• Viruses


Viruses occur in a great variety of shapes and sizes, but for all their diversity in appearance they possess certain characteristics in common: all viruses contain a single nucleic acid molecule – deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) – surrounded by a protective protein coat. Among other things, the protein coat enables the genetic information stored in the nucleic acid to enter a host cell in a usable state, where it can initiate the reproduction of identical virus particles. After penetration of the cell the foreign genetic material of the virus particle first induces the synthesis of macromolecules not normally present in the cell: the viral nucleic acid undergoes replication and very many copies are produced, the protein of the virus coat is synthesized, and then these components are assembled to form a new generation of infectious virus particles. Most viruses also exhibit certain common structural features: their protein coat is built up from subunits arranged in helical or icosahedral fashion around the genetic material.