Virus Structure and Classification

Viruses have several common characteristics: they are small, have DNA or RNA genomes, and are obligate intracellular parasites. The virus capsid functions to protect the nucleic acid from the environment, and some viruses surround their capsid with a membrane envelope. Most viruses have icosahedral or helical capsid structure, although a few have complex virion architecture. An icosahedron is a geometric shape with 20 sides, each composed of an equilateral triangle, and icosahedral viruses increase the number of structural units in each face to expand capsid size. The classification of viruses is very useful, and the International Committee on Taxonomy of Viruses is the official body that classifies viruses into order, family, genus, and species taxa. There are currently seven orders of viruses.