viriony

A virion is the complete, infectious form of a virus outside a host cell, and the form that mediates transmission between hosts. It consists of a nucleic acid genome encased in a protective protein shell called the capsid; many virions also possess a lipid envelope derived from a host cell membrane, with viral glycoprotein spikes embedded in the envelope. Outside cells, virions are typically metabolically inert and rely on host cells to replicate.

The genome of a virion can be composed of DNA or RNA, and may be single-stranded or double-stranded, linear or segmented. Inside the capsid, virions may carry enzymes such as polymerases, reverse transcriptase, or proteases that are needed for initiating replication after infection. The outer layers may include a matrix or tegument region in enveloped viruses, which contains proteins important for early steps of infection, while naked (nonenveloped) virions consist mainly of the capsid and genome.

Virions vary in size, generally from about 20 to 300 nanometers, and in morphology. Common shapes include icosahedral and helical; some viruses have complex or multipart structures. Enveloped virions are more sensitive to desiccation and detergents than naked virions, reflecting their lipid envelope.

Infection begins when a virion attaches to specific receptors on a host cell, followed by entry and uncoating to release the genome. The host cell then transcribes and replicates the viral genome, producing new virions that assemble and exit, often by budding (enveloped viruses) or lysis (nonenveloped viruses).