Viruses are the only acellular organisms on Earth today.
Viruses are very simple and small beings (measuring less than 0.2 µm), formed basically by a protein capsule involving the genetic material, which, depending on the type of virus, may be the DNA, RNA or both together (cytomegalovirus).
The word virus comes from Latin virus that means poisonous fluid or toxin. It is currently used to describe biological viruses, and metaphorically designates anything that reproduces parasitically as ideas. The term computer virus was born by analogy. The word virion or viron is used to refer to a single viral particle that is outside the host cell.
Of the 1,739,600 species of known living things, viruses represent 3,600 species.
Virus is a basically protein particle that can infect living organisms. Viruses are obligatory parasites of the cellular interior and this means that they only reproduce themselves by invading and possessing control of the cellular self-reproducing machinery. The term virus usually refers to the particles that infect eukaryotes (organisms whose cells have a library), while the term bacteriophage or phage is used to describe those that infect prokaryotes (bacteria and archaea domains).
Illustration of the HIV virus showing the capsid proteins responsible for adherence in the host cell.
Typically, these particles carry a small amount of nucleic acid (either DNA or RNA, or both) always encased in a protein capsule called capsid. The proteins that make up the capsid are specific to each type of virus. The capsid plus the nucleic acid it surrounds are called nucleocapsid. Some viruses are formed only by the capsid nucleus, but others have a wrapper or envelope outside the nucleocapsid. These viruses are called encapsulated or enveloped viruses.
The envelope consists mainly of two layers of lipids derived from the host cell plasma membrane and virus-specific protein molecules immersed in the lipid layers.
It is the viral protein molecules that determine which cell type the virus will infect. Generally, the group of cells that a type of virus infects is quite restricted. There are viruses that only infect bacteria, called bacteriophages, those that only infect fungi, called mycophages; those that infect plants and those that infect animals, called, respectively, plant viruses and animal viruses.
HIV Virus Scheme
Viruses are not made up of cells, although they depend on them for their multiplication. Some viruses have enzymes. For example the HIV have the enzyme Reverse transcriptase that makes the process of Reverse transcription performed (formation of DNA from viral RNA). This process of forming DNA from viral RNA is called retrotranscription, which is called retrovirus viruses that perform this process. The other viruses that have DNA do the transcription process (passing the language from DNA to RNA) and only after the translation. These latter viruses are called adenoviruses.
Viruses are obligate intracellular parasites: lack of hyaloplasma and ribosomes prevent them from having their own metabolism. Thus, to run its life cycle, the virus needs an environment that has these components. This environment must be the interior of a cell that, containing ribosomes and other substances, will synthesize virus proteins and simultaneously allow viral genetic material to multiply.
In many cases viruses modify the metabolism of the parasitizing cell, which can cause its degeneration and death. To do this, the virus must first enter the cell: often it adheres to the cell wall and "injects" its genetic material or enters the cell by engaging - by a process that resembles phagocytosis, the cell "swallows" the virus and introduces it into it.
Viruses, living or not?
Viruses do not have any metabolic activity when outside the host cell: they cannot capture nutrients, utilize energy or perform any biosynthetic activity. They obviously reproduce, but unlike growing cells, they duplicate their contents and then divide into two daughter cells, viruses replicate through a completely different strategy: they invade cells, which causes the dissociation of the components of the cell. viral particle; These components then interact with the host cell's metabolic apparatus, subverting cellular metabolism to produce more viruses.
There is much debate in the scientific community about whether viruses should be considered living beings or not, and this debate is primarily a result of different perceptions of what life is, in other words, the definition of life. Those who hold the idea that viruses are not alive argue that living organisms must possess characteristics such as the ability to import nutrients and energy from the environment, must have metabolism (a set of highly interrelated chemical reactions through which living things build and maintain their bodies, grow and perform numerous other tasks, such as locomotion, reproduction, etc.); Living organisms are also part of a continuous lineage, necessarily originating from similar beings and, through reproduction, generating other similar beings (progeny or offspring), etc.
Viruses fulfill some of these criteria: they are part of continuous strains, reproduce and evolve in response to the environment, through variability and selection, like any living being. However, they have no metabolism of their own, so they should be considered "infectious particles" rather than living beings themselves. Many, however, disagree with this view, and argue that since viruses are capable of reproducing, they are living organisms; they depend on the metabolic machinery of the host cell, but until then all living things depend on interactions with other living things. Still others take into account the massive presence of viruses in all kingdoms of the natural world, their origin - apparently as old as life itself - their importance in the natural history of all other organisms, and so on. As already mentioned, different concepts about what life becomes are at the heart of this discussion. Defining life has always been a big problem, and since any definition is likely to be elusive or arbitrary, making it difficult to accurately define viruses.