A virion is the complete, infectious form of a virus, existing as a particle outside of a host cell. This particle is the vehicle that moves viral genetic information from one host to another. Once it enters a suitable cell, it initiates replication by taking over the host’s cellular machinery to create more viruses.
Anatomy of a Virion
The core of every virion is its nucleic acid genome, which carries the complete set of instructions for making new viruses. This genetic material can be either DNA or RNA, and it may be single-stranded or double-stranded. The genome is enclosed by a protective protein shell called a capsid, which safeguards the viral genes from environmental damage, such as from enzymes called nucleases.
Capsids are built from multiple copies of one or more types of protein subunits. These subunits assemble into symmetrical structures, most commonly in helical (rod-like) or icosahedral (a 20-sided polygon) shapes. The specific arrangement and number of these protein units are characteristic of a particular virus. For instance, adenoviruses have 252 of these structural units, known as capsomeres, forming their icosahedral capsid.
Some virions have an additional layer called an envelope, a lipid membrane derived from the previous host cell as the virus exited. This envelope is studded with viral proteins, often glycoproteins, which can appear as spikes on the virion’s surface. Virions possessing this outer layer are termed “enveloped,” while those without it are referred to as “naked” or “non-enveloped.”
The Infection Cycle
A virion’s purpose is to deliver its genome into a host cell so that replication can occur. This process begins with attachment, where the virion physically binds to specific receptor molecules on the surface of a host cell. The proteins on the virion’s capsid or envelope fit into these cellular receptors, an interaction that determines which types of cells a virus can infect.
Following attachment, the virion or its genetic material enters the cell in a step called penetration. Enveloped virions often achieve this by fusing their lipid envelope directly with the host cell’s membrane, releasing the internal components into the cell. Other viruses may be taken into the cell through a process called endocytosis, where the cell membrane engulfs the entire particle.
Once inside, the virion ceases to be a complete particle. The final preparatory step of infection is uncoating, where the capsid breaks down and releases the viral nucleic acid into the host cell’s cytoplasm. For some viruses, this process is triggered by the acidic environment inside cellular compartments. With the genome now free, the virus begins to hijack the cell’s machinery for replication.
Virion vs. Virus Clarified
The terms “virion” and “virus” are often used interchangeably, but they have distinct meanings in a biological context. “Virion” refers specifically to the physical, infectious particle that exists outside of a host cell. It is the assembled package of genetic material and proteins that is structurally complete and capable of transmission from one cell to another.
“Virus,” on the other hand, is a broader term that encompasses the entire life cycle of the entity. This includes the virion particle itself, as well as the non-particulate, replicative stage that occurs inside the host cell. During this intracellular phase, the virus exists as replicating nucleic acids and newly synthesized proteins that have not yet been assembled into new virions.
An effective way to understand the difference is to compare it to a fungus and its spore. The spore is the dormant, transmissible unit that can survive in the environment, much like a virion. The term “fungus” describes the entire organism, including its growth and reproductive phases.