We explain what viruses are and what types of viruses we can find. Also, what is its structure and some examples.
What is a virus?
A virus, in biology, is a microscopic, acellular parasitic agent that is, of a size much smaller than what is visible and is not composed of cells, but capable of reproducing only inside a host cell, taking advantage of the genetic replication mechanisms that it has and, generally, causing damage. in the process.
Viruses can infect different forms of life: animals, plants, bacteria and even other viruses (the so-called virophages), since they cannot survive on their own. There are them in almost all existing ecosystems 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 purpose: inject your 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 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 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 adapt and change constantly, and of which there is no fossil record: Known virus species date back no more than 90 years.
See also: Adaptation of living beings
Types of viruses
There are two ways of classifying viruses. The first contemplates four types, according to the structure they have:
- Helical They are shaped like a helix and have a central cavity where their genetic material is located (consisting of RNA or DNA).
- Icosahedral Moderately spherical and symmetrical viruses. They are the most abundant of those that infect animals.
- Wrapping 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. 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:
- DNA viruses Those that have a deoxyribonucleic acid molecule inside, whether single or double chain. They need to introduce said DNA into the nucleus of the cell in order to start its replication.
- RNA viruses Those that have ribonucleic acid and can replicate directly in the cell cytoplasm, without needing to reach the nucleus of the invaded cell.
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 gyrus). They are usually 100 times smaller than bacteria and have very simple bodies, little more than protein frameworks that cover the viral genetic material.
In some cases, the outside of their bodies has specialized disguise proteins, which allow them to change their chemical appearance and not being recognized by immune system cells. That is why viral diseases are recurrent and do not have further treatment, with the exception of certain retroviral medications, such as those used to combat AIDS.
Examples of viruses
Some examples of known viruses are:
- Human enterovirus type 72 The cause of hepatitis A, one of the curable and least dangerous forms of the disease, is thus known.
- Human papillomavirus This 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 causes 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 their leaves. It was the first virus discovered.
