**3. Coronaviridae family: structure and classification**

Coronaviridae family members are large, enveloped, single-stranded positivesense RNA types of viruses. Their genomic material made up of nucleotide sets ranging from 25 to 32 kb and the virus diameter varies from 118–136 nm [30, 60]. The virus is roughly spherical in shape and has obvious proteins on its cell membrane such as the large spike (S) protein extended 16–21 nm from the virus envelope, membrane protein (M-protein) plays a major role in promoting membrane curvature, envelope protein (E-protein), in low quantity, and hemagglutininesterase (HE) as shown in **Figure 1** [27, 61].

Coronaviridae family divided into two subfamilies: the Coronavirinae and the Torovirinae. Coronavirinae is categorized into four geniuses, alpha-coronavirus, beta-coronavirus, gamma-corona-virus and delta-coronavirus. On the other hand, Torovirinae has only one genus which is Torovirus. In contrast to Coronavirinae subfamily, Toroviruses have a helical, doughnut-shaped nucleocapsid in their structure. Unlike Toroviruses, Coronavirinae subfamily is prevalent among mammals which cause mild respiratory illnesses such as Severe Acute Respiratory Syndrome 1–2 (SARS-1-2) and Middle East Respiratory Syndrome (MERS) or enteric infections [30, 64].

**Figure 1.** *Structural features shared by Coronaviridae family.*
