In 1962, Lwoff, Horne and Tournier proposed a classification system in which viruses were grouped by shared characteristics among the particles themselves rather than the host cells. While their classification system is no longer used, this basic premise is the foundation of modern classification systems. Some of the main features of viruses used in their classification are type of genetic material and capsid shape.
Viruses may carry DNA or RNA as their genetic material. DNA may be single- or double-stranded (ssDNA and dsDNA), and it may be circular or linear. The code found in viral genetic material is usually very complex, with many genes overlapping each other to conserve space in the tiny capsid.
Though capsid shape varies greatly, there are two main categories: helical and icosahedral. Helical capsids are formed by a helix of structural proteins, forming a long tube in which the genetic material resides. Icosahedral capsids are shaped like polygons, forming a ball around the genetic material. The geometry of the polygons is used for further classification. In addition, viruses are capsids are grouped into those with envelopes and those without. /PARGRAPH
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