Most viruses vary in diameter from 20 nanometres (nm 0.0000008 inch) to 250–400 nm the largest, however, measure about 500 nm in diameter and are about 700–1,000 nm in length. The genomes of Mimiviruses and Pandoraviruses, which are some of the largest known viruses, range from 1 to 2.5 Mb (1 Mb = 1,000,000 base pairs of DNA). The protein and nucleic acid constituents have properties unique for each class of virus when assembled, they determine the size and shape of the virus for that specific class. Penetrating the membrane are additional proteins that determine the specificity of the virus to host cells. Some viruses have more than one layer of protein surrounding the nucleic acid still others have a lipoprotein membrane (called an envelope), derived from the membrane of the host cell, that surrounds the nucleocapsid core. The nucleic acid and proteins of each class of viruses assemble themselves into a structure called a nucleoprotein, or nucleocapsid. The amount and arrangement of the proteins and nucleic acid of viruses determine their size and shape.
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