capsid

Meaning: The capsid is the protein shell that encloses and protects a virus's nucleic acid genome, DNA or RNA. It is built from many copies of a few viral-encoded protein subunits called capsomeres, so a small genome can code for a large structure. Common shapes are icosahedral, helical and complex (bacteriophages have a head and tail). Capsid plus genome is the nucleocapsid. Some viruses also have an envelope, a lipid bilayer taken from the host membrane during budding, with the capsid inside it. Naked viruses have no envelope.

On the MCAT: Expect questions on viral structure, lytic versus lysogenic cycles and retroviruses. In the Hershey–Chase experiment, ³⁵S labeled capsid protein, which stayed outside the bacterium, while ³²P labeled DNA, which entered. Enveloped viruses are generally more sensitive to detergents and lipid solvents than naked ones.

Common confusion: The capsid is not the envelope. The capsid is protein encoded by the virus; the envelope is lipid derived from the host cell, though it carries viral glycoproteins. A capsid is also not a cell wall or membrane, since a virus is not a cell and has no ribosomes or metabolism of its own.