Structure of viruses

1. Size. The size range of viruses is from about 20 to 300 nm. On the whole, viruses are much smaller than bacteria. Most animal viruses and all plant viruses and phages are invisible under the light microscope.

2. Simple structure. Viruses have very simple structures. The simplest viruses are nucleoprotein particles consisting of genetic material (DNA or RNA) surrounded by a protein capsid. In this respect they differ from typical cells which arc made up) of proteins, carbohydrates, lipids and nuc1eicacids.

The more complex viruses contain lipids and carbohydrates in addition to proteins and nucleic acids, e. g. the enveloped viruses.

These viruses are surrounded by a membranous envelope which is derived from the host cell. It protects the virus and also serves for transmission from one host to another. The envelope consists of a lipid bilayer and proteins with special functions.

The membrane proteins are of two types, glycoproteins and matrix: proteins. Glycoproteins have a hydrophobic end fixed in the lipid bilayer and a hydrophilic glycosylated end which protrudes into the medium.

The spikes on the outer surface of the virions consist of glycoproteins. In the orthomyxoviruses, paramyxoviruses and rhabdoviruses, there is an unglycosylated matrix protein layer on the inner surface of the envelope. This layer appears to connect the envelope with the capsid.

The envelope and capsid proteins are specified by viral genes. The lipid and carbohydrate of the glycoprotein are derived from the host cell. Since some viruses can be grown in different cell types, they often have different lipid and carbohydrate moieties.

The surface of the virion may therefore contain polysaccharide-determined cellular antigens. The Pseudomonas phage 96 has a lipid envelope, of which the lipid bilayer appears to be assembled de novo, and is not budded from the membranes.