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

Rosalind Franklin

James Watson Francis Crick

Maurice Wilkins

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:

2) Four Base Chemicals (Attached in Pairs)

1) A Sugar Phosphate Backbone

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

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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 1929 1944 1950

The Early Efforts

Thomas Hunt Morgan: Discovers that genes are responsible for inheritance

1865 1909 1911 1929 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

1865 1909 1911 1929 1944 1950

The Early Efforts

However, this idea was not universally accepted

Erwin Chargaff: Shows that: A + G = T + C = 50%

1865 1909 1911 1929 1944 1950

The Early Efforts

Chargaff’s Rule is an important equation in the discovery of the structure of DNA

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.

VS

James Watson Francis Crick

Cambridge University

Rosalind Franklin Maurice Wilkins

King’s College

Caltech University

Linus Pauling

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

Franklin Wilkins

King’s College

Franklin’s Work

In 1951 Rosalind Franklin discovers the Two Forms of DNA through her X-ray diffraction images.

A – Dry Form B – Wet Form

TWO FORMS OF DNA

Photo 51

X-Ray Crystallography

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)

Watson and Crick’s Work

Watson & Crick’s early model of DNA showed that the structure:

However, Franklin disproves their model.

• Was a Triple Helix• Had the Bases on the Outside• Had the Sugar Phosphate Backbone on the Inside

This sends them back to the drawing board.Watson and Crick’s supervisor stops them from model building.

• 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

Social and Cultural Environment

In 1951, Maurice Wilkins gives James Watson Rosalind Franklin’s work

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

They used Chargaff’s Rule to figure out how the 4 Bases match up in pairs

Photo 51

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

Watson & Crick quickly published their Scientific Journal called “Nature” on April 25th 1953

“Nature”

• In 1962 Watson, Crick & Wilkins won the Nobel Prize for their • discovery of the structure of DNA

The Nobel Prize

• However, there is no mention of Franklin’s key work.

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

• Cloning• Genetically Modified Foods• Genetic Engineering• DNA Fingerprinting/Analysis

Science and Technology have advanced astronomically since the discovery in 1953

Here are some of the achievements since that time: