Fig. 3

Diagrammatic representation of the life cycles of the viruses causing respiratory infections. The life cycle includes 1. Endocytic uptake of virus inside the cell, 2. Release of virus from Endosome in cytosol, 3. Microtuble mediated Trafficking of viral particle 4. Uncoating and Release of viral genome into the nucleus for further replication, 5. Viral particles assembled in the nucleus and 6. Cell lysis and release of new virions outside cell. [(Viruses of the families A, B and C enters the cell by binding through cell receptors (CAR for Adenoviridae, VP1u-interacting protein for Parvoviridae, S for Orthomyxoviridae). Internalization takes place by receptor mediated endocytosis inside the cells encapsulated by endosome. The internalised virus is further released from endosome and partially uncoated virus will be directed to nucleus through microtubule network. The virus disassembly occurs in nucleus. The viral genome will be released and replicates inside the nucleus. The transcribed mRNA will be translated in cytosol and expressed proteins will be imported to nucleus, where new virus assembly takes place. Newly assembled virus particles will be released outside the cell by cell lysis]. Whereas the viruses of the families D, E and F binds to the cells through cell surface receptors but Paramyxoviridae will be released directly into cytoplasm, whereas Coronaviridae and Picornaviridae will be internalised inside endosome and released in cytoplasm by endosome lysis. In Paramyxoviridae, following release of negative sense RNA genome, positive sense RNA will be produced for protein synthesis. Whereas in Picornaviridae and Coronaviridae, the positive sense RNA genome is replicated into intermediate –ve sense RNA which then serves as template to form positive sense RNA genome of the progeny virus. The assembly of viral particles takes place at ER-Golgi intermediate compartment but in Paramyxoviridae and Picornaviridae virus assembly takes place at cytosolic double membrane compartment and plasma membrane, respectively. Nascent virion budding takes place through plasma membrane in Paramyxoviridae but in Coronaviridae the virus is transported in outer space through exocytic vesicles. In Picornaviridae the newly synthesized virus is directly released through cell lysis. (ER: endoplasmic reticulum)