Hershey-Chase Experiment and the Discovery of DNA Structure

Introduction

Hershey-Chase Experiment (1952): Provided strong evidence that DNA contains genetic material and is the molecule that is inherited.
1950s Scientific Focus: Understanding the 3D structure of DNA to hypothesize its function, replication, and inheritance.

Nucleic Acids: Polymers made of nucleotides (monomers).
Nucleotides Components:
- Phosphate groups
- Five-sided sugar (Deoxyribose in DNA, Ribose in RNA)
- Nitrogenous base (Purines - Adenine & Guanine; Pyrimidines - Cytosine & Thymine in DNA; Uracil replaces Thymine in RNA)

Francis Crick & James Watson: Credited with the first accurate 3D model of DNA (1953).
Maurice Wilkins: Worked with Crick and Watson, won Nobel Prize.
Rosalind Franklin: Provided critical x-ray crystallography evidence (Photograph 51) supporting the helical structure of DNA.
Raymond Gosling: PhD student who assisted Franklin.
Erwin Chargoff: Discovered complementary base pairing rules (Adenine = Thymine, Guanine = Cytosine), crucial for Watson and Crick's model.
Linus Pauling: Main competitor in the DNA structure race.

Watson & Crick’s Model relied on:
- Chargoff's base pairing rules.
- X-ray crystallography by Franklin & Gosling confirming a helical structure.

Phosphodiester Bonds: Link nucleotides in a 5’ to 3’ direction, forming single DNA strands.
Anti-parallel Strands: The complementary strand runs in the opposite direction (3’ to 5’).
- Two strands connected by hydrogen bonds between complementary bases (A-T, G-C).

Twisted Ladder: Characterized by a uniform helical shape.
- Uniformity: 3.4 nanometers per turn, 2 nanometers width throughout.
- Model validated through biochemistry and experiments.

Initial skepticism faced by Watson and Crick.
Model eventually accepted, enhancing understanding of DNA function and replication.