A virion represents the complete, individual particle of a virus existing outside a host cell. It is the inert, transmissible form that allows the virus to spread between hosts or cells. While the term “virus” broadly encompasses the entire infectious agent and its life cycle, including its active replication within a host cell, a virion specifically refers to this extracellular, infectious particle. Think of a virion like a seed; it contains the necessary genetic information and protective structures to initiate a new infection, but it is not actively replicating until it finds a suitable environment.
The Structure of a Virion
A virion has fundamental components that protect its genetic material and infect a host. At its core is the nucleic acid, the virion’s genetic blueprint. This genetic material can be either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), and it may exist as single-stranded or double-stranded molecules. Surrounding this nucleic acid is a protective protein shell called the capsid. The capsid is constructed from repeating protein subunits known as capsomeres, which self-assemble to form a stable structure.
Some virions have an additional outer layer, an envelope. This lipid bilayer is derived from the host cell membrane during release. This envelope often contains viral proteins, such as glycoproteins, that extend from its surface. Virions without this outer membrane are referred to as “naked.” These structural components give virions distinct shapes, which are broadly categorized into four main types.
- Icosahedral virions appear roughly spherical (e.g., herpesviruses, poliovirus), characterized by a 20-sided geometric shape of equilateral triangles.
- Helical virions (e.g., tobacco mosaic virus) are long and cylindrical, with nucleic acid coiled inside a hollow protein tube.
- Enveloped virions (e.g., influenza, HIV) have an outer membrane surrounding their capsid, often appearing spherical or pleomorphic.
- Complex virions (e.g., bacteriophages) combine multiple symmetries, featuring an icosahedral head attached to a helical tail.
The Virion’s Role in Infection
A virion’s main role is to deliver its genetic material into a host cell to initiate replication. This process begins with attachment, where the virion recognizes and binds to a permissive host cell’s surface. Binding occurs through interactions between specialized virion proteins (e.g., glycoproteins on enveloped virions, capsid proteins on naked virions) and specific receptor molecules on the host cell’s membrane. This precise “key-and-lock” mechanism determines which cell types or species a particular virion can infect.
After attachment, the virion enters the host cell. Enveloped virions enter by fusing their outer lipid envelope directly with the host cell’s plasma membrane, releasing the inner nucleocapsid into the cytoplasm. Alternatively, many virions, both enveloped and non-enveloped, enter through endocytosis. In endocytosis, the host cell engulfs the virion, enclosing it within a vesicle that moves into the cell’s interior.
Once inside the host cell, the final step for the virion is uncoating. During uncoating, the virion’s capsid breaks down, releasing the genetic material (DNA or RNA) into the host cell’s cytoplasm. This uncoating is often triggered by changes in the cellular environment, such as a drop in pH within endosomes or host enzymes. The genetic material’s release signals the end of the virion’s journey as an intact particle, making its blueprint accessible for replication by the host cell’s machinery.
Assembly and Release of New Virions
After a virion delivers its genetic material, the viral blueprint takes control of the host cell’s machinery. The host cell is reprogrammed to produce viral proteins and nucleic acids, diverting cellular resources from normal functions. These newly synthesized components then assemble to form progeny virions. This assembly can occur in various locations within the host cell, including the cytoplasm or the nucleus, depending on the specific type of virus.
Newly assembled virions must be released from the infected cell to spread infection to other cells or hosts. There are two main methods of virion release. The first method, characteristic of many non-enveloped virions, involves the accumulation of new particles within the host cell, eventually causing the cell to rupture, a process known as lysis. This bursting releases a large number of virions simultaneously, leading to host cell death.
The second method of release, typical for enveloped virions, is budding. During budding, the assembled nucleocapsid moves to a cellular membrane (often the plasma membrane) where viral proteins have been inserted. The nucleocapsid then pushes outward, acquiring a portion of the host cell’s lipid membrane as its outer envelope, pinching off from the cell. This process allows enveloped virions (e.g., influenza, HIV) to exit the host cell without immediately causing its death, potentially allowing the infected cell to continue producing new virions for an extended period.