DNA Decoded
From the first evidence that DNA carries heredity, through the double helix, replication, repair — to PCR and sequencing.
Module 01
The Hunt for the Genetic Material
How scientists proved DNA — not protein — is the hereditary molecule, through experiments from 1928 to 1952.
Background
Early Circumstantial Evidence (1920s)
Chromosomes contain both DNA and proteins. A new dye — the Feulgen stain — specifically colored DNA and revealed three clues:
Right place
DNA was in chromosomes, where genes reside.
DNA was in chromosomes, where genes reside.
Varied by species
DNA composition differed between species.
DNA composition differed between species.
Right amount
Diploid cells had 2× the DNA of haploid cells.
Diploid cells had 2× the DNA of haploid cells.
Still circumstantial — direct proof required experiments.
Experiment 1 — Griffith's Transformation (1928)
Frederick Griffith used two strains of Streptococcus pneumoniae:
S strain (smooth) — Virulent. Has polysaccharide capsule. Kills mice.
R strain (rough) — Non-virulent. No capsule. Mice survive.
✦ A "transforming principle" from dead S cells permanently converted R into virulent S. But what substance was it?
Experiment 2 — Avery's Identification (1944)
Avery systematically destroyed each candidate molecule in S strain filtrate to see which one was essential: