5.2.1. The Genetic Material is DNA

The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages. The bacteriophage attaches to the bacteria and its genetic material then enters the bacterial cell. The bacterial cell treats the viral genetic material as if it was its own and subsequently manufactures more virus particles. Hershey and Chase worked to discover whether it was protein or DNA from the viruses that entered the bacteria. They grew some viruses on a medium that contained radioactive phosphorus and some others on a medium that contained radioactive sulfur. Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not. Similarly, viruses grown on radioactive sulfur contained radioactive protein but not radioactive DNA because DNA does not contain sulfur.

In the Hershey and Chase experiment, scientists aimed to determine whether DNA or proteins were the genetic material transferred from bacteriophages to bacteria. They labeled bacteriophages with radioactive phosphorus (which labels DNA) and radioactive sulfur (which labels proteins). After allowing the labeled phages to infect E. coli bacteria, they used a blender to separate the phage coats from the bacterial cells. Centrifugation revealed that only bacteria infected with the phages containing radioactive DNA were radioactive, indicating that DNA was indeed the genetic material, as it was the only component entering the bacteria.

Think of it like a treasure chest and its key. If you’re trying to figure out how to open the chest (the cell), you can either have the chest itself (the protein) or the key (the DNA). The Hershey and Chase experiment is like conducting a test to find out whether the key or the chest helps to unlock it. By seeing which part goes into the chest and makes it work, they discovered that the key (DNA) is essential for its function.

Radioactive phages were allowed to attach to E. coli bacteria. Then, as the infection proceeded, the viral coats were removed from the bacteria by agitating them in a blender. The virus particles were separated from the bacteria by spinning them in a centrifuge. Bacteria which was infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria. Bacteria that were infected with viruses that had radioactive proteins were not radioactive. This indicates that proteins did not enter the bacteria from the viruses. DNA is therefore the genetic material that is passed from virus to bacteria.

The process involved allowing the bacteriophage to infect E. coli. After the phages attached, the researchers removed the phage coats using a blender. The mixture was centrifuged, separating the heavier bacterial particles from the lighter viral particles. The bacteria that had been infected with phages containing radioactive DNA were found to be radioactive, while the bacteria infected with phages containing radioactive proteins were not. This demonstrated that it was the viral DNA, not the protein, that was responsible for the infection and replication inside the bacteria.

Imagine a delivery service. The delivery truck (the bacteriophage) arrives at a house (the bacteria) and drops off a package (the DNA). The truck leaves, and the house starts using the package to make more packages. After checking the house, you find out that only the packages from the delivery (the DNA) have been used, not the truck itself (the protein). This shows that the vital instruction (genetic material) for making more packages came from the dropped off package.

The results of the Hershey and Chase experiments provided critical evidence for DNA being the hereditary material. This conclusion was fundamental in the scientific understanding of genetics, leading to the understanding that DNA carries the genetic blueprint necessary for constructing organisms, guiding their development, function, and reproduction.

The Hershey and Chase experiments led to a definitive conclusion that DNA, not proteins, serves as the genetic material for living organisms. This finding was pivotal as it shifted the scientific community’s prior beliefs and opened the doors for further genetic research, including the study of how DNA is replicated and how genetic information is expressed.

Consider the blueprint of a house. The house itself represents life (the organism), while the blueprint (DNA) contains all the necessary information for constructing and maintaining the house. Just like how you can't build without blueprints, organisms can't develop or function properly without DNA. The experiments showed that DNA is this blueprint—crucial for everything that happens in living beings.

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