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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)
 Rosalind Franklin (1953)
 Meselson & Stahl (1958)

AP Biology
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
AP Biology
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?!
AP Biology
Mendel, Morgan, Beadle and
Tatum
 http://www.youtube.com/watch?v=EGz
Fsme22s8&feature=related (Bill Nye)
AP Biology
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
AP Biology
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?

AP Biology
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?
AP Biology
35S
radioactivity
found in the medium
32P
radioactivity found
in the bacterial cells
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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!
AP Biology
1952 | 1969
Hershey
Hershey & Chase
AP Biology
Martha Chase
Alfred Hershey
Video (Mcgraw-hill)
 http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=s
wf::535::535::/sites/dl/free/0072437316/1
20076/bio21.swf::Hershey%20and%20C
hase%20Experiment
AP Biology
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%
AP Biology
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
 Maurice Wilkins
 Linus Pauling
AP Biology
Franklin
Wilkins
Pauling
1953 article in Nature
Watson and Crick
Watson
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Crick
Rosalind Franklin (1920-1958)
AP Biology
H&C, W&C, Franklin, Wilkins
 http://www.youtube.com/watch?v=iLx_
Uz_xiAQ&feature=related (Bill Nye)
AP Biology
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
AP Biology
Can you design
a nifty experiment
to verify?
semiconservative
dispersive
Meselson & Stahl
Matthew Meselson
Franklin Stahl
Franklin Stahl
Matthew Meselson
AP Biology
 http://www.pbs.org/wgbh/nova/photo51
/media/anatomy.swf
Rosalin Franklin animation- on own time
AP Biology
Scientific History
 March to understanding that DNA is the genetic material

T.H. Morgan (1908)
 genes are on chromosomes

Frederick Griffith (1928)
 a transforming factor can change phenotype

Avery, McCarty & MacLeod (1944)
 transforming factor is DNA

Erwin Chargaff (1947)
 Chargaff rules: A = T, C = G

Hershey & Chase (1952)
 confirmation that DNA is genetic material

Watson & Crick (and Franklin) (1953)
 determined double helix structure of DNA

AP Biology
Meselson & Stahl (1958)
 semi-conservative replication
Science …. Fun
Party Time!
Any Questions??
AP Biology
2006-2007