Unit 3: Nucleic Acids & DNA Structure

Syllabus Reference: Discovery of DNA (Griffith, Hershey-Chase, etc.); DNA Structure, cot curves; Organization (Prokaryotes vs Eukaryotes). Nucleosome; [cite_start]Chromatin [cite: 511-514].

1. Discovery of DNA as Genetic Material

• Griffith's Experiment (1928): "Transformation Principle". Heat-killed S-strain (virulent) bacteria transformed live R-strain (non-virulent) into virulent ones in mice.
• Avery, MacLeod & McCarty (1944): Proved the transforming principle was DNA. DNase enzyme stopped transformation; Protease/RNase did not.
• Hershey & Chase (1952): "Blender Experiment". Used radioactive Phosphorus (³²P for DNA) and Sulfur (³⁵S for Protein) in Bacteriophages. Only ³²P entered bacteria, proving DNA is the genetic material.

2. Structure of DNA

Watson & Crick Model (1953):

• Double Helix: Two polynucleotide chains coiled around a central axis.
• Anti-parallel: One strand runs 5' → 3', the other 3' → 5'.
• Base Pairing (Chargaff's Rule): Adenine binds to Thymine (A=T) with 2 H-bonds. Guanine binds to Cytosine (G≡C) with 3 H-bonds.
• Backbone: Sugar-Phosphate backbone is on the outside; bases are inside.

Cot Curves

Cot analysis measures DNA renaturation kinetics. It helps estimate the genome size and complexity (amount of repetitive vs unique DNA sequences).

3. Forms of DNA

4. DNA Organization (The Nucleosome)

In Eukaryotes, huge DNA molecules must fit into a tiny nucleus.

• Nucleosome (Beads-on-string): The fundamental unit of packaging.
  Consists of a Histone Octamer (2 copies each of H2A, H2B, H3, H4) wrapped by approx 146 bp of DNA.
  H1 Histone acts as a linker.
• Chromatin Types:
  Euchromatin: Loosely packed, transcriptionally active, stains light.
  Heterochromatin: Tightly packed, transcriptionally inactive, stains dark.