeth are
nitrogenous
bases.
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phosphate
deoxyribose
bases
Nucleotides
 One deoxyribose together
with its phosphate and
base make a nucleotide.
O
O -P O
O
Phosphate
Nitrogenous
base
O
C
C
C
O Deoxyribose
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Thymine and Cytosine are
pyrimidines
 Thymine and cytosine each have
one ring of carbon and nitrogen
N
atoms.
O
N
O
N
C
C
C C
N
C
thymine
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O
C
C
C
N
C
cytosine
Adenine and Guanine are purines
 Adenine and guanine each have
two rings of carbon and nitrogen
atoms.
O
N
N
N
C
C
N
C
C
C
C
N
N
N
Adenine
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N
C
N
C
Guanine
C
C
N
N
Hydrogen Bonds

C
C
N
N
C
N
C
N
C
C
C
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N
N
C
O

each other because
of hydrogen bonds.
Hydrogen bonds are
weak but there are
millions and millions
of them in a single
molecule of DNA.
(The bonds between
cytosine and guanine
are shown here.)
C
 The bases attract
N
O
Hydrogen Bonds, cont.
 When making


hydrogen bonds,
cytosine always
pairs up with
guanine,
And adenine
always pairs up
with thymine.
(Adenine and
thymine are shown
here.)
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O
N
O
C
C
C C
N
C
Important:
• Adenine and Thymine always
join together in a double
bond
A
T
• Cytosine and Guanine
always join together in a
triple bond
C
G
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DNA Replication
 When cells reproduce, then need to be

able to pass the “cell template” to the
new cell they are creating
In order to accomplish this, they need
to make an entirely new copy of the
genes within the nucleus.

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DNA Replication: the process by which
DNA in a cell is copied before it
undergoes cell division
DNA replication begins at sites of origin
along the DNA and proceeds in both
directions.
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DNA Replication
 Once the DNA is separated,
complementary base pairs bind to the
now separated DNA strands
The enzyme called DNA polymerase is
used to facilitate this process
 This process is creating two identical
strands

 One strand is from the original double helix
 The other strand is being newly created by
the DNA polymerases
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The two
strands of DNA
are antiparallel.
Problem:
DNA
polymerase
only add
nucleotides
to the 3'
end. Goes
from 5’  3’
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All DNA is
replicated at
special sites
called
“origins of
replication”
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DNA Replication
 Since DNA is in a double helix, it is first
necessary to separate the two strands
from each other
This is done by an enzyme known as a
helicase
 The helicases “unzip” the DNA double
helix
 The point at which the helicases work is
called the replicating fork

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Helicases
unwind the
helix.
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Singlestrand
binding
proteins
hold the
strands
apart.
Topoisomerases
“cut” one
strand to
allow the
snarls to
untangle.
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• Since DNA
polymerase
can only add
bases to the
3’ end of the
DNA, you
need RNA
primers to
begin the
process by
placing an
RNA “primer”
at the
beginning.
Priming DNA synthesis
DNA polymerase can’t start a new strand – it
can only add to the 3' end of an alreadystarted
strand.
The primer is
a short
segment of RNA
synthesized by the enzyme PRIMASE. Each
primer is replaced by DNA.
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LEADING STRAND VS. LAGGING
STRAND
 When DNA is replicated, there are 2
directions in which it is replicated

Leading strand (goes from 3’  5’
direction)
 It is continuous

Lagging strand (also goes from 3’  5’,
but the replicating fork is “in the way”
 Not continuous
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LAGGING STRAND
 Since the lagging strand is not a
continuous synthesis, it is built in
smaller segments
RNA primers start at several areas on
the DNA template and are extended by
DNA polymerase
 These newly synthesized fragments of
DNA are called Okazaki fragments
 They create gaps in the newly formed
DNA strand that are filled in later with
an enzyme called DNA ligase

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DNA Polymerase I vs DNA
Polymerase III
DNA Polymerase III is the enzyme
that adds the nucleotides from the
5’  3’
DNA Polymerase I is the enzyme
that removes the RNA primers on
the lagging strand and replaces it
with DNA
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ERRORS IN REPLICATION
 For every billion base pairs that are
created, there is an error in replication
(one base is added to the strand or
deleted that shouldn’t be)
 The reason for this low error is because
the DNA polymerase has a proofreading function
The DNA is “proofread” as it is
created
 If an error is detected, the DNA
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polymerase can repair the error

Excision
repair of DNA
damage.
Ex: Thymine
dimers are
formed from UV
radiation.
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