virionit

A virion is the complete infectious form of a virus outside a host cell. It consists of a nucleic acid genome, which can be DNA or RNA, enclosed within a protective protein shell called a capsid. In many viruses, the genome–capsid complex, or nucleocapsid, is further packaged within a lipid envelope derived from a host cell membrane; envelope-embedded glycoproteins mediate attachment and entry into susceptible cells. Some virions, known as non-enveloped or naked virions, lack this lipid envelope.

Virions exhibit structural diversity. Capsids may display icosahedral or helical symmetry, and the envelope’s presence and composition vary among virus families. Size is variable, typically about 20 to 300 nanometers in diameter.

The virion's primary role is to protect the genome and to deliver it to a new host cell. Entry occurs when virion surface proteins bind to cell receptors; uptake is often by endocytosis or fusion of the viral envelope with cellular membranes. After uncoating, the viral genome is released for replication. Newly formed virions are assembled within the cell and released by lysis (for many non-enveloped viruses) or budding through membranes (for enveloped viruses).

Studied in virology and routinely used in vaccine development and diagnostic assays, virions are central to understanding viral transmission, stability, and host range. Handling requires appropriate biosafety measures due to pathogenic risk.