Evidence from the Hershey–Chase Experiment

Background

• Year: 1952
• Scientists Involved: Dr. Hershey and Martha Chase
• Objective: To determine whether DNA or protein is the genetic material.

Importance of the Experiment

• Confirmed that DNA is the material basis for heredity.
• Preceded the discovery of DNA’s structure by Watson & Crick in 1953.

Context

• Chromosomes are known to be composed of DNA and proteins.
• Proteins, consisting of 20 amino acids, were initially thought to be the genetic material due to their complexity.
• DNA is composed of only 4 bases, making proteins seem like the more likely candidate.

Experiment Methodology

• Materials Used: T2 phage virus, radioactive isotopes of Sulfur (35S) and Phosphorus (32P).
• Concept: Differentiate between protein and DNA using radioactive isotopes.
  • Proteins contain sulfur but no phosphorus.
  • DNA contains phosphorus but no sulfur.

Findings

• Result: DNA, not protein, was identified as the genetic material through the use of isotopes.

DNA Structure and Stability

Purine-to-Pyrimidine Bonding

• Purines: Adenine (A), Guanine (G) - two-ring structure.
• Pyrimidines: Cytosine (C), Thymine (T), Uracil (U) - one-ring structure.
• Complementarity:
  • A pairs with T
  • G pairs with C
• Reason for Stability: Equal number of purines and pyrimidines, allowing tight packing of the DNA helix.
• X-ray Diffraction: Showed tightly packed DNA helix requiring purine-pyrimidine pairing.

Base Pairing and Stability

• Hydrogen Bonds: Form between complementary base pairs, enhancing stability.
• Electrochemical Attraction: Tight bonding due to hydrogen acceptors and donors.
• Partial Charges: Influence the number of hydrogen bonds between base pairs.

Chargaff’s Contributions

Chargaff's Findings

• Disproving Tetranucleotide Hypothesis: DNA does not have a repeating sequence with equal nucleotides.
• Chargaff’s Rule: Analyzed DNA from different species to show variation in nucleotide composition.

These notes provide an overview of the Hershey–Chase experiment, its methodology, and its implications for understanding DNA as the genetic material, as well as related concepts in DNA structure and stability.