The demonstration by Wendel Stanley in 1935 that a preparation of tobacco mosaic virus could be crystallised was an indication of the relative chemical homogeneity of viruses, and meant that they could not be thought of in the same terms as other living things. Compared to even the most primitive cellular organism, viruses have a very simple structure (Figure 10.1). An intact viral particle, or virion, has in essence just two components: a core of nucleic acid, surrounded and protected by a protein coat or
Box 10.1 Where do viruses come from?
Three major mechanisms have been proposed for the evolution of viruses:
• 'Escaped gene' theory: Viruses derive from normal cellular nucleic acids and 'gain independence' from the cell. DNA viruses could come from plasmids or transposable elements (see Chapter 12), while RNA viruses could derive from mRNA.
• Regressive theory: Gradual degeneration of procaryotes living parasitically in eucaryotic cells. Enveloped forms such as poxviruses are most likely to have been formed in this way.
• Coevolution theory: Independent evolution alongside cellular forms from primor-
Some scientists consider it unlikely that the same mechanism could account for the diversity of viruses we see today, and therefore propose that viruses must have evolved many times over. A study published in 2004 conversely proposes that all viruses share a common ancestor and may even have developed before cellular life forms.
capsid, the combination of the two being known as the nucleocapsid. In certain virus types, the nucleocapsid is further surrounded by a membranous envelope, partly derived from host cell material. Most viruses are smaller than even the smallest bacterial cells; Figure 10.2 shows the size of some viruses compared to that of typical bacterial and eucaryotic cells.
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