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Biology---Yippee!
DNA
The Genetic Material
AP Biology
2008-2009
Scientific History
 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)

1908 | 1933
Genes are on chromosomes
 T.H. Morgan
working with fruit flies
 Determined: genes are on
chromosomes
 The question then was: is it
the protein or the DNA of the
chromosomes that are the
genes?

 through 1940 proteins
were thought to be
genetic material because their
structure is more complex
Frederick Griffith

was working to find cure for
pneumonia (Streptococcus
pneumonia bacteria)
harmless live bacteria mixed
with heat-killed infectious
bacteria causes disease in
mice
 substance passed from dead
bacteria to live bacteria =
“Transforming Factor”

1928
The “Transforming Factor”
live pathogenic
strain of bacteria
A.
mice die
live non-pathogenic heat-killed
strain of bacteria
pathogenic bacteria
B.
C.
mice live
mice live
mix heat-killed
pathogenic &
non-pathogenic
bacteria
D.
mice die
Transformation
something in heat-killed bacteria could still transmit
disease-causing properties to the harmless bacteria
 Avery, McCarty & MacLeod
1944
purified both DNA & proteins from
Streptococcus pneumonia bacteria to
determine which will transform nonpathogenic bacteria?
 injected protein into bacteria
What’s the

 no effect

injected DNA into bacteria
 transformed harmless bacteria
into virulent bacteria
 Concluded  DNA is the
transforming factor
conclusion?
1952 | 1969
Confirmation of DNA
 Hershey & Chase
classic “blender” experiment
 worked with bacteriophage

 viruses that infect bacteria

grew phage viruses in 2 media,
radioactively labeled with either

35S
in their proteins
 32P in their DNA

infected bacteria with
labeled phages
Hershey
Blender experiment
 Radioactive phage & bacteria in blender

35S
phage
 radioactive proteins did NOT enter bacteria
 32
P phage
 radioactive DNA did enter bacteria

Confirmed DNA is “transforming factor”
Taaa-Daaa!
Summary
 Used radioactive labels to identify





genetic material
Labeled DNA with radioactive
phosphorus (32P)
Labeled protein coats with radioactive
sulfur (35S)
Blender
Examined newly infected host cells
(pellet) and found only radioactive
phosphorus label (not sulfur)
***DNA is the genetic material
1952 | 1969
Hershey
Hershey & Chase
Martha Chase
Alfred Hershey
Nucleic Acids
(review) section 10.2
 Examples:

RNA (ribonucleic acid)
 single helix

DNA (deoxyribonucleic acid)
 double helix
 Structure:

monomers = nucleotides
DNA
RNA
Nucleotides
3 parts
nitrogen base
1.

2.
Adenine, guanine, cytosine & thymine
pentose sugar (5 Carbon)
ribose in RNA
deoxyribose in DNA
3.
phosphate (PO4) group
Nitrogen base
I’m the
A,T,C,G or U
part!
Erwin Chargaff
1947
 DNA composition: “Chargaff’s rules”
varies from species to species
 Amount of thymine = amount of adenine
 Amount of cytosine = amount of guanine

1953 | 1962
Structure of DNA
 Watson & Crick

developed double helix model of DNA
 other scientists working on question:
 Rosalind Franklin
 Maurice Wilkins
 Linus Pauling
Franklin
Wilkins
Pauling
Rosalind Franklin (1920-1958)
Rosalind Franklin & Maurice Wilkins
(1950)
1. used X-ray crystallography to study the structure
of DNA.


In this technique, X-rays are diffracted as they
passed purified DNA.
The diffraction pattern can be used to deduce the
three-dimensional shape of molecules.
1953 article in Nature
Watson and Crick
Watson
Crick
James Watson and Francis Crick
1. Concluded that the structure of DNA is a
2.
3.
double helix. (After Watson viewed
Franklin’s x-ray diffraction photo)
Backbone consists of alternating sugar &
phosphate units.
Attached to the backbone are four kinds of
bases.
I. Adenine
III. Cytosine
II. Guanine
IV. Thymine
Twist
Fig. 16.5
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copying DNA
 Replication of DNA

base pairing allows
each strand to serve
as a template for a
new strand
Semi-conservative replication
1958
Franklin Stahl
Matthew Meselson
 Meselson & Stahl experiment used radioactive
nitrogen to determine that DNA replication is
semi-conservative (each of the 2 new DNA
molecules is half original or parent DNA and
half newly made)
DNA Replication
AP Biology
2008-2009
DNA Replication
 DNA used to make DNA
 Making an exact copy of the DNA
before the cell divides
 original strand serves as a template for
the new strand
 Each resulting double-stranded DNA
molecule is made of one original and
one new strand ( ½ parent template and
½ new DNA)  semi-conservative
replication
Anti-parallel strands
DNA molecule has
“direction”
 complementary strand runs
in opposite direction
 Replication only occurs in
the 5’ to 3’ direction

5
3
3
5
Bonding in DNA
5
weak bonds
hydrogen
bonds
3
covalent
phosphodiester
bonds
strong bonds
3
5
Base pairing in DNA
 Pairing

A:T
 2 bonds

C:G
 3 bonds
Replication: 1st step
 Unwind DNA

Helicase enzyme
 unwinds part of DNA helix
 stabilized by single-stranded binding proteins
helicase
single-stranded binding proteins
replication fork
Replication: 2nd step
 Build daughter DNA
strand


DNA
Polymerase III

add new complementary
bases to 3’’ end of
growing DNA strand
Enzyme 
DNA polymerase III
strand only grows
53
Loss of DNA
 With each replication, small segments
at the end of our chromosomes (called
telomeres) are lost
This may be part of the aging process
 When enough DNA is lost the cell can
no longer divide

 An enzyme called telomerase is able to
add on to the end of chromosomes –
but it is inactivated in most of our cells

Cancer cells keep the enzyme active
and can divide forever