Notes for A1.2.14 Evidence from the Hershey–Chase experiment for DNA as the genetic material (HL only) - IB | RevisionDojo
A1.2.14 Evidence from the Hershey–Chase experiment for DNA as the genetic material (HL only) Notes
How the Hershey-Chase Experiment Demonstrated DNA as the Genetic Material
Scientists in the 1950s were debating a critical question: What molecule carries the genetic instructions for life?
Is it protein, known for its complexity and versatility? Or is it DNA, a simpler molecule often underestimated?
This debate ended with a groundbreaking experiment by Alfred Hershey and Martha Chase that changed our understanding of genetics.
Hershey and Chase
The Experiment: Tracing DNA and Protein with Radioisotopes
Hershey and Chase devised an ingenious experiment to distinguish between DNA and protein as the genetic material.
Their approach relied on two key tools:
Radioactive Isotopes:
Phosphorus-32 ($^{32}P$)was used to label DNA because DNA contains phosphorus (in its phosphate backbone) but no sulfur.
Sulfur-35 ($^{35}S$)was used to label proteins because proteins contain sulfur (in amino acids like cysteine) but no phosphorus.
Bacteriophages:
They grew bacteriophages in media containing either $^{32}P$ or $^{35}S$, ensuring that the DNA in one group of phages was radioactive and the protein coat in the other group was radioactive.
Bacteriophage
Labeling with radioisotopes
Viruses were grown in medium containing radioactive ³²P (phosphorus) → labeled the DNA.
Other viruses were grown in medium containing radioactive ³⁵S (sulfur) → labeled the protein coat.
Infection of bacteria
Radioactively labeled viruses infected E. coli cells.
Blender step
A blender shook the virus–bacteria mixture, dislodging virus coats from bacterial surfaces.
Centrifugation step
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Questions
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Question 1
Recap question
Why is the entry of DNA into bacterial cells, rather than retention of protein outside, essential for concluding that DNA is the genetic material?
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Note
Historical context of genetic material
Before the Hershey-Chase experiment, scientists were uncertain about which molecule carried genetic information. Two main candidates were:
Proteins: Complex and diverse molecules with many functions
DNA: A simpler molecule with a repetitive structure
In the early 20th century, many scientists believed that proteins were the genetic material because of their complexity.
AnalogyThink of proteins as a detailed instruction manual and DNA as a simple code - which would you expect to carry important information?
DefinitionGenetic MaterialThe molecule that carries the instructions for inheritance and functioning of living organisms.
ExampleBefore the discovery of DNA as the genetic material, some scientists speculated that inheritance might be carried by unknown substances.
