1.2.6Inquiry Process
The Hershey and Chase Experiment — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 3 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4The Question and Why Viruses Were Used
Before the 1950s many biologists thought protein carried genetic information because proteins are built from twenty different amino acids, whereas DNA seemed to offer only four bases.
Bacteriophage T2
- Bacteriophage T2 A virus that infects the bacterium Escherichia coli and is made of only a DNA core inside a protein coat.
Simple structure
- Simple structure Because the virus has just two components, one of them must carry the instructions for making new viruses.
Chemical difference
- Chemical difference DNA contains phosphorus but no sulfur, whereas proteins contain sulfur but almost no phosphorus.
Radioisotopes
- Radioisotopes Radioactive ³²P marks DNA and radioactive ³⁵S marks protein, so each molecule can be followed separately.
Part 2 of 4The Procedure
The experiment was carried out twice in parallel, once with each label.
1
Bacteria were grown on a medium containing either ³⁵S or ³²P, and phages were then grown in these bacteria so that they became labelled.
2
The labelled phages were mixed with fresh, unlabelled E. coli and given a few minutes to attach to the cells and inject their genetic material.
3
The mixture was agitated in a blender to shear the empty phage coats off the surface of the bacteria.
4
The sample was spun in a centrifuge so that the small phage particles stayed in the liquid supernatant while the heavier bacteria formed a pellet.
5
Radioactivity was measured in the supernatant and in the pellet, and the infected bacteria later released new phages.
Part 3 of 4Results and Conclusion
Results and Conclusion
³⁵S result
- ³⁵S result Most of the radioactivity from labelled protein stayed in the supernatant, so the protein coats had not entered the bacteria.
³²P result
- ³²P result Most of the radioactivity from labelled DNA was found in the pellet, so viral DNA had entered the bacterial cells.
Conclusion
- Conclusion DNA is the molecule that is passed on and directs the making of new viruses, so DNA is the genetic material.
Nature of science
- Nature of science The availability of radioisotopes after the Second World War made this type of experiment possible for the first time.
| Label | Molecule labelled | Where radioactivity was found |
|---|---|---|
| ³⁵S | Protein coat | Mainly supernatant |
| ³²P | DNA | Mainly pellet |
Part 4 of 4Exam Checklist & Tips
1.2.6 – Key Points for the Exam
- Bacteriophage A virus that infects bacteria.
- Radioisotope An unstable isotope that emits radiation and can be used as a tracer or label.
- Supernatant The liquid that remains above the solid pellet after centrifugation.
- Pellet The dense material, here the bacterial cells, collected at the bottom of the tube after centrifugation.
- Heritable material The molecule that carries information from one generation to the next.
Common Mistakes
- Swapping the labelsphosphorus marks the DNA, while sulfur marks the protein.
- Saying the blender was used to break open the bacteria, when it only detached the phage coats from the cell surface.
- Claiming that no radioactivity at all appeared in the other fraction;
- the separation was good but not perfect.
- Stating that the experiment proved DNA is the genetic material of every organism, rather than showing it for this virus.
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