Coronavirus Morphology

 

Coronavirus 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.
The viral envelope consists of a lipid bilayer where the membrane (M), envelope (E) and spike (S) structural proteins are anchored.
A subset of coronaviruses (specifically the members of Betacoronavirus subgroup A) also have a shorter spike-like surface protein called hemagglutinin esterase (HE).
Inside the envelope, there is the nucleocapsid, which is formed from multiple copies of the nucleocapsid (N) protein, which are bound to the positive-sense single-stranded RNA genome in a continuous beads-on-a-string type conformation.

The genome size for coronaviruses ranges from approximately 27 to 34 kilobases. 
 

The lipid bilayer envelope, membrane proteins, and nucleocapsid protect the virus when it is outside the host cell.
 

Coronavirus Replication

Infection begins when the virus enters the host organism and the spike protein 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 cell entry through 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.
A number of the nonstructural proteins coalesce to form a multi-protein replicase-transcriptase complex (RTC). 
The main replicase-transcriptase protein is the RNA-dependent RNA polymerase (RdRp). 
It is directly involved in the replication and transcription of RNA from an RNA strand. The other nonstructural proteins in the complex assist in the replication and transcription process. The exoribonuclease non-structural protein for instance provides extra fidelity to replication by providing a proofreading function which the RNA-dependent RNA polymerase lacks.
One of the main functions of the complex is to replicate the viral genome. RdRp directly mediates the synthesis of negative-sense genomic RNA from the positive-sense genomic RNA. This is followed by the replication of positive-sense genomic RNA from the negative-sense genomic RNA.
The other important function of the complex is to transcribe the viral genome. RdRp directly mediates the synthesis of negative-sense subgenomic RNA molecules from the positive-sense genomic RNA. This is followed by the transcription of these negative-sense subgenomic RNA molecules to their corresponding positive-sense mRNAs.
The replicated positive-sense genomic RNA becomes the genome of the progeny viruses. 
The mRNAs are gene transcripts of the last third of the virus genome after the initial overlapping reading frame. These mRNAs are translated by the host's ribosomes into the structural proteins and a number of accessory proteins.
RNA translation occurs inside the endoplasmic reticulum. The viral structural proteins S, E, and M move along the secretory pathway into the Golgi intermediate compartment. There, the M proteins direct most protein-protein interactions required for assembly of viruses following its binding to the nucleocapsid.
Progeny viruses are then released from the host cell by exocytosis through secretory vesicles.