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DNA
(Deoxyribonucleic Acid)
History and Structure
What material is responsible for
heredity?
 Early scientists knew that there was some
material responsible for passing on traits from
parent to offspring.
 But what was the material?
Fred Griffith
 Performed an experiment with bacteria and
mice




Injected Smooth strain (with capsule) of
bacteria into a mouse → mouse died
Injected Rough strain (no capsule) of bacteria
into a mouse → mouse survived
Heat killed S strain injected into mouse →
mouse survived
Heat killed S strain mixed with living R strain
→ mouse died
 Griffith’s questioned what caused
the bacteria to change or transform.
 Protein
or DNA?
Avery, McLeod, & McCarty
 Repeated Griffith’s experiment

Before injecting heat killed S cells into the
mice, removed the protein and left the DNA


Transformation occurred → mice died
Before injecting heat killed S cells into the
mice , removed the DNA and left the protein

Transformation did not occur → mice survived
Hershey & Chase
 Used viruses (bacteriophage) and bacteria
 Batch 1:




Stained virus’s protein capsid with radioactive
sulfur
Allowed virus to infect the bacteria
Blended and centrifuged the solutions to
separate the viruses from the bacteria
Determined the radioactivity was found in the
liquid, not with the bacteria in the pellet.
 Batch 2:




Stained the DNA of the viruses with
radioactive phosphorus
Allowed virus to infect the bacteria
Blended and centrifuged the solutions to
separate the viruses from the bacteria
Determined the radioactivity was found in the
pellet with the bacteria.
 DNA is confirmed as the transforming factor.
 What is the structure of DNA?
Rosalind Franklin
 Used X-Ray diffraction to study the shape of
DNA
 Produced an image called Photo 51
Erwin Chargaff
 Determined that the percentage s of purines
are equal to the percentages of pyrimidines


Adenine percentage = Thymine percentage
Guanine percentage = Cytosine percentage
COMPOSITION OF DNA IN SEVERAL SPECIES
Purines
Source
Human
Ox
Salmon Sperm
Wheat germ
E. coli
Sea Urchin
Adenine
30.9%
29.0%
29.7%
28.1%
24.7%
32.8%
Guanine
19.9%
21.2%
20.8%
21.8%
26.0%
17.7%
Pyrimidines
Cytosine
19.8%
21.2%
20.4%
22.7%
25.7%
17.3%
Thymine
29.4%
28.7%
29.1%
27.4%
23.6%
32.1%
Watson, Crick, and Wilkins
 Used Photo 51
(without Franklin’s
permission) to
construct the first
DNA model
 Won the Nobel Prize
for discovering the
structure of DNA in
1962
Structure of DNA
Nucleotide


The smallest unit used to form DNA.

Made of 3 Parts



Phosphate Group
5-Carbon (C) sugar molecule (deoxyribose)
Nitrogen Base
Nucleotide

Nitrogen (N) Base


N Base makes the nucleotides different
4 Bases: Adenine (A), Guanine(G),
Cytosine(C), Thymine(T)

2 main groups of bases:
 Purines
- A double ring of C and N atoms
- Includes adenine and guanine
 Pyrimidines
- A single ring of C and N atoms
- Includes thymine and cytosine
Nitrogen Bases
 Adenine
Thymine
 Guanine
Cytosine
Base Pairing Rules
1. Purines only bond to pyrimidines
2.Adenine always bonds to Thymine
3.Guanine always bonds to Cytosine
Double Helix
 Structure of DNA is a double helix (a spiral
staircase of 2 strands of nucleotides twisting
around a central axis)
 Also known as a twisted ladder


The sides of the ladder are made up of
alternating sugar and phosphate units held
together by strong covalent bonds
The rungs (Steps) are made up of a purine
and a pyrimidine held together by weak
Hydrogen (H) bonds
 Base pairing rules results in 2 strands that
are complementary to each other
T
C
G
A
T
A
Double Helix