What are Viruses?

Viruses are microscopic parasites, generally smaller than bacteria. They have difficulty reproducing and surviving outside of a carrier body. Viruses are generally known as the cause of transmission of diseases. The worldwide illness and death cases have greatly increased the reputation of viruses. The Ebola virus, which caused an epidemic in West Africa in 2014, and the H1N1 virus, which caused a worldwide pandemic in 2009, are among the first viruses that come to our mind. Some viruses are the nemesis of doctors and healthcare professionals, while others are used as a tool in research. In other words, they help us better understand certain cellular processes such as protein synthesis.

Discovery of Viruses

How small are viruses compared to most bacteria? A little.

The measles virus, with a diameter of 220 nanometers, is 8 times smaller than an E. Coli bacterium. The 45-nanometer hepatitis virus is 40 times smaller than the E. Coli bacteria. According to David R. Wessner, a biology professor at Davidson College, published in Nature Education in 2010, the 30-nanometer diameter poliovirus is 10,000 times smaller than a grain of salt. The size difference between viruses and bacteria was an important clue to the existence of viruses. Towards the end of the 19th century, it was known that microorganisms, especially bacteria, could cause disease. But researchers could not find the cause of a disease found in tobacco, namely tobacco mosaic disease.

In the article “On Tobacco Mosaic Disease” in 1886, German chemist and agricultural researcher Adolf Mayer published the results of his extensive experiment. Mayer, when he crushed sick tobacco leaves and injected the poisonous liquid into the veins of healthy tobacco leaves, he saw that the healthy leaves turned yellow and discolored. Mayer concluded that the cause of the disease was in the toxic fluid. But more concrete results did not help him find a definitive answer. Mayer was certain it was some kind of bacteria that caused the disease.

But he could not isolate the substance that caused the disease or observe it under a microscope. In addition, he could not reveal the disease again by injecting a group of bacteria into the leaves. A student named Dmitri Ivanovsky reruned Mayer’s experiment in 1892, but it was a little different from his. According to an article published in the journal Bacteriological Reviews in 1972, Ivanovsky passed the liquid from the sick tobacco leaves through the Chamberland filter (a filter that can hold bacteria and other microorganisms).

Despite this process, the liquid from the filter was still contagious. This showed that the cause of the disease was small enough to pass through the filter. But Ivanovsky suggested that the liquid that came out contained “bacteria or soluble toxin”. So he concluded that the cause of tobacco mosaic disease was bacterial. However, the existence of viruses was recognized in 1898. Dutch scientist Martinus Beijerinck, while confirming the results of Ivanovsky’s experiment, suggested that the cause of the disease was not bacterial, but “living fluid virus” or, as it was called, “virus that can pass through the filter”.

The experiments of Ivanosky, Beijerinck and the scientists who followed their path showed that viruses existed. Viruses would be visible after only a few decades. According to the article published in the journal Clinical Microbiology Reviews in 2009, photographs of a virus were taken for the first time thanks to the electron microscope, which was invented by German scientists Ernst Ruska and Max Knoll, thanks to the high-resolution technology. This virus, photographed in 1939, was the tobacco mosaic virus. Thus, viruses were discovered.

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Structure of Viruses

It cannot be decided whether viruses are alive or not. On the one hand, it contains nucleic acids (DNA or RNA) found in all living organisms. Viruses, on the other hand, do not have sufficient capacity to independently identify and act on the information contained in these nucleic acids. According to Jaquelin Dudley, a professor of molecular biology at the University of Texas, a virus is a parasite that must reproduce in the host cell. “A virus cannot reproduce in an environment other than a carrier because it does not have the complex mechanism that the carrier cell has.” The cellular mechanism of the carrier helps viruses to produce RNA (transcription) using their own DNA and to produce proteins (translation) according to the data in their RNA.

The virus that is fully formed and can be transmitted is called “virion”. According to the authors of the book Medical Microbiology (4th Edition) published by the University ofTexas, a simple virion has RNA in its structure and proteins known as capsids on its outer layer. Capsids protect infectious nucleic acids against host cell enzymes known as “nuclease”. Some viruses have a second protective layer, the membrane. This layer usually consists of the cell membrane of the carrier. So the virus can use some parts of the carrier for its own purposes. DNA or RNA found in viruses can be single-stranded or double-stranded. It contains the genome or all of the genetic information of the virus. Virotic genomes are usually small. And it only codes for important proteins (capsid proteins, enzymes, or proteins to aid proliferation).

Function of Viruses

According to the Medical Microbiology book, the main task of viruses or virions is to transfer the DNA or RNA genome to the host cell and transcribe and transcribe that genome by the host cell. First, viruses must enter the host cell. Airways and open wounds are gateways for viruses. Sometimes insects take on this task. Certain viruses enter the body through insects’ saliva. According to the authors of the book Molecular Biology of a Cell (4th Edition), viruses can reproduce both in the insect’s cells and in the host cells, and thus can easily pass from one to another. Viruses that cause yellow fever and dengue are examples. Viruses are then fixed on the surface of the host cell.

They do this by binding to receptors on the cell surface. Different viruses can bind to the same receptor or a single virus can bind to different receptors. Viruses use the receptors for their own benefit. However, the function of the receptors is to help the cell. When the virus is fixed on the surface of the host cell, it can act on the cell membrane. There are many different ways of entering the cell. The sheathed HIV virus merges with the cell membrane and enters. Influenza virus, another virus with a sheath, is drawn in by the cell.

Some non-sheathed viruses, such as the poliovirus, create a porous channel and enter the cell. Once inside, the viruses release their genomes and disrupt the cell’s machinery. Virotic genomes enable host cells to produce virotic proteins and often prevent the cell from synthesizing RNA or protein. Viruses that provide the necessary environment for their reproduction are in an advantageous position. For example, when someone with cold sneezes, 20,000 droplets containing rhino virus or corona virus comes out of their mouth. Inhaling these droplets or coming into contact with them is enough for the virus to spread.

Viruses; by examining their sizes-shapes, forms, and nucleic acids, the relationship between them has begun to be understood. As we sort and compare viral genomes with better methods and new scientific data increases, we learn more about viruses. The fact that viruses are smaller than bacteria was accepted until 1992. According to Wessner, a bacteria-like microorganism was discovered in amoebas inside a cooling tower that year. This microorganism is not a type of bacteria, it is actually a large virus they call “Mimivirus”. The size of the virus is about 750 nanometers and has the same characteristics as gram-positive bacteria. Immediately afterward, other large viruses such as Mamavirus and Megavirus were discovered. Dudley points out that it is not known how these large viruses evolved and sees them as “elephants of the virus world”.

He suggests that these may be immature cells that have become parasites of other cells (because Mimivirus infects amoebae), or more typical viruses that take over extra carrier genes. Mimiviruses, like other small viruses, need the cellular machinery of the carrier to produce proteins, according to Wessner. But there are genes leftover from the translation process in the genome of Mimiviruses.

The fact that mimiviruses were once independent viruses may be true. Or, in the simplest way, they captured and accumulated some of the carrier genes. With such discoveries, new questions and new areas of research arise. These studies will uncover important issues in the future, such as the origins of viruses, their current parasitic state, and whether viruses are included in the tree of life.