March 25, 2020

Coronavirus- A crash course

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1, What is a Coronavirus?
Like most organism viruses too are classified by their phenotypic characteristics (morphology), Coronavirus belongs to the subfamily Orthocoronavirinae- family coronaviridae- order Nidovirales - & realm Riboviria. The genus Alpha, Beta, Gamma, and Delta-Coronavirus constitutes the sub-family Orthocoronavirinae.
CoV is a faction of viruses that cause diseases in mammals and birds. Diseases include mild/ fatal respiratory tract infection (for humans), Diarrhea (for bovines & pigs) and URTI (in chickens).

2, How does it look?
Derived from the Greek word "korṓnē" which means "Garland". the Coronavirus typically looks like a spikey crown medieval kings wore. They have fringes of large, bulging surface projections that resemble a solar corona. The spikes are due to the peplomers- which are proteins residing on the surface of the virus.

C.S. of coronavirus (Credits: Scientific Animations)
3, What is it made of?
Size ranging from 27-34 kilobases & a diameter of 120nm, Coronavirus is an enveloped virus that is composed of a positive-sense (positive-strand- 5'-3'), single-stranded RNA genome. The viral envelope is made up of a bilipid layer (fat membrane and spikes anchored together) and Nucleocapsid i.e. a protein shell is present inside the envelope that is bound to the positive-stranded RNA- which resembles a beads-on-string type conformation.
The bilipid envelope protects the virus when it is outside the host cell. The glycoproteins on the surface of the envelope help the virus to adapt/change in a short time thereby avoiding the host immune system.

4, What does it do?
When a virus enters the host system, the glycoprotein-viral spikes (Peplomers) attach itself to the host cell receptor. The virus then sneaks into the host cell through endocytosis. Once it enters, it sheds its outer coating thereby releasing its Genome into the host cell's cytoplasm, ribosome (for translation). This directly results in the virus actively replicating inside the host's body.

5, How does it spread from an Animal?
Most bacteria, viruses or parasites cross the xenographic barrier i.e. they mutate/ they are capable of spread from non-human animals (typically vertebrates) to humans. Direct transmission between animal to human is known as Direct Zoonosis (e.g. Rabies, Influenza) and indirect transmission occurs when a vector or intermediate carries the diseases from an infected host to humans. (e.g. HIV, SARS)

6, Are all coronavirus fatal?
Not all coronaviruses are capable of inducing fatal diseases. We know 7 strains of human coronavirus, of which 4 reportedly cause mild symptoms (such as- common cold). The remaining three strains such as the Middle East respiratory syndrome-related coronavirus (MERS-CoV), Severe Acute respiratory syndrome coronavirus (SARS-CoV) & Novel coronavirus 2019 (SARS-CoV-2) are relatively severe.

7, What happens to your lungs when you're infected with fatal CoV?
Mammals are evolved in such a way that our lungs deliver oxygen to the bloodstream via air sacs. The air we breathe in enters our trachea, which divides at left and right bronchi, then it branches further at each bronchus until it becomes narrower & narrower. Finally, they end at the smallest possible airways called 'alveoli'. Our body has 300-600 million alveoli (air-sacs) which are surrounded by tiny blood vessels. Oxygen from the air we just inhaled moves across the walls of alveoli and gets picked up by the blood, thereby starting its journey to power our body. The excess waste or carbon dioxide gas is exhaled via the same route.
Novel Coronaviruses target our lungs & immune system by binding itself to various receptors. Severe cases of CoV results in ARDS- Acute Respiratory Distress Syndrome, pneumonia- an inflammatory condition that affects the alveoli. CoV damages the alveolar walls, cell linings, and the surrounding capillaries. As the damaged/ dead air sacs accumulate at their respective openings, it becomes harder & harder for the lungs to transfer oxygen to the blood. This is when we feel shortness in breath and other lethal symptoms. Our body is entirely capable of fighting back, but it's relatively harder for people with underlying diseases (such as Diabetes, Heart conditions, Immunocompromised individuals).

8, What exactly happens? (With examples from MERS & COVID19)
MERS-CoV (Credits:
As mentioned above, Coronaviruses enters the host cell by attaching itself to "receptors".
In the case of MERS-CoV, the virus enters the host cell by binding itself to DPP4 receptors (DPP4 Gene is an enzyme affiliated to immune regulation. apoptosis, etc) in the lung tissues, thereby replicating & damaging the alveoli.
While in SARS-CoV-2/ 2019 Novel coronavirus, the virus enters the host cells via ACE2 enzyme (ACE2 enzymes are predominantly found in the outer surface of lungs, kidney, and intestines). ACE2 enzymes are copiously present in type II alveolar cells of the lungs. (in case of a potential infection- The percentage of ACE2 enzymes present in each lung tissue is directly related to the severity of the disease)



  1. Family Coronaviridae". In King AM, Lefkowitz E, Adams MJ, Carstens EB, International Committee on Taxonomy of Viruses, International Union of Microbiological Societies. Virology Division (eds.). Ninth Report of the International Committee on Taxonomy of Viruses. Oxford: Elsevier.
  2. Homology-Based Identification of a Mutation in the Coronavirus RNA-Dependent RNA Polymerase That Confers Resistance to Multiple Mutagens Nicole R. Sexton,a,b Everett Clinton Smith,b,c Hervé Blanc, Marco Vignuzzi,d Olve B. Peersen,e and Mark R. Denison 
  3. Coronaviruses: An Overview of Their Replication and Pathogenesis , Anthony R. Fehr and Stanley Perlman

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