What are viruses?

In this step we are going to learn about the size, the composition and the shape of viruses. Viruses are subcellular infectious agents, i.e., smaller and simpler than cells. Most of them, at least those that affect animals, are only visible with the electron microscope, since their size ranges between 20 nm (e.g. picornavirus or parvovirus) and 300 nm (e.g. the poxvirus, or herpes virus or paramyxovirus). If a virus were the size of a flea, a corresponding person would be the size of Mount Everest. Viruses have only one type of nucleic acid, DNA or RNA, but not both. We will call these “DNA viruses” or “RNA viruses”, respectively. Both DNA and RNA can be single-stranded or double-stranded.

There is always a protein coat around the nucleic acid that protects it; otherwise, it would degrade quickly. This protein coat is called capsid. It is composed of subunits called capsomers. Some viruses are also surrounded by a lipid envelope derived from the host cell. The majority of proteins inserted into the envelope are encoded by the viral genome, and are often grouped into clusters called peplomers or spikes. Viruses with an envelope are called enveloped viruses, and those without, naked viruses. Due to their simplicity, especially since they have only one type of nucleic acid, viruses are obligated intracellular parasites, which must infect living cells that provide them with the compounds that they lack to complete their biological cycle.

Therefore, the viral cycle is composed of two phases: an extracellular phase, called virion or viral particle, being metabolically inert; and an intracellular phase, to complete the replication cycle, which produces the pathogenic effect. For some viruses, the extracellular phase is unnecessary, since they can spread from cell to cell without being in the extracellular environment; but no virus can do without the intracellular phase. Lastly, they are not sensitive to antibiotics. Surprisingly, if the viral capsid proteins are provided the right conditions, they gather spontaneously to form the capsid. They may adopt different shapes, typical of each family of viruses.

One of these shapes is the icosahedron, which constitutes a physical solution to a biological problem, as the icosahedron is the geometrical figure that incorporates the maximum volume with a minimum surface. Therefore, with a low number of proteins it can lodge a relatively large genome inside. Often it may be seen as a rounded figure, almost spherical. In reality, if we think about it, a soccer ball is an icosahedron, with the vertices represented by the pentagonal patches and faces by the hexagonal ones. All animal naked viruses have icosahedral symmetry. On other occasions, the capsid is shaped as a helix. In animal viruses, the helical capsids surround the viral genome very closely, always RNA and in enveloped viruses, are called nucleocapsids.

Finally, other animal viruses do not have a specific form

and are said to be complex or amorphous.

This is important! Don’t forget that viruses are obligated intracellular parasites, with only one type of nucleic acid, protected by proteins and sometimes by lipids.

They are very small and their capsid can have three main shapes: icosahedral, helical or amorphous. In addition, they cannot be treated with antibiotics.