The Hershey–Chase experiments:
The Hershey–Chase experiments were a series of experiments conducted in 1952 by Alfred Hershey and Martha Chase, confirming that DNA was the genetic material, which had first been demonstrated in the 1944 Avery–MacLeod–McCarty experiment. While DNA had been known to biologists since 1869, most assumed at the time that proteins carried the information for inheritance.
Hershey and Chase conducted their experiments on the T2 phage, a virus whose structure had recently been shown by electron microscopy. The phage consists of a protein ****l containing its genetic material. The phage infects a bacterium by attaching to its outer membrane and injecting its genetic material and leaving its empty ****l attached to the bacterium.
In their first set of experiments, Hershey and Chase labeled the DNA of phages with radioactivePhosphorus-32 (the element phosphorus is present in DNA but not present in any of the 20 amino acids from which proteins are made). They allowed the phages to infect E. coli, and through several elegant experiments were able to observe the transfer of P32 labeled phage DNA into the cytoplasm of the bacterium.
In their second set of experiments, they labeled the phages with radioactive Sulfur-35 (Sulfur is present in the amino acids cysteine and methionine, but not in DNA). Following infection of E. coli they then sheared the viral protein ****ls off of infected cells using a high-speed blender and separated the cells and viral coats by using a centrifuge. After separation, the radioactive S35 tracer was observed in the protein ****ls, but not in the infected bacteria, supporting the hypothesis that the genetic material which infects the bacteria was DNA and not protein.
Hershey shared the 1969 Nobel Prize in Physiology or Medicine for his “discoveries concerning the genetic structure of viruses.”
Decoding DNA BY James Watson and Francis Crick:
James Watson and Francis Crick get the credit for unlocking the mystery of DNA, but their discovery depended heavily on the work of others, like Alfred Hershey and Martha Chase, who, in 1952, conducted a now-famous experiment that identified DNA as the molecule responsible for heredity. Hershey and Chase worked with a type of virus known as a bacteriophage. Such a virus, made up of a protein coat surrounding a strand of DNA, infects a bacteria cell, programs the cell to make more viruses, then kills the cell to release the newly made viruses. The two knew this, but they didn't know which component -- protein or DNA -- was responsible until their ingenious "blender" experiment directed them to DNA's nucleic acids.
After Hershey and Chase's experiment, scientists like Rosalind Franklin focused on DNA and rushed to decipher its molecular structure. Franklin used a technique called X-ray diffraction to study DNA. It involves shooting X-rays at aligned fibers of purified DNA. As the X-rays interact with the molecule, they are diffracted, or bent, off their original course. When allowed to strike a photographic plate, the diffracted X-rays form a pattern that's unique to the molecule being analyzed. Franklin's famous photo of DNA shows an X-shaped pattern that Watson and Crick knew was a signature of a helical (or spiral-shaped) molecule. They could also determine the width of the helix from looking at Franklin's image. The width suggested that two strands made up the molecule, leading to the double-helix shape we all take for granted today.