Viruses are mainly thought to be infectious diseases that spread easily, originating from a cough or sneeze, or even out of nowhere. Although this is partially true, there is much more behind the development of a virus. Tracing the origins of viruses can be quite difficult because they do not necessarily leave behind any ‘fossils.’ Rather, they only make copies of themselves which means studying their ancestry requires pinpointing the host cell and trying to deduce its origins.
A virus is a non-living microscopic agent that has attachment proteins that act as receptors. Viruses are extremely small, approximately 20 to 400 nanometers in diameter. By comparison, a human red blood cell is about 6,000 to 8,000 nanometers in diameter. The structure of a virus has a center of nucleic acid (either DNA or RNA), and is enclosed in a defensive layer of protein called the capsid. A capsid is composed of protein subunits also known as capsomeres. Its envelope and the cell membrane are also made up of similar material.
Viruses have either single-stranded DNA, single-stranded RNA, double-stranded DNA, or double-stranded RNA. The type of genetic material found in a particular virus depends on the nature and function of the specific virus. The viral genome can consist of very few genes or up to hundreds of genes depending on the type of virus. The genetic material is not typically exposed but is covered by the capsid in order to be protected from damage.
Scientists have been able to theorize how viruses may develop based on the fact that the genes of many viruses, like those that cause herpes, share similar characteristics with the genes from cells. This theory is known as the Cellular Theory. This theory implies that viruses first started as big pieces of cellular DNA and eventually became independent. Others speculate that viruses came along very early in evolution, and some of their DNA stayed in cells’ genomes. This is known as the Theory of Evolution for Viruses. The fact that viruses infecting humans share similar structural features with viruses could mean that all of these viruses have a common origin, dating back billions of years.
Not all viruses look exactly the same in appearance, but they all share a similar structure. The shape of viruses varies widely. In general, viruses fit usually into two different visual categories. Viruses of humans, animals, and plants are typically spherical and rod-shaped, sometimes consisting of many sides. Viruses of bacteria (bacteriophages) are often shaped almost like a spaceship.
Figure 2: Visual of the structure of a virus that has a “spaceship” appearance.
Because viruses can not exist independently, they must take over a living cell in order to survive and reproduce. When a virus comes into contact with a susceptible host cell, it will latch itself onto the surface using its receptors. The virus will then inject its nucleic acid into the host cell. After, the virus will enter the cell in order to take control of the host by using the protoplasm,the living part of a cell that is surrounded by a plasma membrane, inside of it to create new viruses. Once these new virus particles assemble, they will leave the original host cell and find new host cells and repeat the same process, called the lytic cycle. There is no cell division within the development and replication of viruses. Viruses are only able to be replicated through the chemical synthesis of viral nucleic acid and capsid proteins. In the ending stage of virus replication, host cells that have been attacked may be completely destroyed or suffer little or no harm.
Few viruses go through an additional phase before replication, known as the lysogenic or dormant phase. During this phase, the virus can remain inside the host cell for extended periods of time without causing any changes to the cell. Once activated, however, these viruses can immediately enter into the lytic cycle.
Figure 3: Diagram of a lytic cycle, the process of virus replication.
Viruses cause a number of diseases in the organisms they infect. Human infections and diseases caused by viruses include Ebola fever, chickenpox, influenza, AIDS, and herpes. Plant diseases include mosaic disease, ringspot, leaf curl, and leafroll. Viruses known as bacteriophages cause disease in bacteria and archaeans. Humans can contract viruses through ingestion, sexual transmission, from the air, and many other ways.
Figure 4: Illustration of how airborne diseases can spread.
Vaccines have been effective in preventing some types of viral infections, such as smallpox and the flu. They work by helping the body build an immune system response against these specific viruses. However, it is important not to forget that many viruses can still cause serious damage to living things, and some can actually be fatal. Viruses like COVID-19, are examples of diseases that morphed into a strong and deadly virus that attacks the body and has the ability to kill.
Coronaviruses are a family of viruses that can cause illnesses such as the common cold and it is essentially made up of Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS). However, the newly discovered coronavirus, COVID-19 or SARS-CO-V-2, became so severe that it caused a worldwide pandemic and has resulted in hundreds of thousands of deaths. Experts say SARS-CoV-2 originated in China and was passed from bats to humans.
Figure 5: Diagram depicting the human-to-human transmission of coronavirus and its effects on the body.
The name “coronavirus” is derived from Latin: corona, meaning “crown” or “wreath.” Coined by June Almeida and David Tyrrell who studied the human coronavirus, the name was meant to refer to the appearance of the virus. Coronaviruses are large, roughly spherical shaped particles with viral spike peplomers which are actually proteins that lie on the surface of the virus. On average a coronavirus particle has 74 surface spikes all around. The average diameter of a virus particle is around 125 nm (.125 μm). They also contain a positive-sense, single-stranded RNA genome.
Figure 6: Image depicts the COVID-19 virus and its anatomy.
Inside their envelope lies the nucleocapsid, which is formed from multiple copies of nucleocapsid protein. They are bound to the positive-sense single-stranded RNA genome in a “continuous beads-on-a-string type conformation.” The lipid bilayer envelope, membrane proteins, and nucleocapsid protect the virus when it is outside the host cell.
Figure 7: Infographic detailing the structure and makeup of a COVID-19 virus particle.
A virus infects the body by entering healthy cells and going through the lytic cycle. There, the infector makes copies of itself and multiplies throughout the body. The new coronavirus would then latch its spiky surface proteins to receptors on healthy cells, especially those in the lungs, and this is why those who have contracted the coronavirus experience respiratory issues. Eventually, the virus kills off some of the healthy cells.
COVID-19 begins with droplets from an infected person’s cough, sneeze, or breath. They could travel through the air or be on a surface that someone may touch before touching the eyes, nose, or mouth. That gives the virus a passage into the mucous membranes and allows it to segway into the throat. Within 2 to 14 days, the immune system may respond with symptoms including a fever, cough, body aches, headaches, shortness of breath, chills, loss of taste, loss of smell, nausea, and other symptoms. The virus moves down into the respiratory tract, the airway that is connected to the mouth, nose, throat, and lungs. Because the lower airways have more ACE2 receptors compared to the respiratory tract, COVID-19 is more likely to travel deeper than the average cold.
Viruses are very complex in the way they manifest, and there is still much research that has yet to be discovered about viral infections. Doctors are continuously working close with researchers to develop stronger and more efficient ways of combating these infections. Currently, there are different vaccines being made in order to eventually fight COVID-19 virus cases.
Figure 8: A visual referencing the symptoms that may come with COVID-19.
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