A1.2.14 (HL)—Hershey-Chase experiment
The Hershey-Chase experiment used radioactive labels in phage DNA and protein to show that DNA enters bacteria as hereditary material.
- Syllabus
- First assessment 2025
- Objective
- A1.2.14
- Level
- HL
Exam analysis
Chance of appearing7%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–3
Common command terms
- Outline
- Describe
- Explain
Scoring notes
- Common mistake
- Remembering the experiment name without explaining what each radioactive label tracked.
Recent exam appearances
November 2025Paper1A HL · TZ12[ 1 ]A1.2.14 (HL)—Hershey-Chase experiment
May 2025Paper1B HL · TZ23(b)[ 3 ]A1.2.14 (HL)—Hershey-Chase experiment
May 2024Paper2 HL · TZ25(c)[ 3 ]A1.2.14 (HL)—Hershey-Chase experiment
May 2021Paper1 HL · TZ226[ 1 ]A1.2.14 (HL)—Hershey-Chase experiment
May 2019Paper1 HL · TZ211[ 3 ]A1.2.14 (HL)—Hershey-Chase experiment
Practice this objective
Coverage 2017–2025 · Updated 14 Jul 2026
Hershey–Chase and Chargaff evidence
HL only
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.
Hershey-Chase experiment
HL onlyAssessment in practice
1–3 marksHow it is assessed
This objective is assessed through structured response, commonly using Outline / Describe / Explain.
Command terms
Outline / Describe / Explain
What earns marks
Build the answer around this relationship: Hershey-Chase experiment should be described using precise molecular vocabulary.
Watch for
Remembering the experiment name without explaining what each radioactive label tracked.
Representative question
Question 1
Explain how this data provides evidence that DNA is the genetic material of cells.
Retrieve The HL Extensions
HL onlyHL 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.
- Directionality: phosphodiester bonds create 5′ and 3′ ends, and DNA strands are antiparallel.
- Helix stability: purine-pyrimidine pairing keeps width constant and hydrogen bonds stabilize base pairs.
- Packaging: DNA wraps around histone octamers and H1 binds linker DNA.
- Evidence: Hershey-Chase supports DNA as genetic material; Chargaff supports complementary pairing.
- Chargaff found purines equal pyrimidines across DNA samples.
Concept essentials
- Hershey-Chase experiment should be described using precise molecular vocabulary.
- Structure and information must be kept separate: backbones provide continuity while bases carry sequence meaning.
- Exam answers should match the command term and avoid adding unsupported extra claims.