**5. How virus replicate**

As viruses are obligate pathogen they cannot replicate on their own but they need the host to replicate and produce multiple copies of them. This typically occurs by the virus inserting its genetic material in host cells, co-opting the proteins to create viral replicates, until the cell bursts from the high volume of new viral particles.

There are six fundamental stages that are essential for viral replication (**Figure 3**).


**Figure 3.** *Stages of viral replication.*

*Role of Anti-Viral Drugs in Combating SARS-CoV-2 DOI: http://dx.doi.org/10.5772/intechopen.99599*


CoVs are positive stranded RNA enveloped viruses. Upon entry into the host, viral RNA replication begins with synthesis of polyprotein 1a/1ab(pp1a/pp1ab). By the synthesis of subgenomic RNAs sequences and replication-transcription complex, transcription occurs. Termination of transcription occurs at transcription regulatory sequences. Six open reading frames (ORF) are present on CoV genome. Frameshift between ORF 1a and ORF1b helps in production of both pp1a and pp1ab polypeptides which are further processed by chymotrypsin like protease (3CLpro) or main protease or may include papain like proteases which produces non-structural proteins (NSPs 1–16). Besides ORF1a and ORF1b, other ORFs encode structural proteins including nuclecapsid proteins, membrane, envelope, spike and accessory proteic chains [17]. Dedicated sgRNAs translate accessory and structural proteins. Researchers have also reported NSPs in stalling hosts immunity [18]. The envelop plays crucial role in viral pathogenesis as it encourages viral release and assembly [19].
