A1.2.15 Chargaff’s data on pyrimidine and purine bases across diverse life forms (HL only) Notes

How Chargaff’s Data Falsified the Tetranucleotide Hypothesis

1. Erwin Chargaff’s chemical analyses of DNA showed that base composition varies among species and follows consistent pairing patterns (A=T, G=C).
2. These results falsified the earlier tetranucleotide hypothesis, illustrating how science advances through falsification rather than proof.

The Problem of Induction

1. The problem is that no number of consistent observations can guarantee truth. one black swan disproves the claim.
2. This is known as the problem of induction.

The Certainty of Falsification

1. If a prediction fails, the hypothesis must be rejected or revised.
2. So, if induction collects evidence; falsification tests it.

The Tetranucleotide Hypothesis

1. In the early 1900s, scientists proposed that DNA was made of a repeating sequence of four nucleotides (A, T, G, C) in equal proportions.
2. If this were true, DNA would be too simple to encode genetic information, so proteins were believed to be the genetic material instead.

Chargaff’s Findings

1. By chemically analyzing DNA from many species, Chargaff discovered:
   1. The total number of purines = total number of pyrimidines.
      1. Purines: Adenine (A), Guanine (G).
      2. Pyrimidines: Cytosine (C), Thymine (T).
   2. Within each DNA sample:
      1. %A ≈ %T.
      2. %G ≈ %C.
   3. The relative proportion of A+T vs. G+C varies between species.
      1. Example: Humans ~31% A, 31% T, 19% G, 19% C.
      2. Mycobacterium tuberculosis: ~15% A, 15% T, 35% G, 35% C.

How Chargaff Falsified the Hypothesis

1. Chargaff’s results disproved the tetranucleotide hypothesis because:
   1. The bases were not present in equal amounts (A, T, G, C ≠ 25% each).
   2. The proportions differed among species, showing DNA had structural complexity.
2. These findings directly falsified the prediction made by the hypothesis.