The discovery of DNA Double Helix is one of the greatest
findings of all time, but it’s also one of the most controversial
KEY SCIENTISTS INVOLVED
James Watson
Francis Crick
Rosalind Franklin
Maurice Wilkins
Issues to Cover:
What is DNA?
Scientific Thinking before 1953
Social and Cultural Background
Watson and Crick’s Contribution
Franklin’s Contribution
• 2 Forms of DNA
• X-ray Crystallography
Paradigm Shift
DNA Applications
What is DNA?
DNA (Deoxyribonucleic Acid)
DNA is a double stranded
molecule that is twisted into a
Helix (Spiraling Staircase)
DNA Double Helix
Each strand consists of:
1) A Sugar Phosphate Backbone
Each strand consists of:
1) A Sugar Phosphate Backbone
2) Four Base Chemicals
(Attached in Pairs)
Perkins Model
1. Long Search:
The search for the secret of life dates back nearly a century
prior to Watson and Crick’s ground breaking discovery in 1953
1865
1909
1911
1929
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1950
The Early Efforts
Gregor Mendel:
Introduces the concept of heredity
1865
1909
1911
1929
1944
1950
The Early Efforts
Wilhelm Johannsen:
Coins the term “Gene”
1865
1909
1911
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1944
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The Early Efforts
Thomas Hunt Morgan:
Discovers that genes are responsible for inheritance
1865
1909
1911
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1944
1950
The Early Efforts
Phoebus Levene:
Discovers that DNA is made up of nucleotides, phosphates,
sugars and 4 bases
1865
1909
1911
1929
1944
1950
The Early Efforts
Oswald Avery:
Shows that DNA can transform the property of cells
However, this idea was not universally
accepted
1865
1909
1911
1929
1944
1950
The Early Efforts
Erwin Chargaff:
Shows that: A + G = T + C = 50%
Chargaff’s Rule is an important equation in the
discovery of the structure of DNA
1865
1909
1911
1929
1944
1950
Background
Soon after WWII the race to discover the “Secret of Life” was on.
Scientists knew this would be the discovery of the century and
would guarantee a Nobel Prize.
King’s College
Caltech University
Rosalind Franklin Maurice Wilkins
VS
Cambridge University
Linus Pauling
James Watson
Francis Crick
King’s College
• Rosalind Franklin accepts a job at King’s College to work in Maurice Wilkins’ Lab
• At the time of her arrival Wilkins is away on other matters
• When Wilkins returns, he discovers that his lab is taken over by Franklin
• He notices that Franklin is very intelligent and is able to work independently
• She spends hundreds of hours perfecting her X-ray Diffraction images and
hand written calculations
• Wilkins is still under the impression that Franklin is his assistant
• There is a lot of tension in the lab due to miscommunication and personality
clashes
King’s College
Franklin
Wilkins
Franklin’s Work
TWO FORMS OF DNA
In 1951 Rosalind Franklin discovers the Two Forms of DNA
through her X-ray diffraction images.
A – Dry Form
B – Wet Form
Photo 51
X-Ray Crystallography
Watson and Crick’s Work
Watson and Crick’s Work
In 1951 James Watson traveled from the United States to work with
Francis Crick at Cambridge University
Watson and Crick used the “Model Building” approach
They physically built models out of wire, sheet metal, nuts and bolts to
come up with the structure of DNA
Why did they build models?
“Sometimes the fingers can grasp what the mind cannot”
(Biology the Science of Life)
Perkins Model
2. Little Apparent Progress
Watson & Crick’s early model of DNA showed that the structure:
• Was a Triple Helix
• Had the Bases on the Outside
• Had the Sugar Phosphate Backbone on the Inside
However, Franklin disproves their model.
This sends them back to the drawing board.
Watson and Crick’s supervisor stops them from model
building.
Social and Cultural Environment
• Science at the time was a male dominated field
• Women did not receive equal treatment
• Women were forbidden in the lunch room
• Franklin is unhappy at King’s College
• She has no one to share ideas and collaborate with
• She is fed up and decides to leave King’s College
But before she does, Franklin promises to finish writing up her research
papers
In the process of her move, Wilkins gets hold of Franklin’s work which
contained the famous photo 51
Perkins Model
3. Precipitating Event
In 1951, Maurice Wilkins gives James Watson
Rosalind Franklin’s work
Perkins Model
4. Cognitive Snap
Watson: “The instant I saw the picture my mouth fell open and
my pulse began to race.”
Page 196: Rosalind Franklin The Dark Lady of DNA
How Watson and Crick Solved the DNA Structure
They already knew from Franklin and Wilkins’ work
that DNA was in the form of a Double Helix
Photo 51
They used Chargaff’s Rule to figure out how the 4 Bases
match up in pairs
How Watson and Crick Solved the DNA Structure
They discovered that:
• The Phosphate Backbone was on the outside
• Which protected the Bases on the inside
• DNA acts as a Template or a Copying Mechanism for reproduction
“Nature”
Watson & Crick quickly published their Scientific Journal
called “Nature” on April 25th 1953
The Nobel Prize
• In 1962 Watson, Crick & Wilkins won the Nobel Prize for their
• discovery of the structure of DNA
• However, there is no mention of Franklin’s key work.
Perkins Model
5. Transformation
Paradigm Shift:
Prior to Watson and Crick’s discovery it was believed that:
• Proteins were responsible for heredity because of their complex structure
• DNA was too simple for heredity because it only contained 4 bases
Discovering the Structure of DNA enabled the Scientific
Community to understand the “Secret of Life” and how all life
on earth is passed down from generation to generation
DNA Applications
Science and Technology have advanced astronomically since
the discovery in 1953
Here are some of the achievements since that time:
• Cloning
• Genetically Modified Foods
• Genetic Engineering
• DNA Fingerprinting/Analysis
Thank You!