Viruses are microscopic organisms, smaller than bacteria, that are only capable of reproducing in a host. They are extremely small, most ranging from 20 to 400 nanometers. They are of particular interest to scientists because some of their characteristics resemble those of living organisms. For instance, they all contain a nucleic acid genome and a protein capsid. However, they are unable to reproduce without the help of a host.
A general misconception is that viruses and bacteria are the same. However, they are incredibly different. One notable difference between the two is their sizes. Bacteria, on average, are hundreds of times smaller than viruses, which rely on hosts to reproduce. Bacteria, on the other hand, are able to reproduce without a host, truly making them living organisms.
With viruses being so different, the question arises: how did they come to be in the first place? Did they evolve from another organism? Unfortunately, there is not a confirmed theory, currently, but scientists are running various experiments to try to uncover this mystery. The three main hypotheses that virologists have developed over the years include the regressive, progressive, and virus-first hypotheses.
The regressive hypothesis essentially claims that viruses were once cells that eventually became what they are today, having just a genome and protein capsid. The basis of this theory comes from the fact that some viruses have double-stranded DNA, similar to humans. The progressive hypothesis tackles the shortcoming of the regressive hypothesis: the inability to explain the origins of viruses with RNA. It essentially says that DNA and RNA acquire the ability to transfer from cell to cell. Lastly, the virus-first hypothesis — like its name implies — suggests that viruses must have evolved first, predating even cells. The support for this theory comes from the fact that viruses are simpler than cells and thus evolution may have caused more complex cells to form. However, viruses rely on hosts, so if viruses did evolve first, then it begs the question of how they managed to survive.
Tobacco Mosaic Virus
In the second half of the 19th century, a prevalent disease was causing the discoloration of tobacco leaves. Due to this discoloration, farmers faced huge losses as over half of their crops were gone to waste. To try to discover what was causing this, Adolf Mayer, in 1879, tried to spot the virus causing this. However, as technology for viewing viruses had not been developed at the time, Mayer was unable to identify the cause. In 1892, biologist Dmitry Ivanovsky discovered that whatever was causing this disease was able to pass through porcelain filters, which meant it was smaller than bacteria. Finally, in 1898, Martinus Beijerinck was able to prove the culprit causing the disease was in an entirely new family of its own, now known as a virus.
This discovery was the basis for understanding viruses and eventually led to Wendell M. Stanley winning the Nobel Prize in Chemistry (1946) for being able to “show that the tobacco mosaic virus is composed of protein and ribonucleic acid, or RNA.” Each discovery since the discovery of the tobacco mosaic virus has been crucial to understanding more about viruses and the natural world. In fact, Dr. Harvey J. Alter, Michael Houghton, and Charles M. Rice were awarded The Nobel Prize in Physiology or Medicine last year for discovering the hepatitis C virus.
Our understanding of viruses has come a long way since the discovery of the tobacco mosaic virus, but with improving technology and more research being conducted, there is no doubt our understanding of viruses will continue to expand in the years to come.
Saharsh Satheesh, Youth Medical Journal 2021
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