Viruses are microscopic infectious entities that contain genetic material (DNA or RNA) and must infect a host to reproduce.
Viruses can infect bacteria, plants, and animals, among other living organisms. Viruses are so small that they require the use of a microscope to see them, and they have a very simple structure. When a virus particle is unattached from its host, it consists of a viral genome, or genetic material, enclosed in a capsid, a protein shell. The protein shell of some viruses is encased in a membrane known as an envelope. Because viral genomes can be DNA or RNA, single-stranded or double-stranded, linear or circular, and vary in length and quantity of DNA or RNA molecules, they are extremely diverse.
Viruses are primarily known for their ability to cause disease, as they have caused major outbreaks of illness and death throughout human history. The 2014 Ebola outbreak in West Africa, the 2009 swine flu pandemic, and the COVID-19 pandemic, which was triggered by a coronavirus discovered in late 2019, are all recent examples of virus-driven outbreaks.
How was Virus discovered?
According to the Smithsonian Magazine, the idea that microbes, particularly bacteria, may cause disease was well established by the end of the nineteenth century. According to “Discoveries in Plant Biology, researchers investigating a serious illness in tobacco plants known as tobacco mosaic disease were perplexed as to its cause.
Adolf Mayer, a German chemist and agricultural researcher, published the results of his extensive experiments on tobacco plants affected by the disease, which caused the plants’ leaves to break out in dark green, yellow, and gray splotches, in an 1886 research paper titled “Concerning the Mosaic Disease of Tobacco.” Mayer discovered that by crushing infected tobacco leaves and injecting the resultant juice into the veins of healthy leaves, the once-healthy leaves developed the diseased plants’ speckling and discoloration. Mayer was correct in his assumption that the cause of tobacco mosaic disease was in the leafy juice.
More solid outcomes, on the other hand, eluded him. Mayer reasoned that he should be able to identify and culture the pathogen that causes tobacco mosaic disease on lab dishes based on the work of German physician Robert Koch, who found the bacteria that causes tuberculosis. He was, however, unable to isolate or identify the disease-causing pathogen under a microscope. According to Smithsonian Magazine, he also couldn’t reproduce the sickness by injecting healthy plants with a variety of recognized germs.
In 1892, a Russian student named Dmitri Ivanovsky (sometimes spelled Ivanowski) effectively replicated Mayer’s juicing experiments with a twist. A close-up of a large green leaf with dark brown patches on it, indicating that it has been infected by the tobacco mosaic virus.
Ivanovsky ran the juice from sick leaves through a Chamberland filter, which is thin enough to trap bacteria and other known pathogens, according to a 1972 study published in the journal Bacteriological Reviews. The liquid filtrate remained infectious despite the screening, implying a new piece of the puzzle: whatever was causing the sickness was small enough to pass through the filter.
Ivanovsky, on the other hand, came to the conclusion that tobacco mosaic disease was caused by bacteria, implying that the filtrate “included either bacteria or a soluble toxin.”
Mayer, Ivanovsky, Beijerinck, and others’ experiments after that merely hinted at the presence of viruses; it would be decades before anyone really saw one.
According to Smithsonian Magazine, scientist Wendell M. Stanley crystallized a sample of the tobacco mosaic virus in 1935, allowing the pathogen to be seen on X-ray. The first unambiguous images of the unaltered virus, however, were not acquired until 1939. According to a 2009 article published in the journal Clinical Microbiology Reviews, this achievement was made possible by the introduction of the electron microscope, a tool that uses beams of negatively charged electrons to make images of incredibly small things.
Size of Viruses
In comparison to bacteria, how much smaller are most viruses? Quite a bit, actually.
The measles virus is nearly eight times smaller than Escherichia coli bacterium, with a diameter of 220 nanometers; one nanometer equals 0.000000039 inches. Hepatitis virus is around 40 times smaller than E. coli at 45 nm. In a 2010 paper published in the Journal Nature Education, David R. Wessner, a professor of biology at Davidson College, presents an analogy to show how little this is: The poliovirus is about 10,000 times smaller than a grain of salt, with a diameter of 30 nanometers.
Despite the fact that most viruses are far smaller than bacteria, scientists discovered gigantic viruses that rivaled bacteria in size in the 1990s according to the Nature Education report.
Scientists detected bacteria-like structures in amoebas from a water-cooling tower in 1992. A later investigation of the bacteria-like organisms, published in 2003, found that these unusual formations were actually enormous viruses, not bacteria. The massive virus was given the name Acanthamoeba polyphaga mimivirus by the researchers (APMV).
According to the Nature Education study, after the discovery of APMV, which has a diameter of 750 nanometers, researchers discovered more huge viruses, including a second strain of APMV nicknamed “mamavirus.” Mollivirus, Megavirus, Pithovirus, and Pandoravirus are the four known gigantic virus families. Giant viruses have been discovered in a variety of unusual locations, ranging from melting permafrost in Siberia to the depths of the Antarctic ocean, and have been shown to mostly infect amoebas and phytoplankton.
Despite the fact that laboratory studies suggest they could infect animal cells. Giant viruses may develop genes and proteins that aren’t found anywhere else on Earth, according to research, and they spew these genes out via a star-shaped gate on their surfaces.
Are Viruses a living thing?
Viruses teeter on the edge of what qualifies as life. On the one hand, viruses carry the nucleic acids DNA or RNA, which are found in all living beings. Viruses, on the other hand, lack the ability to read and act on the information contained inside those nucleic acids on their own; as a result, they aren’t regarded “alive.”
The Structure of Virus?
A virion is a virus that has been fully formed and is capable of infection. Simple virions, according to “Medical Microbiology (University of Texas Medical Branch in Galveston, 1996), have an inner nucleic acid core covered by a capsid (an outer casing of proteins). Capsids protect viral nucleic acids from being chewed up and destroyed by nucleases, which are specific enzymes found in the host cell.
The envelope is a second layer of protection that some viruses have. This layer is frequently made up of stolen bits from a host’s cell membrane that are changed and repurposed for the virus’s usage.
The virus’s genome, or the sum total of its genetic material, is made up of the DNA or RNA located in the inner core. Viral genomes are typically short, coding just for essential proteins including capsids, enzymes, and proteins required for reproduction within a host cell.
Virus: How do they work?
According to Jaquelin Dudley, a professor of molecular biosciences at the University of Texas at Austin, a virus requires a host cell to replicate, or generate new copies of itself. “The virus can’t reproduce outside the host because it lacks the complex machinery that a [host] cell has,” she explained . The cellular machinery of the host cell enables viruses to make RNA from their DNA (a process known as transcription) and build proteins using the instructions encoded in their RNA (a process called translation).
According to “Medical Microbiology,” a virus’s principal job is to “transport its DNA or RNA genome into the host cell so that the genome can be expressed (transcribed and translated) by the host cell.”
In the case of animals and humans, viruses first infect the host cell, which may be part of a larger organism. Viruses can enter the body through the respiratory tract and open wounds. Insects can also provide an entry point; certain viruses can travel in an insect’s saliva and then enter the host’s body after the bug bites. Such viruses can proliferate within both insect and host cells. This ensures a smooth transfer from one to the other. Viruses that cause yellow fever and dengue fever are examples of such pathogens.
Viruses attach themselves to the surface of host cells once inside an organism. They do so by identifying and binding to cell surface receptors, or proteins that protrude from the cell surface; viral proteins fit into these receptors like puzzle pieces. A single virus can bind to multiple cell surface receptors and many distinct viruses can bind to the same receptor. Viruses take advantage of cell surface receptors, yet they are actually designed to help the cell.
Viruses disrupt or hijack numerous components of the cellular machinery once inside the host cell. Host cells are directed to manufacture viral proteins by viral genomes, which often prevents the creation of RNA and proteins that the host cell may employ for its own functions.