1. The Race to Discover the Structure of DNA

2. POINT > Describe the results of Chargoff’s experiments
POINT > Review a timeline of genetics discoveries
POINT > Describe the people and events surrounding the discovery of the DNA double helix
POINT > Describe Rosalind Franklin’s contributions
POINT > Describe Watson & Crick’s model-building
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3. POINT > Describe the results of Chargoff’s experiments
Recall that DNA is made up of two strands of nucleotides- A,G, C, and T’s
In 1950, studying DNA nucleotides in cells, Erwin Chargoff found that: amount of A + G = C + T
Why?
ALWAYS! Every cell type!

4. WB CHECK:
If a cell’s DNA contains 20% cytosine, what is the percentage of guanine?
If a cell’s DNA contains 20% cytosine, what is the percentage of adenine?
If a cell’s DNA contains 15% thymine, what is the percentage of guanine?

5. POINT > Review a timeline of genetics discoveries
The scientific search for the secret of heredity dates back nearly a century prior to Watson and Crick’s ground breaking discovery in 1953
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1865
1909
1911
1929
1944
1950

6. POINT > Review a timeline of genetics discoveries
Gregor Mendel:
Introduces the concept of heredity
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1865
1909
1911
1929
1944
1950

7. POINT > Review a timeline of genetics discoveries
Wilhelm Johannsen:
Coins the term “Gene”
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1909

8. POINT > Review a timeline of genetics discoveries
Thomas Hunt Morgan:
Discovers that genes are arranged in groups
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1911

9. Discovers a “factor” that can transform bacteria genetically
POINT > Review a timeline of genetics discoveries
Frederick Griffith:
Discovers a “factor” that can transform bacteria genetically
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1928

10. POINT > Review a timeline of genetics discoveries Phoebus Levene:
Discovers that DNA is made up of nucleotides: phosphates, sugars and 4 bases
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1929

11. POINT > Review a timeline of genetics discoveries
Oswald Avery:
Shows that DNA is the hereditary material
However, this idea was not universally accepted
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1944

12. POINT > Review a timeline of genetics discoveries
Erwin Chargaff:
Shows that: A + G = T + C
Chargaff’s Rule is an important equation in the discovery of the structure of DNA
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1950

13. After WWII the race to discover the “Secret of Life” was on
POINT > Describe the people and events surrounding the discovery of the DNA double helix
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
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14. KEY SCIENTISTS INVOLVED
POINT > Describe the people and events surrounding the discovery of the DNA double helix
KEY SCIENTISTS INVOLVED
pat

15. James Watson
Francis Crick
Rosalind Franklin
Maurice Wilkins
pat

16. Many teams of Scientists were involved in this race. Most Importantly:
Rosalind Franklin
Maurice Wilkins
King’s College
Caltech University
Linus Pauling VS
James Watson
Francis Crick
Cambridge University
Many teams of Scientists were involved in this race. Most Importantly:

17. POINT > Describe Franklin’s contributions
Rosalind Franklin accepts a job at King’s College to work in Maurice Wilkins’ Lab. Wilkins is away on other matters
When Wilkins returns, he discovers that his lab is taken over by Franklin
Wilkins notes that Franklin is very intelligent and works independently
matt
Franklin
Wilkins
King’s College

18. POINT > Describe Franklin’s contributions
Franklin 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 tension in the lab due to miscommunication and personality clashes
Franklin
Wilkins
King’s College
matt

19. POINT > Describe Franklin’s contributions
In 1951 Rosalind Franklin discovers “two forms of DNA” through her X-ray diffraction images
Mat
A – Dry Form
B – Wet Form
**Photo 51**

20. WB CHECK:
What scientific technique did Rosalind Franklin use?
Whose lab did Franklin work in at King’s college?

21. Watson and Crick’s Work
POINT > Describe the people and events surrounding the discovery of the DNA double helix
Watson and Crick’s Work

22. Watson and Crick used a “model building” approach
POINT > Describe the people and events surrounding the discovery of the DNA double helix
Watson and Crick used a “model building” approach
They built models out of wire, sheet metal, nuts and bolts to try to determine the structure of DNA
Why they built models:
“Sometimes the fingers can grasp what the mind cannot”
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23. Watson & Crick’s early model of DNA predicted that the DNA structure:
POINT > Describe the people and events surrounding the discovery of the DNA double helix
Watson & Crick’s early model of DNA predicted that the DNA structure:
was a triple helix
had the bases on the outside of the helix
- had the sugar-phosphate backbone on the inside of the helix
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24. POINT > Describe the people and events surrounding the discovery of the DNA double helix
Rosalind Franklin’s work showed that the Watson and Crick model was not structurally possible
Her research showed that the sugar-phosphate backbone had to be on the outside edges of the helix
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25. WB CHECK:
What was wrong with Watson and Crick’s early DNA models?
a) they thought DNA was a double helix
b) they thought DNA was a triple helix
c) they thought the sugar-phosphate backbone was on the inside of the helix, with bases on the outside
d) they thought the sugar-phosphate backbone was on the outside of the helix, with bases on the inside
e) b and c
f) b and d
g) a and d

26. WB CHECK:
Whose work convinced Watson and Crick that their models needed to be revised?

27. Social and Cultural Environment
POINT > Describe the people and events surrounding the discovery of the DNA double helix
Social and Cultural Environment
Science at the time was a male dominated field and women did not receive equal treatment
For example, women were forbidden in the lunchroom!
Franklin was unhappy at King’s College. Feeling isolated and frustrated, she decides to leave King’s
In the process of her move, Wilkins examines Franklin’s work, which contained Photo 51
will

28. In 1951, Wilkins gives Rosalind Franklin’s work to James Watson
POINT > Describe the people and events surrounding the discovery of the DNA double helix
In 1951, Wilkins gives Rosalind Franklin’s work to James Watson
Watson: “The instant I saw the picture my mouth fell open and my pulse began to race.”*
*From: “Rosalind Franklin, The Dark Lady of DNA”
Matt

29. POINT > Describe the people and events surrounding the discovery of the DNA double helix
Watson and Crick knew from Franklin’s work that DNA was a double helix with bases on the inside
Photo 51

30. POINT > Describe the people and events surrounding the discovery of the DNA double helix
They used Chargaff’s Rule to figure out how the 4 nitrogenous bases match up in pairs:

31. POINT > Describe the people and events surrounding the discovery of the DNA double helix
Watson & Crick published their findings in the scientific journal Nature on April 25th 1953

32. In 1962 Watson, Crick & Wilkins won the Nobel Prize for their discovery of the structure of DNA
There is no mention of Rosalind Franklin’s key work

33. DNA science and technology have advanced astronomically since the discovery in 1953:
Countless advances in medicine and disease etymology:
Cloning
Genetically modified foods
Genetic engineering
DNA fingerprinting/analysis
Watson and Crick received recognition and fame from their discovery, and they earned it. Many others, like Rosalind Franklin also deserved credit. This is common in the history of science!
Will

34. Question that arose after the discovery of DNA structure: DNA never leaves the nucleus Proteins do almost all of the work all around the cell DNA contains the information to build the proteins How does the DNA code lead to protein production?

35. Homework:
Read
Assess #1-3 page 348
Prepare for Exam tomorrow!