Structure of Viruses

Structure of Viruses

Chemically, a virus particle (or virion) is built from just two essential components: a core of nucleic acid, which carries the virus's genetic information, and a surrounding protein coat called the capsid, made up of repeating protein subunits called capsomeres, which protects the nucleic acid and helps the virus attach to and enter a host cell. Unlike a cell, a single virus particle contains only one type of nucleic acid - either DNA or RNA, never both - and this nucleic acid may be single-stranded or double-stranded depending on the virus. Some viruses additionally carry an outer envelope, an extra lipid-containing layer derived from the membrane of a previous host cell, wrapped around the capsid, though many viruses (including the two described here) lack such an envelope.

The Tobacco Mosaic Virus (TMV) is a rigid, rod-shaped virus that illustrates helical symmetry particularly clearly. Its genetic material is a single strand of RNA, which runs along the central axis of the rod. Around this RNA strand, identical protein subunits (capsomeres) are arranged in a helical, spiral pattern, stacking together to build up the capsid as a rigid protein tube enclosing the RNA core along its entire length. …