Biology · Ch 5 — Molecular Basis of Inheritance
The Hershey–Chase Blender Experiment
The Hershey–Chase Blender Experiment
Even after Avery, MacLeod and McCarty's 1944 biochemical proof, a substantial part of the scientific community remained unconvinced that DNA, rather than protein, was the genetic material — the doubt persisted mainly because DNA's simple four-base chemical alphabet still seemed, to many biologists, too limited to encode the vast complexity of hereditary information. The experiment that finally settled the argument, in 1952, was designed by Alfred Hershey and Martha Chase, and it used an entirely different experimental system: bacteriophages, which are viruses that infect bacteria.
Hershey and Chase worked with the T2 bacteriophage, which infects the bacterium Escherichia coli. A bacteriophage particle is structurally very simple, consisting of essentially just two components: an outer protein coat and an inner core of DNA — which made it an ideal system for testing which of these two components actually enters the host bacterial cell and directs the production of new phage particles. To distinguish the protein coat from the DNA core experimentally, Hershey and Chase exploited a key chemical difference between the two molecule types: protein contains sulphur (in the amino acids cysteine and methionine) but essentially no phosphorus, while DNA contains phosphorus (in its phosphate backbone) but no sulphur.
They therefore grew two separate batches of phage, one in a growth medium containing the radioactive isotope of sulphur, 35S, which labelled only the phage's protein coat, and a second batch in a growth medium containing the radioactive isotope of phosphorus, 32P, which labelled only the phage's DNA. Each radioactively labelled batch of phage was then allowed to infect separate cultures of unlabelled E. coli bacteria. After allowing a short time for infection, each culture was violently agitated in a blender, which sheared off any phage material still attached to the OUTSIDE of the bacterial cells (their protein coats), and the mixture was then centrifuged, separating the heavier bacterial cells (which pellet at the bottom) from the lighter, now-empty phage coats (which remain in the supernatant liquid above). …
What this figure shows. A two-branch flow diagram of the Hershey-Chase 1952 experiment. Top branch: T2 bacteriophage grown with radioactive 35S (labelling only the phage's protein coat, shown shaded) infects E. coli; after blending and centrifuging, the radioactivity is shown concentrated in the supernatant (empty phage coats), not in the bacterial cell pellet at the bottom of the centrifuge tube. Bottom branch: T2 bacteriophage grown with radioactive 32P (labelling only the phage's DNA core, shown shaded inside the phage head) infects E. coli; after the same blending and centrifuging steps, the radioactivity is shown concentrated in the bacterial cell pellet, not the supernatant. A phage particle is drawn simply as a hexagonal head with a DNA core and a protein coat/tail, and the centrifuge tube in each branch is split into an upper supernatant zone and a lower pell …