Mimivirus
Mimivirus is a large and complex virus that primarily infects amoebas. Discovered in 1992 and identified as a virus a decade later, it is notable for its size, approaching that of certain bacteria, which led to its name meaning "mimicking microbe." With a capsid diameter of about 500 nanometers and a total width of approximately 750 nanometers when accounting for its fibrous tendrils, the mimivirus is one of the largest viruses ever identified. It contains both DNA and RNA, with around 1,000 genes, which is significantly more than many other viruses. This genetic complexity has sparked discussions among scientists regarding the nature of viruses and their classification as living organisms. While the mimivirus has been detected in some human cases of pneumonia, no direct link to the disease has been established. The exploration of mimivirus has contributed to ongoing debates about the relationship between viruses and more complex forms of life, particularly as even larger viruses, like megavirus and pandoravirus, have been discovered since its identification. Overall, mimivirus plays a crucial role in advancing our understanding of viral biology and evolution.
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Mimivirus
The mimivirus is a complex type of virus that is also one of the largest ever identified. While most viruses are extremely small, the mimivirus approaches the size of many bacteria. This aspect is what gave the virus its name as scientists thought its size “mimicked” the larger microorganisms. The mimivirus was discovered in 1992, but was not identified as a virus until a decade later. The virus mainly infects amoebas and is not known to have any effect on humans. The discovery of the mimivirus and other large viruses changed the fundamental understanding of viruses and prompted some researchers to reexamine the scientific definition of life.
Background
Viruses are microscopic biological agents that need to find and infect a living host cell to reproduce. Viruses consist of a central core of genetic material such as DNA or RNA. This core is surrounded by a protective coat of proteins called a capsid. A virus does have the capability to replicate proteins and make genetic copies of itself. To reproduce, a virus must come into physical contact with a host cell and insert its genetic material into the cell’s nucleus. By doing so, the virus takes over the cell’s functions and essentially rewires its basic instructions, ordering the cell to make copies of the virus. After the replication process is complete, the new virus leaves the cell, killing it and moving on to find a new host. While viruses are made up of biological material, their inability to reproduce on their own has led many scientists to question whether viruses are true living organisms.
Viruses come in different shapes and sizes and can vary in their complexity. They are believed to be the most abundant biological agents on Earth, with an estimated number of about 1031—a 1 followed by 32 zeroes. Most viruses are extremely small. The smallest known virus has a capsid diameter of 17 nanometers, a figure that measures billionths of a meter. In comparison, the rhinovirus—the leading cause of the common cold—is 30 nanometers wide, the average influenza virus is about 100 nanometers wide, and the smallpox virus is about 250 nanometers wide. Bacteria, which are single-celled microorganisms, are significantly larger. One E. coli bacterium is about 2,000 nanometers wide, while a salmonella bacterium—a major cause of food poisoning—can be as large as 5,000 nanometers. The smallest known bacteria are about 400 nanometers in size.
Overview
In 1992, medical researchers were investigating the source of a pneumonia outbreak in Bradford, England. While taking samples from a water tank at a hospital, they discovered what they thought was a bacteria inside an amoeba, a tiny single-celled animal that lives in water. Based on the organism’s size and past instances of amoeba-borne illnesses, researchers assumed they had found a new type of pneumonia-causing bacteria. The sample was stored in a laboratory for ten years until it was reexamined by French biologists in 2003. They discovered that it was a giant virus, the largest ever discovered up to that time. They named it Acanthamoeba polyphaga mimivirus, or more simply, mimivirus—short for “mimicking microbe”—because of its bacteria-like size.
The mimivirus has a capsid diameter of about 500 nanometers and is surrounded by fibrous tendrils that give it a fuzzy or hairy appearance. When these tendrils are taken into account, the virus has a total width of about 750 nanometers. While the typical virus can contain either DNA or RNA, the mimivirus is unique in that it has both. The virus has a single strand of DNA that is responsible for rewriting the host cell’s genetic information, and about twenty-six strands of RNA that perform other functions.
After mapping its genetic structure, scientists found the mimivirus contained about 1,000 genes. By contrast, the smallpox virus contains fewer than 200 genes, while the influenza virus has 11. Several mimivirus genes are also found in bacteria and other more advanced microorganisms. Among these are genes used for DNA repair and genetic translation—the instruction process of replicating proteins. However, as with all viruses, the mimivirus does not replicate its own proteins. These genes are inactive in the mimivirus, leading scientists to speculate on why they are found in the virus.
One theory holds that the mimivirus represents a new evolutionary branch of microbial life. The branch could be an entirely new line of organisms, or a sort of “missing link” between viruses and more complex single-celled organisms. Other researchers think that the mimivirus may have acquired those genes from their amoeba hosts or from bacteria within the amoeba. The complexity of the mimivirus reignited debate among scientists as to whether viruses are actually living organisms or just biological agents built to transfer genetic material.
While the mimivirus has been found inside some human patients suffering from pneumonia, scientists have not been able to link the virus to the disease. The mimivirus predominantly infects amoebas, attaching itself to the surface walls of the host and injecting its DNA into the interior. Within a few hours, the nucleus of the amoeba disappears and its interior begins to fill up with replicated copies of the virus. Eventually, the cell walls burst open and release the new mimiviruses.
At the time of its discovery, the mimivirus was the largest virus ever found. However, in 2010, scientists discovered another amoeba-borne virus that was more complex than the mimivirus. The megavirus has a slightly larger capsid diameter than the mimivirus and contains more than 1,100 genes. In 2013, an even larger virus dubbed the pandoravirus was identified off the coast of Chile and in Australia. The virus has a diameter of 1,000 nanometers and contains more than 2,500 genes. Like the mimivirus, the pandoravirus had previously been observed by scientists, but was mistaken for a bacterium. It also primarily infects amoebas and is typically found only in water.
Bibliography
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