Half Hollow Hills High School
Ms. Foglia
AP Biology
Chapter 16.
DNA
The Genetic Material
Replication
AP Biology
2004-2005
Scientific History
 The march to understanding that DNA
is the genetic material
T.H. Morgan (1908)
 Frederick Griffith (1928)
 Avery, McCarty & MacLeod (1944)
 Hershey & Chase (1952)
 Watson & Crick (1953)
 Meselson & Stahl (1958)

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Half Hollow Hills High School
Ms. Foglia
AP Biology
Genes are on chromosomes
 T.H. Morgan
1908|1933
working with Drosophila (fruit flies)
 genes are on chromosomes
 but is it the protein or the DNA of the
chromosomes that are the genes?

 through 1940 proteins were thought to be
genetic material… Why?
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1928
The “Transforming Factor”
 Frederick Griffith

Streptococcus pneumonia bacteria
 was working to find cure for pneumonia
harmless live bacteria mixed with heatkilled infectious bacteria causes disease
in mice
 substance passed from dead bacteria to
live bacteria = “Transforming Factor”

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Ms. Foglia
AP Biology
The “Transforming Factor”
Transformation: change in genotype & phenotype
due to assimilation of external DNA by a cell
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2004-2005
1944
DNA is the “Transforming Factor”
 Avery, McCarty & MacLeod

purified both DNA & proteins 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
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What’s the conclusion?
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Ms. Foglia
AP Biology
Avery, McCarty & MacLeod
Oswald Avery
Colin MacLeod
Maclyn McCarty
AP Biology
Confirmation of DNA
 Hershey & Chase
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1952|1969
classic “blender” experiment
 worked with bacteriophage

 viruses that infect bacteria

grew phage viruses in 2 media,
radioactively labeled with either:



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2004-2005
35S
32P
in their proteins
in their DNA
infected bacteria with
labeled phages
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Ms. Foglia
AP Biology
Hershey & Chase
AP Biology
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Martha Chase
Alfred Hershey2004-2005
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Half Hollow Hills High School
Ms. Foglia
AP Biology
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2004-2005
Blender experiment
 Radioactive phage & bacteria in blender
 35
S phage
 radioactive proteins stayed in supernatant
 therefore protein did NOT enter bacteria
 32
P phage
 radioactive DNA stayed in pellet
 therefore DNA did enter bacteria

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DNA is confirmed as “transforming
factor”
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Half Hollow Hills High School
Ms. Foglia
AP Biology
Chargaff
 DNA composition: “Chargaff’s rules”
1947
varies from species to species
 amounts of 4 bases not equal
 bases present in characteristic ratio

 humans:
A=30.9%
T=29.4%
G=19.9%
C=19.8%
AP Biology
2004-2005
Structure of DNA
 Watson & Crick

1953|1962
developed double helix model of DNA
 other scientists working on question:
 Linus Pauling
 Maurice Wilkins
 Rosalind Franklin
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Ms. Foglia
AP Biology
Rosalind Franklin (1920-1958)
AP Biology
Watson and Crick
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1953 article in Nature
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AP Biology
Double helix structure of DNA
the structure of DNA suggested a mechanism for
2004-2005
how DNA is copied by the cell
AP Biology
Anti-parallel strands
 Phosphate to sugar bond
involves carbons in 3’ & 5’
positions
DNA molecule has
“direction”
 complementary strand
runs in opposite direction

“It has not escaped our notice that
the specific pairing we have
postulated immediately suggests a
possible copying mechanism for the
genetic
material.”
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Half Hollow Hills High School
Ms. Foglia
AP Biology
Bonding in DNA
5’
hydrogen
bonds
3’
phosphodiester
bonds
3’
5’
….strong or weak bond?
How
do the bonds fit the mechanism for copying2004-2005
DNA?
AP
Biology
Base pairing in DNA
 Purines
adenine
 guanine

 Pyrimidines
thymine
 cytosine

 Pairing
A:T
C : G

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