We explain what viruses are and what types of viruses we can find. Also, how is its structure and some examples.
What is a virus?
A virus, in biology , is a microscopic and acellular parasitic agent , that is to say, of a size much smaller than what is visible and that 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 usually causing damage to her in the process .
Viruses can infect different forms of life: animals , plants , bacteria , and even other viruses (so-called virophages ), since they cannot survive on their own. They exist in almost all existing ecosystems , they are the most abundant biological form on the planet: more than 5,000 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 is the question of whether they are really alive, given how simple they are, little more than a genetic code in search of a cell to synthesize it. That seems to be its only task: to inject its DNA or RNA into a host cell and force it to synthesize new copies of the virus instead of the proteins it normally builds.
Some theories suggest that viruses evolved from plasmids, that is, free-living DNA fragments ; while other theories prefer to think that they are bacteria or other cellular organisms that have evolved, 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 for mutation that allows them to constantly adapt and change, and of which there is no fossil record: the known virus species date back no more than 90 years .
There are two ways of classifying viruses. The first contemplates four types, according to the structure they possess:
- Helical They have a helix shape and a central cavity where their genetic material (consisting of RNA or DNA) is found.
- Icosahedral Medium spherical and symmetric viruses. They are the most abundant of those that infect animals.
- wrapper Viruses that have a lipid layer or envelope , which they obtain from the cell membrane of their host cells, and which is used to inject genetic material into the cell.
- Complexes There are viruses with more complex shapes that combine the previous types and may even have additional components, such as protein tails to move around. In many cases, they serve to inject the virus’s genetic material into the cell.
The second form of classification is based on the type of genetic material they contain:
- DNA viruses – Those that have within them a molecule of deoxyribonucleic acid, either single or double chain. They need to introduce said DNA to the nucleus of the cell in order to initiate its replication.
- RNA viruses – Those that have ribonucleic acid and can replicate directly in the cell cytoplasm , without the need to reach the nucleus of the invaded cell.
Structure of a virus
Most viruses are so tiny that they cannot be seen through light microscopes , except for a few large species (called gyrus ). 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 outside of their bodies have specialized proteins in their disguise, which allow them to change their chemical appearance and not be recognized by cells of the immune system . That is why viral diseases are recurrent and do not have further treatment, except for certain retroviral drugs, such as those used to combat AIDS.
Examples of viruses
Some examples of known viruses are:
- Human enterovirus type 72. This is the cause of hepatitis A, one of the curable and least dangerous forms of the disease.
- Human papilloma virus. It is a family of viruses that are extremely common in men, some sexually transmitted (HPV) and others by touch, which usually cause warts and are fairly 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.