The Race to Discover the Structure of DNA

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 sanaz

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!

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?

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 sanaz 1865 1909 1911 1929 1944 1950

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

POINT > Review a timeline of genetics discoveries Wilhelm Johannsen: Coins the term “Gene” sanaz 1865 1909 1911 1929 1944 1950 1909

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

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 sanaz 1928

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

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

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 sanaz 1950

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 sanaz

KEY SCIENTISTS INVOLVED POINT > Describe the people and events surrounding the discovery of the DNA double helix KEY SCIENTISTS INVOLVED pat

James Watson Francis Crick Rosalind Franklin Maurice Wilkins pat

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:

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

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

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**

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

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

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” sanaz

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 sanaz

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 sanaz

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

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

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

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

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

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:

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

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

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

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?

Homework: Read 344-349 Assess #1-3 page 348 Prepare for Exam tomorrow!