SARS және COVID-19 коронавирусының түрлері

Species of coronavirus causing SARS and COVID 19
a species of coronavirus comprising multiple strains

Severe acute respiratory syndrome–related coronavirus (SARSr CoV or SARS CoV) is a species of virus consisting of many known strains. Two strains of the virus have caused outbreaks of severe respiratory diseases in humans: severe acute respiratory syndrome coronavirus 1 (SARS CoV or SARS CoV 1), the cause of the 2002–2004 outbreak of severe acute respiratory syndrome (SARS), and severe acute respiratory syndrome coronavirus 2 (SARS CoV 2), the cause of the pandemic of COVID 19. There are hundreds of other strains of SARSr CoV, which are only known to infect non human mammal species: bats are a major reservoir of many strains of SARSr CoV; several strains have been identified in Himalayan palm civets, which were likely ancestors of SARS CoV 1. These enveloped, positive sense single stranded RNA viruses enter host cells by binding to the angiotensin converting enzyme 2 (ACE2) receptor. The SARS related coronavirus was one of several viruses identified by the World Health Organization (WHO) in 2016 as a likely cause of a future epidemic in a new plan developed after the Ebola epidemic for urgent research and development before and during an epidemic towards diagnostic tests, vaccines and medicines. This prediction came to pass with the COVID 19 pandemic.

Phylogenetic analysis showed that the evolutionary branch composed of Bat coronavirus BtKY72 and BM48 31 was the base group of SARS–related CoVs evolutionary tree, which separated from other SARS–related CoVs earlier than SARS CoV 1 and SARS CoV 2.]] The morphology of the SARS related coronavirus is characteristic of the coronavirus family as a whole. The viruses are large pleomorphic spherical particles with bulbous surface projections that form a corona around the particles in electron micrographs. The size of the virus particles is in the 80–90 nm range. 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) proteins are anchored. The spike proteins provide the virus with its bulbous surface projections, known as peplomers. The spike protein's interaction with its complement host cell receptor is central in determining the tissue tropism, infectivity, and species range of the virus. 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 (~30 kb) 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.

SARS related coronavirus follows the replication strategy typical of all coronaviruses.

The attachment of the SARS related coronavirus to the host cell is mediated by the spike protein and its receptor. The spike protein receptor binding domain (RBD) recognizes and attaches to the angiotensin converting enzyme 2 (ACE2) receptor. The attachment of sarbecoviruses to ACE2 has been shown to be an evolutionarily conserved feature, present in many species of the taxon. The first path the SARS coronavirus can take to enter the host cell is by endocytosis and uptake of the virus in an endosome. The receptor attached spike protein is then activated by the host's pH dependent cysteine protease cathepsin L. Activation of the receptor attached spike protein causes a conformational change, and the subsequent fusion of the viral envelope with the endosomal wall. In the SARS coronavirus, the activation of the C terminal part of the spike protein triggers the fusion of the viral envelope with the host cell membrane by inducing conformational changes which are not fully understood.

A number of the nonstructural replication proteins coalesce to form a multi protein replicase transcriptase complex (RTC). One of the main functions of the replicase transcriptase 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 other important function of the replicase transcriptase 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.

When two SARS CoV genomes are present in a host cell, they may interact with each other to form recombinant genomes that can be transmitted to progeny viruses. Recombination likely occurs during genome replication when the RNA polymerase switches from one template to another (copy choice recombination).

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 released from the host cell by exocytosis through secretory vesicles.