Chargaff’s data showed consistent base-ratio patterns in DNA, supporting complementary base pairing and challenging the tetranucleotide hypothesis in IB Biology exam contexts.
Hershey–Chase experiment (1952)
T2 bacteriophages were prepared with either ³²P-labelled DNA or ³⁵S-labelled protein. After infection of E. coli, the ³²P signal was associated with the bacterial pellet and was found in progeny phages, whereas the ³⁵S protein label remained outside the bacteria.
Observation → conclusion: DNA entered the bacterial cells and was passed to new viruses; protein did not. Therefore DNA, rather than protein, is the genetic material in this system.
Chargaff’s data
Across DNA samples, the amount of purines equalled the amount of pyrimidines, with A = T and G = C. These results supported complementary base pairing and falsified the tetranucleotide hypothesis of a repeating, fixed base pattern.
Evidence rule: state the result first, then explain what it allows us to conclude; do not present the isotope labels as the conclusion itself.
This objective is assessed through structured response, commonly using Explain.
Explain
Build the answer around this relationship: Chargaff's data should be described using precise molecular vocabulary.
Representative question
Explain the reasons for expecting a thymine percentage of 32.4 %.
HL adds directionality, helix geometry, DNA packaging, and classic evidence. These ideas stay separate: 5′/3′ explains strand direction; purine-pyrimidine pairing explains width; nucleosomes explain packaging; Hershey-Chase and Chargaff explain evidence for DNA and base pairing.