The Double Helix: Discovery and Structure of DNA

Initial Understanding of Genetic Material

• Early 1950s: Proteins believed to carry genetic info due to diversity.

Oswald Avery's Experiments

• Claimed DNA, not proteins, carried genetic info.
• Evidence: DNA presence led to transformation; DNA destruction halted it.

DNA Nucleotide Structure

• Composed of a phosphate group, deoxyribose sugar, and a nitrogenous base.

DNA Double Helix Structure

• Two strands linked by hydrogen bonds between complementary bases: Adenine ($A$) with Thymine ($T$), Guanine ($G$) with Cytosine ($C$).
• Genetic traits determined by nucleotide sequence.

Watson and Crick's Model Evolution

• Initial triple helix model with bases inside.
• Revised after Rosalind Franklin's Photo 51 and reports, showing phosphates exterior and helical structure.

Chargaff's Rules

• Observed that in DNA, $A \approx T$ and $G \approx C$.

Photo 51 by Rosalind Franklin

• X-shape diffraction indicated a helix and dimensions.
• Confirmed DNA was a helix, phosphates exterior, and two strands.

Final Watson and Crick Claim (Double Helix Model)

• Claim: DNA is a double helix with antiparallel strands; $A$ pairs with $T$, $G$ with $C$.
• Key Evidence: Photo 51 (Franklin) for helical shape; Franklin's reports for measurements; Chargaff's Rules for base pairing.

Properties of Genetic Material Explained by DNA Structure

• Information Storage: Base sequence ($A, T, C, G$) stores genetic data.
• Consistent Replication: Double helix allows semi-conservative replication, passing info.
• Allow for Changes (Evolution): Base sequence mutations provide variation for evolution.