What Are Viruses?

A virus is an infectious agent consisting of a nucleic acid strand within a protein coat. It is a unique and nonliving particle that replicates only in cells of other organisms. Most viruses are smaller than prokaryotic cells, although they range in size and shape. Each virus is essentially a deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) genome surrounded by a protein coat. A genome is the genetic material of an organism. Viruses invade the host cell and use the host cell's system and energy to replicate. Viruses also have an extracellular form, which functions independently of a host and is called the virion or viral particle. The virion moves a virus from one host to another. The nucleic acid that makes up the virus genome—DNA or RNA—can be single-stranded or double-stranded, as well as linear or circular. Both of these characteristics affect the viral replication process. A viral capsid is the protein coat surrounding and protecting the viral genome. The viral genome directs the synthesis of capsid proteins and some enzymes necessary for viral penetration in some viruses. The capsid often includes multiple proteins, with molecules arranged around the DNA or RNA into capsomeres. A capsomere is the collection or assembly of protein molecules making up a viral capsid. Many capsomeres can make up a single virion. The entire complex of capsid and nucleic acid is the nucleocapsid. Some viruses are "naked," with no layers around the nucleocapsid, whereas others have a viral envelope. The viral envelope is a structure that consists of lipid-containing layers that surrounds the nucleocapsid of a virus. Viruses with a viral envelope are described as "enveloped." Capsomeres are arranged into two types of symmetry, helical and icosahedral, that determine viral shape. Helical is the type of symmetry of capsomere arrangement associated with spiral-shaped viruses. Icosahedral is the type of symmetry of capsomere arrangement associated with spherical viruses. An icosahedron is a roughly spherical geometric structure with 20 triangular faces and the most efficient arrangement of capsomeres in a viral capsid. Some viruses, including large bacteriophages—viruses that infect bacteria—have a complex structure. Many bacteriophages, for instance, have icosahedral heads and helical tails.

Because viruses are unique and highly diverse, their classification is complex. Viruses are not grouped into a kingdom but are typically classified into ten orders and 134 families. A species is a unique member of a monophyletic group (descended from the same ancestor) that shares distinctive properties different from other species. Viruses are classified primarily by which nucleic acid they carry and by the nucleic acid strandedness (single or double). Genomes of most double-stranded (ds) RNA viruses are segmented, whereas dsDNA viruses typically consist of one large molecule. Single-stranded (ss) DNA viruses tend to be small. RNA viruses are also subdivided by the different roles RNA plays in transcription: directly as mRNA, as a template for mRNA synthesis, and as a template for DNA synthesis. ssDNA viruses and some ssRNA viruses replicate through an intermediary nucleic acid, which acts as a template for generating new viral genomes. These include a unique group of RNA viruses, called retroviruses, that use reverse transcriptase, an enzyme that uses RNA as a template to make a DNA copy.