Viral structures

Viral structures are the physical arrangements of a virus’s genetic material and surrounding components. They include a protein capsid and, in some viruses, an outer lipid envelope.

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Viral genome: A virus’s genetic material, composed of DNA or RNA and organized in one or more molecules. Its type, shape, and packaging are central features of viral architecture.
Icosahedral symmetry: A symmetry with 20 triangular faces, 12 vertices, and 60 rotational symmetry operations. Many capsids use this arrangement to make a large shell from repeated proteins.
Tobacco mosaic virus: A plant virus with a helical protein coat surrounding a single-stranded RNA genome. It is a classic example of an elongated helical virus.
Enveloped virus: A virus particle surrounded by a lipid membrane bearing viral proteins. It is the structural class that includes an outer lipid envelope.
Capsid: The protein shell that encloses and protects a virus’s genome. It is the main structural component surrounding viral genetic material.
Helical symmetry: An arrangement in which repeating units follow a spiral around an axis. Helical nucleocapsids package genomes along a repeating protein structure.
Adenovirus: A nonenveloped virus with an icosahedral capsid and a double-stranded DNA genome. Its capsid displays a distinctive icosahedral design with projecting fibers.
Non-enveloped virus: A virus particle whose capsid is the outermost structural layer. It lacks the membrane layer present in enveloped viral structures.
Viral envelope: A lipid membrane surrounding some viruses, usually bearing viral proteins that mediate attachment and entry. It adds an outer membrane beyond the capsid in enveloped viruses.
Quasi-equivalence: A principle describing how similar protein subunits occupy slightly different environments in large icosahedral capsids. It explains how repeated proteins can form capsids larger than 60 subunits.