Final answer:
A virion is a complete, extracellular virus particle that includes a nucleic acid core, a protective protein capsid, and sometimes an outer envelope. It is the infectious form of a virus outside the host cell. Virions require a host cell to replicate as they are acellular and cannot reproduce independently.
Step-by-step explanation:
A virion is a complete, extracellular virus particle. Specifically, a virion is an individual virus particle outside a host cell, comprising a nucleic acid core, which contains the virus genome, an outer protein coating referred to as a capsid, and sometimes, an outer envelope derived from the host cell. This structure is the infectious form of a virus outside the host cell and is responsible for transmitting the viral genome from one cell to another.
Viruses are acellular, meaning they lack a cellular structure, and therefore, they do not have cell membranes, cytoplasm, or ribosomes. A virion can be quite small, ranging from about 20-250 nanometers. Due to their size, they are not typically visible with a standard light microscope, except for the largest kinds like the poxvirus family.
While all viruses have a nucleic acid and a capsid, not all are enveloped; thus, a virion is not necessarily an enveloped virus particle, as they can either have this lipid envelope or not. It's also important to note that a virion contains either DNA or RNA (not both at the same time), and viruses require a host cell to replicate, as they cannot replicate on their own outside of the cell.