Do you want to know more about these curious non-beings? We explain them here.
Viruses are known to all as tiny causes of disease , but there is little knowledge among the population about their characteristics.
In this article, we will talk about different types of viruses, how we can classify them, how they work, and some associated diseases.
The taxonomy (the way we classify species according to their proximity) of viruses is complex, due to several factors. By not sharing some basic characteristics common to living beings (since they are not considered as such) it is difficult to separate them in the same way. Their classification is complex and depends on a combination of relatedness and similarities in certain characteristics .
Because of this, in this article we will introduce some forms of viral classification, to better understand the world of these microscopic non-beings.
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What is a virus?
Viruses are cellular parasites that can only reproduce using the cells of the beings they infect . This process affects the normal functioning of the cells that they infect, and can cause diseases of greater or lesser severity depending on the tissue they affect and the way they infect.
We can imagine a virus as a protein box, whose characteristics will vary from species to species, containing genetic material . This genetic material, which comes in different formats that we will talk about later, is introduced into cells to take advantage of their genetic and protein-forming machinery.
Thanks to this infectious process, the viruses, unable by themselves to generate more viruses, multiply inside the cells where they have injected their genetic material. These cells are loaded with viral particles (called virions), made up of their protein case and the genetic material (the instructions for creating more virions) within it.
Once the infection cycle is complete, the viruses can remain latent inside the cells (little by releasing viral particles, or waiting for an external signal to generate them) or they can simply overload the cellular machinery, causing their death (lysis). and releasing the virus particles.
virus types
Part of the complexity of classifying viruses is their apparent lack of a common ancestor . Unlike living beings, which could theoretically trace their origin to a common ancestor for all of them, viral lineages seem to have originated separately on multiple occasions, which precludes the use of usual taxonomic methods. Today, the origin of viruses remains a mystery .
We can find viruses in all kinds of environments, since some of them are extremely resistant and can withstand various disinfection methods. Infectious viruses, however, usually have a shorter half-life, especially those with more complex structures (such as those that, in addition to protein capsid, are surrounded by lipid membranes).
Below we will mention different ways of classifying viruses, while showing examples of some of them and the diseases with which they may be associated.
1. According to the organism they infect
This is a very simple classification of viruses, where we separate them according to the related cells for their particular infective process. Any known living organism is susceptible to viral infection, but viruses often specialize for a particular class of organism.
This is partly because the defense mechanisms of different species are different, and viruses cannot adapt to all of them, but they can adapt to a few similar defense systems (or to one unique to a species). We will find, for example, depending on the type of infected organism:
- Viruses that infect plants, such as tobacco mosaic virus, the first virus discovered.
- Viruses that infect animals, such as HSV-1, which causes herpes in humans.
- Viruses that infect bacteria, which we call bacteriophages.
- Fungal viruses, called mycoviruses or mycophages.
Although this classification can go into much more detail, depending on the organisms that we use to make the characteristics. We could make, for example, categories of viruses that only infect mammals, birds, amoebas …
This type of classification is relatively useful, since viruses that infect similar organisms will tend to be more similar to each other than viruses that infect very different organisms (although this is not always the case).
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2. According to its morphology
Viruses exhibit an enormous amount of morphological variation. That is, they come in different forms. Some distinctions between viruses are made based on the geometry of their capsid, the protein box that contains the genetic material . Other distinctions will be made according to whether they have a lipid membrane or not, or their type of genetic material (whether it is DNA or RNA, the morphology of the molecule, the characteristics of the infectious process…)
Below we list some morphological characteristics that are used to classify viruses and some viruses of interest.
2.1. Classified according to their capsid
We will find four types of viruses classified according to the morphology of their capsid:
- Helical viruses, common form of viruses that infect plants
- Icosahedral viruses, such as poliovirus
- Enveloped viruses, which usually infect mammals. HIV is one of these.
- Head and tail viruses, again mainly bacteriophages.
2.2. Classified according to the Baltimore method
The Baltimore method of viral classification is a method based on biochemical parameters. Specifically, depending on how their genome is structured and the methods they use to pass their genetic material into mRNA, which will be read within the infected cell to form virions. They are separated into the following seven families:
- I- Double-stranded DNA viruses, such as Adenoviruses, which can cause gastroenteritis.
- II- Single-stranded DNA virus, common in marine viruses.
- III- Double-stranded RNA virus, the Reoviruses. Some of them have been linked to celiac disease.
- IV- RNA virus of a positive reading strand.
- V- negative reading single stranded RNA virus
- VI- RNA retrotranscription virus. The HIV virus is one of them.
- VI- DNA retrotranscription virus.
The explanation of some of these types requires advanced genetic knowledge, so we will only present them . As an example we can highlight type VI viruses, also called retroviruses:
These retroviruses convert their genetic material from RNA to a copy of DNA, which they then insert into the genome of the affected cells, ensuring a long-lasting production of virions, since being integrated into the DNA of the infected cell, the latter continues to produce virus as part of its life cycle.
conclusion
There are many types of viruses and the ways to classify them will vary depending on the purpose of our study. This article simply serves as an introduction to the many ways we have to study these fascinating non-living creatures.
Bibliographic references
- Gelderblom HR. (nineteen ninety six). Structure and Classification of Viruses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; chapter 41
- Baltimore, D. (1971). Expression of Animal Virus Genomes. Bacteriological Reviews 35, no. 3.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.