Morphology

Coronaviruses are large pleomorphic spherical particles with bulbous surface projections. The diameter of the virus particles is around 120 nm. The envelope of the virus in electron micrographs appears as a distinct pair of electron dense shells as it is studded with projecting glycoproteins.

The viral envelope consists of a lipid bilayer where the membrane, envelope and spike structural proteins are anchored. A subset of coronaviruses (specifically the members of beta coronavirus subgroup A) also have a shorter spike-like surface protein called hemagglutinin esterase.

The envelope glycoproteins are responsible for attachment to the host cell and also carry the main antigenic epitopes, particularly the epitopes recognized by the neutralizing antibodies.

Inside the envelope, there is the nucleocapsid, which is formed from multiple copies of the nucleocapsid protein, which 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.