Viruses that infect eukaryotic cells consist of a nucleic acid, either DNA or RNA, covered by a protein coat, a capsid. A single protein subunit of the capsid is referred to as a capsomere. The capsids of most eukaryotic viruses consist of a number of different proteins. The complex of nucleic acid and capsid is designated the nucleocapsid. Many animal viruses are surrounded by a membrane (lipid bilayer) derived from the host cell in which they proliferate. These viruses are said to be enveloped. The complete, intact virus particle is referred to as a virion.
The primary characteristics used to differentiate eukaryotic viruses are associated with their nucleic acid. First, viruses may be separated based on whether they are DNA viruses or RNA viruses. The nucleic acid may be single-stranded (ss) or double-stranded (ds), depending on the species. If the ssRNA is able to function as mRNA it is referred to as plus strand RNA (+RNA); if it is the equivalent to antisense RNA it is known as minus strand RNA (-RNA). Some of the genomes in plant and animal viruses are fragmented into segments. Virion shape is also used to differentiate among the viruses since they have a number of distinctive forms: cylindrical or helical, spherical, icosahedral, bulletshaped, or even more complex shapes. The presence or absence of an envelope and the virion size are also helpful in distinguishing viruses.
Viruses are not considered "living" organisms.
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