Corona virus
Coronaviruses are a group of related viruses that cause diseases in mammals and birds. In
humans, coronaviruses cause respiratory tract infections that can range from mild to lethal. Mild
illnesses include some cases of the common cold (which has other possible causes,
predominantly rhinoviruses), while more lethal varieties can cause SARS, MERS, and COVID-19.
Symptoms in other species vary: in chickens, they cause an upper respiratory tract disease, while in
cows and pigs they cause diarrhea. There are yet to be vaccines or antiviral drugs to prevent or treat
human coronavirus infections.
Coronaviruses constitute the subfamily Orthocoronavirinae, in the family Coronaviridae,
order Nidovirales, and realm Riboviria. They are enveloped viruses with a positive-sense single-
stranded RNA genome and a nucleocapsid of helical symmetry. The genome size of coronaviruses
ranges from approximately 26 to 32 kilobases, one of the largest among RNA viruses. They have
characteristic club-shaped spikes that project from their surface, which in electron
micrographs create an image reminiscent of the solar corona from which their name derives.
Infection begins when the viral spike (S) glycoprotein attaches to its complementary host cell
receptor. After attachment, a protease of the host cell cleaves and activates the receptor-attached
spike protein. Depending on the host cell protease available, cleavage and activation allows
the virus to enter the host cell by endocytosis or direct fusion of the viral envelop with the host
membrane.
On entry into the host cell, the virus particle is uncoated, and its genome enters the cell cytoplasm.
The coronavirus RNA genome has a 5′ methylated cap and a 3′ polyadenylated tail, which allows
the RNA to attach to the host cell's ribosome for translation The host ribosome translates the initial
overlapping open reading frame of the virus genome and forms a long polyprotein. The polyprotein
has its own proteases which cleave the polyprotein into multiple nonstructural proteins.
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