Biology of Coronaviruses and Predicted Origin of SARS-CoV-2

Coronaviruses are enveloped viruses with a single-stranded, positive-strand RNA genome, belonging to the Coronaviridae family and the Nidovirales order. They are divided into four genera, Alpha-, Beta-, Gamma-, and Delta-coronaviruses isolated from many mammals and birds. Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has an approximately 30 kb-long genome containing 16 non-structural, four structural, and six accessory genes. Its life cycle is characterized by attachment to the ACE2 host receptor, viral entry through endosomes, and RNA release into the cytoplasm for replication and expression of structural proteins. Mature viral particles are assembled and released by exocytosis. SARS-CoV-2 is a virulent, pandemic human pathogen causing Coronavirus Disease 19 (COVID-19) and usually infects the human respiratory tract. Clinical manifestations are variable and range from a mild infection of the upper airways to life-threatening interstitial pneumonia, especially in elderly individuals or patients with comorbidities. Studies on the origin of SARS-CoV-2 are of utmost significance, as they not only provide clues on where, how, and from what species SARS-CoV-2 originated, but also provide crucial means for better preparation for any emerging outbreaks. So far, the hypothesis of its natural animal origin has been favored and an unintentional laboratory leak cannot be excluded.