DNA: The Genetic Material

From "Transforming Principles" to the Double Helix

Griffith Experiment

PART 1: The Zombie Bacteria

1928: Griffith's Discovery

Frederick Griffith mixed harmless Rough (R) strain bacteria with heat-killed Smooth (S) deadly bacteria. To his shock, the harmless ones became killers!

The Transforming Principle: Something moved from the dead S-strain to the live R-strain, "transforming" them forever.
Avery et al Experiment

PART 2: The Process of Elimination

The Chemical Manhunt

Avery, MacLeod, and McCarty used enzymes to systematically destroy Protein, RNA, and DNA to see which one stopped the transformation.

The Verdict

Destroying proteins (Proteases) or RNA (RNases) did nothing. But when they used DNases to destroy DNA, transformation stopped! DNA was the culprit.

Hershey Chase Experiment

PART 3: The Blender Experiment

Radioactive Labeling

Hershey and Chase used S-35 to label viral proteins and P-32 to label viral DNA. They wanted to see what actually enters the bacteria during infection.

The Whirrr!

After blending and centrifuging, the radioactive DNA was found inside the bacterial pellet. The protein stayed in the liquid outside.

DNA Structure

PART 4: The Double Helix

The Final Piece

Using Rosalind Franklin's X-ray data (the famous Photo 51), Watson and Crick finally visualized the "twisted ladder."

The Structure:
  • Double twisted helix.
  • Specific base-pairing rules (A-T, G-C).
  • "Life's Instruction Manual."

DNA WINS!

The quest for the genetic material is complete.

PART 5: Exam Essentials

NEET KEY POINTS
  • Griffith: Transformation (S-strain vs R-strain).
  • Avery et al: Biochemical characterization (DNase).
  • Hershey-Chase: Final proof using Bacteriophages (P-32/S-35).
  • Franklin: X-ray crystallography data.
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