<P> A capsid is the protein shell of a virus . It consists of several oligomeric structural subunits made of protein called protomers . The observable 3 - dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres . The capsid encloses the genetic material of the virus . </P> <P> Capsids are broadly classified according to their structure . The majority of viruses have capsids with either helical or icosahedral structure . Some viruses, such as bacteriophages, have developed more complicated structures due to constraints of elasticity and electrostatics . The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere, while the helical shape resembles the shape of a spring, taking the space of a cylinder but not being a cylinder itself . The capsid faces may consist of one or more proteins . For example, the foot - and - mouth disease virus capsid has faces consisting of three proteins named VP1--3 . </P>

A protein coat that enclosed a viral genome is known as a(n)