The Structure of DNA

An example of scientists building upon each others discoveries.

How was it discovered that DNA was the molecule of heredity?

Where is hereditary information stored in the cell?

• Joachim Hammerling studied the unicellular green algae, Acetabularia

• His experiments included transplanting nuclei between two different species of Acetabularia.

Hammerling’s Conclusion:

(1943)Hereditary

information in Acetabularia is

stored in the foot of the cell,

where the nucleus resides.

Can Hereditary Information Pass between Organisms?

Fredrick Griffith’s experiments in 1928

Griffith concluded that hereditary information can pass from dead cells to living cells, transforming them. He called this transformation, a change in genotype and phenotype due to assimilation of

foreign DNA.

What Is the Genetic Material: Proteins or DNA

• What is the “agent” responsible for transforming the bacteria in Griffith’s experiment?

• In 1940s it was known that chromosomes were made up of DNA and protein.

• Predominant idea was that proteins were the genetic material.

“The Avery Experiments”—What is the “Transforming Molecule?”

• Avery, MacLeod & McCarty purified molecules from heat-killed, pathogenic bacteria & attempted to transform live, harmless bacteria

• Only DNA was able to transform the harmless bacteria into pathogenic ones.

• Conclusion: DNA is the transforming agent• Their results were met with skepticism

The Transforming Principle is DNA

• Avery and his colleagues removed 99.98% of the protein from the bacteria

• Despite the removal of protein, the bacteria were still transforming information to each other

• Chemical analysis revealed that the substance was the same as DNA

• Adding a DNA-digesting enzyme destroyed all transforming activity

Hershey & Chase Experiment, 1952 Conclusion: DNA, not protein, is the hereditary material

What Constitutes DNA?P.A. Levene, 1920’s

Chargaff’s Rules, 1947

• In any species there is an equal amount of Adenine and Thymine bases and an equal amount of Guanine and Cytosine bases

Complementary Base Pairing in DNA

A & T are paired, they form three hydrogen bonds

G & C are paired, they form two hydrogen bonds

What is the structure of DNA?

• Rosalind Franklin performed x-ray diffraction experiments on DNA in 1953

• She concluded that DNA had the shape of a helix, a diameter of 2 nanometers and a completed helical turn every 3.4 nanometers

Aha!• In 1953, James Watson

(age 23) and Francis Crick (age 43) used Rosalind Franklin’s results to determine the structure of DNA

Watson & Crick determined the base pairing rules

Nobel Prize, 1962

• Wilkins, Crick & Watson receive the Nobel Prize for the discovery of the structure of DNA.