A virus must undergo the process of replication in order to create new, infectious virions that are able to continue the chain of infection. The viral replication process takes place over six steps. Depending on the virus, some steps may occur simultaneously or they may occur in a different order. In general, the steps of viral replication are as follows:
Attachment - In this step, the virus attaches to a specific receptor site on the target cell membrane either via glycoproteins embedded in the viral envelope or through attachment proteins present in the capsid.
Penetration - Plant and animal viruses enter the host cell via endocytosis. When these viruses attach to the host cell, they are surrounded and engulfed by the cell membrane. When bacteriophages attach to the host cell, only the nucleic acid penetrates the membrane and enters the cell. The capsid is left behind outside the cell.
Uncoating - The viral capsid degrades when the virus enters the host cell. The viral nucleic acid, which is released, becomes available for replication and transcription.
Replication - The replication mechanism may vary depending on the viral genome. In general, DNA viruses make additional DNA using the host cell’s proteins, whereas RNA viruses synthesize viral genomic RNA and mRNA using the RNA core as a template.
Assembly - The viral proteins are assembled in this step.
Egress or Release - This is the last stage of viral replication. In this step, the newly created viruses are released from the host cell using different mechanisms. Some viruses exit by bursting the host cell in a process known as lysis. Others exit by following the cell’s own pathways, in a process called exocytosis. A few viruses exit by budding from the cell’s plasma membrane.