Viral replication occurs inside a host cell and typically greatly harms or kills that cell. The invading virus redirects the host cell to generate components of new virions. Replication occurs within the six distinctive steps of the virus life cycle.
1. Attachment is the first step in the virus life cycle, in which a virion attaches to a host cell's surface.
2. Penetration is the second step in the virus life cycle, when the virion enters or injects its nucleic acid into the host cell.
3. Uncoating is the third step that occurs in enveloped viruses, when the virion enters a host cell and viral DNA or RNA is freed from the capsid and viral envelope.
4. Synthesis, the fourth step in the virus life cycle, occurs when the virus directs the host cell's metabolism to produce the virus's nucleic acid and protein.
5. Assembly, the fifth step in the virus life cycle, in which new viruses are created by packaging of the replicated genome into capsid.
6. Release is the sixth step of the virus life cycle, when mature virions leave the host cell. The life cycle may repeat several times.
Specific processes involved in the life cycle vary with the type of virus and host cell. For instance, plant viruses cannot penetrate plant cells' tough walls and instead rely on insects to inject viral nucleic acids through their proboscis (tubular mouthparts). In contrast, entire virions of animal viruses typically penetrate animal host cells, which lack cell walls. Most animal viruses carry out penetration through endocytosis. Endocytosis is a form of bulk transport that moves material into a cell by an infolding of the cell membrane around the material, forming a vesicle (small sac) that moves into the cell.
The release stage also varies between viruses. Viral budding is a process by which a mature virion leaves a host cell, borrowing contents of the host membrane to build its viral envelope as it exits. Exocytosis is a form of bulk transport used to move material outside the cell by fusion of a vesicle with the plasma membrane and release of the contents outside the cell. Budding, typical in enveloped viruses, does not kill the host cell. Nor does exocytosis, common in nonenveloped viruses. Often, however, viruses rupture and kill host cells upon release. This event is called cell lysis.