Viruses have been and continue to be a puzzle to solve, and the more we learn about them the more we learn about fundamentals of life in general. Until the electron microscope and the ultracentrifuge became available from 1935 to 1940, no one could see or chemically analyze a virus. But over 40 years earlier, the Russian botanist, Dimitri Ivanovsky made the first major breakthrough. He learned that whatever was infecting tobacco plants was unlike bacteria in that it could not be trapped by a porcelain filter.
In 1898, Beijerinck, a Dutch microbiologist, verified Ivanovsky’s experiments and introduced the term filtrable virus to describe a new classification of infectious agent. Soon clinicians realized that the same filtration property was shared by the agents that caused rabies, polio , herpes simplex, and vaccinia, which had been used for smallpox vaccination. Beijerinck and others also discovered that viruses could not be cultured without living cells. Unlike bacteria, they needed hosts to reproduce.
Eventually, electron micrographs revealed to scientists that viruses came in various shapes and with different surfaces. Oblique evaporation of a thin film of metal over the preparation created a three-dimensional effect , leading to new information about the heights and shapes of viruses. Notice in the following images that vaccinia is rectangular, polio, spherical and tobacco virus is rodlike. (From Robley C. Williams , Ralph W. G. Wyckoff. Science 08 Jun 1945.)
One of the most important insights gained from studying viruses was their composition and mechanism of reproduction. At first it seemed that they were composed of only proteins, but then it was realized that the protein was always accompanied by nucleic acids(DNA or RNA), and that nucleic acids acted as the hereditary material. This also turned out to be true for all living cells.
The idea came from the Alfred Hershey-Martha Chase experiments of 1952. They demonstrated that the genetic material of a phage (a virus that infects bacteria) is DNA, not protein.
The clever experiment used two sets of T2 bacteriophages. In one set, the protein coat is labeled with a radioactive isotope of sulfur (35S), not found in DNA. In the other set, the DNA was labeled with radioactive phosphorus (32P), not found in protein. Only the 32P ended up in the E. coli bacteria, revealing that DNA was the agent needed to make new virus. Later in the decade, after the Chase experiments had inspired Franklin, Crick and Watson to elucidate DNA’s structure and method of replication, two research groups separated the tobacco virus’ protein coat from its RNA interior, and discovered that only the latter was capable of producing symptoms in tobacco leaves.
In 1970, Temin and Baltimore realized that some RNA viruses, retroviruses, could inject their RNA into host cells’ DNA. They do so by using the enzyme reverse transcriptase. The discovery that the information in RNA could be relayed to DNA meant that genetic information is not only transferred in one direction, from DNA to RNA, as previously assumed, but also in the opposite direction.
In more recent decades, virologists have slowly come to realize that only a minority of viruses cause disease. Most are innocuous, some are definitely beneficial, and others are indispensable to their hosts. I explored that theme here.