Polyoma virus particles. Coloured transmission electron micrograph (TEM) of polyoma virus particles, members of the papovirus family. Each particle consists of an icosahedral protein capsid (coat, blue) that encloses the genetic material, DNA (deoxyribonucleic acid). The genetic material of papoviruses is unique amongst viruses as the DNA forms a circular molecule instead of the usual strands. There are a number of polyoma viruses, many are asymptomatic, but some do cause disease, such as the BK virus that causes respiratory illness in children. Magnification: x189,000 when printed 10 centimetres wide.