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POXVIRUS GRAPHICS

Smallpox (variola) virions rotating, showing internal structures (from the surface inwards): outer protein layer (ruffled surface), outer lipid membrane, lateral bodies (discoid objects nestling in the hollows of the biconcave, brick-shaped core), palisade protein layer (red) with scattered pores or channels, inner membrane, and DNA. The slab-shaped Smallpox virus is a large and complex double-stranded DNA (dsDNA) virus that infects only humans. Although extinct in the wild, it's escaped from labs and is a potential bioweapon. Monkeypox (Mpox) looks the same but is milder. Other poxviruses attack different animals.

Smallpox virus life cycle or replication in a cell. At top right, virions land on the cell surface and are taken into the cytoplasm. Viral DNA is released and makes copies of itself. The progeny DNAs go into the developing spherical viral particles which mature internally (bottom left) and become brick-shaped. Most will stay as Intracellular Mature Viruses (IMV) and will be released when the cell bursts. But some get a second coat of host membrane. This happens at the trans Golgi as seen just left of centre of the graphic. Such double-wrapped particles are called Intracellular Enveloped Viruses (IEV). Using actin, which polymerises at their "lower" membrane surface, these IEVs propel themselves through the cytoplasm and arrive at the cell surface. Some exit the plasma membrane and stay attached to the cell surface as Cell-associated Enveloped Viruses (CEVs). Others penetrate the membrane and are released as Extracellular Enveloped Viruses (EEVs). Some continue their travels (on their actin tails), pushing the host-cell plasma membrane out into finger-like projections or microvilli. These villi can become very long and so make contact with other cells, transferring infectious viral particles to them. The mode of entry into cells varies depending on the virus form. IMV particles seem to enter by fusion with the plasma membrane (extreme right of cell surface) whereas EEV particles seem to be taken into the cell by endocytosis, shown here as particles taken in by membrane "bubbles".