1. 7.1 DNA Structure and Replication
Nucleic Acids
7.1 DNA Structure and Replication

2. Data-based question
Analysis of the Hershey-Chase experiment provided evidence that DNA is the genetic material
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3. The Hershey-Chase experiment
Analysis of the Hershey-Chase experiment provided evidence that DNA is the genetic material

4. Watson, Crick, and Franklin
Making careful observations: Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA was a helix
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5. Watson and Crick’s Model
Several lines of experimental evidence came together to lead to the knowledge of the structure of DNA
Molecular modelling of Linus Pauling
X-ray diffraction by Rosaling Franklin
Base composition studies of Erwin Chagall
Insight and imagination were also important…

6. Watson and Crick’s Model
First suggestion:
Sugar phosphate strands wrapped around one another and nitrogen bases pointed out.
PROBLEM?
Nitrogen bases are relatively hydrophobic compared to the sugar-phosphate backbone

7. Watson and Crick’s Model
Franklin’s x-ray diffractions showed that the DNA was tightly packed
Bases must fit closely together
Would work if a pyramidine and purine were paired and upside down relative to each other
But what pairs?

8. Watson and Crick’s Model
Adenine has a surplus negative charge and Thyamine has a surplus positive charge
Cytosine and Guanine have the ability to form 3 hydrogen bonds
SO?

9. Watson and Crick’s Model
The idea of complementary base pairing immediately suggested a mechanism for DNA replication
Semi-conservative replication