T4 bacterial virus, an assembly of protein components, is a
simple biological "nanomachine". The head is a protein membrane,
shaped like a kind of prolate icosahedron with 30 facets and
filled with deoxyribonucleic acid (DNA). It is attached by a
neck to a tail consisting of a hollow core surrounded by a
contractile sheath and based on a spiked end plate to which six
fibers are attached. The spikes and fibers affix the virus to a
bacterial cell wall. The sheath contracts, driving the core
through the wall, and viral DNA enters the cell. Bacteriophages
are being studied in anti-bacterial applications as phage
therapy.