What is a virus In biology?

We explain what viruses are and what types of viruses we can find. Also, how is its structure and some examples.

1. What is a virus?

A virus, in biology , is a microscopic and acellular parasitic agent , that is, much smaller than the visible size and is not composed of cells , but capable of reproducing only inside a host cell, taking advantage of the mechanisms of genetic replication that she possesses and, in general, causing damage to the process .

Viruses can infect different life forms: animals, plants, bacteria and even other viruses (so-called virophages ), since they cannot survive on their own. There are in almost all existing ecosystems , they are the most abundant biological form on the planet: more than 5 thousand species have been known since the discovery of their existence in 1899, and it is believed that there could be millions of species.

The origin of these life forms is uncertain, as well as the question of whether they are really alive, given how simple they are, little more than a genetic code in search of a cell that synthesizes it. That seems to be her only task: to inject her DNA or RNA into a host cell and force her to synthesize new copies of the virus instead of the proteins she normally builds.

Some theories propose that viruses would have evolved from plasmids, that is, free-living DNA fragments ; while other theories prefer to think that they are bacteria or other cellular organisms that they involved, since the structure of any virus is much simpler than that of any cell.

In any case they are very primitive organisms, with an enormous capacity of mutation that allows them to adapt and change constantly, and of which there is no fossil record: the known virus species date back no more than 90 years .

2. Virus types

There are two ways to classify viruses. The first includes four types, according to the structure they have:

• Helical . They have a helix shape and a central cavity where its genetic material is found (consisting of RNA or DNA).
• Icosahedral . Mediumly spherical and symmetric viruses. They are the most abundant of those that infect animals.
• Wrapping . Viruses that have a layer or envelope of lipids , which they obtain from the cell membrane of their host cells, and that is used to inject the genetic material into the cell.
• Complex . There are viruses with more complex forms that combine the above types and may even have additional components, such as protein tails to move around. They, in many cases, serve to inject the genetic material of the virus into the cell.

The second form of classification is based on the type of genetic material they contain:

• Virus DNA . Those that have a deoxyribonucleic acid molecule inside , either single or double stranded. They need to introduce said DNA to the cell nucleus to be able to start their replication.
• RNA virus . Those who have ribonucleic acid and can replicate directly in the cell cytoplasm , without reaching the nucleus of the invaded cell.

3. Structure of a virus

Most viruses are so tiny that they cannot be seen through optical microscopes , except for some cases of large species (called girus ). They are usually 100 times smaller than a bacterium and have very simple bodies, little more than protein scaffolds that cover the viral genetic material.

In some cases, the outer part of their bodies has specialized proteins in the disguise, which allow them to change their chemical appearance and not be recognized by the cells of the immune system . That is why viral diseases are recurrent and do not have more treatment, except for certain retroviral drugs, such as those used to fight AIDS.

4. Virus examples

Some examples of known viruses are:

• Type 72 human enterovirus . The cause of hepatitis A, one of the curable and least dangerous forms of the disease, is thus known.
• Human Papilloma Virus . It is a family of extremely common viruses in man, some sexually transmitted (HPV) and others by touch, which usually cause warts and are mildly harmless, except for certain strains that have been linked to cancer.
• Herpes simplex virus (HSV) . An extremely common human virus that generates skin lesions (known as herpes) and of which there are two variants: one genital and another that prefers the tongue, mouth, eyes and pharynx.
• Tobacco mosaic virus . It is an RNA virus that attacks plants (it was discovered in the tobacco plant) and produces whitish or yellowish spots on its leaves. It was the first virus discovered.