Colds, influenza, herpes, meningitis, encephalitis, polio, pneumonia, hepatitis, AIDS, Ebola, some warts and some cancers. What they have in common all these diseases?
They are caused by infectious virus. In fact, it is estimated that over 90% of human diseases may be caused by them.
Viruses are conspicuous, cosmopolitan, universal and very abundant. Every day we are exposed to them. Are not they more than enough reasons to want to know more about them?
Currently, in many situations we use the word virus: a computer virus it can damage the computer, a viral video is one that has reached many people in a short time and if you’re sick without apparent cause, you’ve taken a “whatever virus“.
Good to know that every minute, we inspired about 6 liters of air. In this volume, reach our lungs large amounts of virus. And not just “visit us” us, also visit our pets, plants we have in the balcony and even drift in the water of seas and rivers.
But what are we talking about when we say virus?
Define a virus is perhaps the most difficult question to answer. Moreover, than to answer: what is life? Because viruses are, somehow, the exception of life.
In origin, the meaning of virus included any poisonous substance, such as snake venom. Virus in Latin, means poison. Later, it was used in a more specific way, to designate the etiological agent of any infectious disease. Louis Pasteur himself referred to infectious bacteria and viruses. In the late nineteenth century had been isolated lots of bacteria causing infectious diseases, but there were a few diseases that were not due to known bacterial agents such as FMD affects animals. Therefore more adjusted denominations to reality were needed as soluble filterable infectious agent or filterable virus.
Viruses are small nucleic acids (DNA or RNA) protected by a capsid protein or wrappings. The larger viruses have an outer wrapper. These wrappers allowed them entry into cells. In themselves they are acellular, because they lack completely of typical cell structure and do not belong to any of the five natural kingdoms of classification of organisms. Once inside the cells take over the machinery to generate copies (genetic material and envelope proteins) of them. Once assembled and ready to disperse, leave the cell, destroying it completely.
We are talking about structures 100-1000 times smaller than a cell, with overwhelming power of destruction. The smallpox virus, which is one of the largest, has a diameter of 200 nanometers. By contrast, the polioviruses, which are the smaller ones, have only 28 nanometers in diameter. Can be crystallized due to its size and limited structural complexity.
Besides its genetic information, it is estimated 100,000 times less than the genetic information of the cell it infects. For example, one of known larger viral genomes is the poxvirus genome with 190,000 base pairs. Most bacterial genomic DNA is between 1,000,000 and 9,000,000 base pairs.
How can they with as little redirected to the destruction of a higher organism?
Well, because in this “little” information they contain, have the essential “instructions” to paralyze all cellular mechanisms and put the synthesis apparatus (ribosomes) and power plant (mitochondria) operating according to their own requirements.
Before we said that they are the exception in life. Let’s talk about this.
Viruses have been in their life cycle, one extracellular and other intracellular. During external phase cell, they are nothing more than a submicroscopic particle formed by a nucleic acid and a protein coat (and occasionally by other macromolecular components). At this stage, the virus particle is also known virion. It is said that is not alive because it is metabolically inert and about to breathing, is zero. The virion conforms the transportation structure of viral genetic information into a new cell.
Once enters the host cell, beginning with the intracellular phase of viral replication, ie, starting the duplication of genetic material. This process of introduction and reproduction, where the virus “seems to be very much alive”, is called infection.
The virus can be classified differently according to a specific term. They can be classified according to the type of genetic material they have, DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). They also have an added twist. Humans have dsDNA (doublé strand DNA), ie two strands or two information copies tangled helically. Virus can have one or two chains of DNA or RNA. However, exist viruses with both DNA and RNA but at different stages of their life cycle, such as retroviruses, which passes RNA when has a virion form to DNA or the virus of human hepatitis B who has DNA when is a virion but to replicate uses RNA.
They can also be classified as they infected hosts (animal virus, plant virus, bacterial virus or bacteriophage …).
At this point, if we compare the virus with chromosomes (a constant part in a cell), they are not living organisms, because both structures need the host cell to able to replicate. Therefore, lacking independence, can not be considered as organisms.
But if we consider the maxim: “an organism is the unit of a continuous lineage with a constant evolutionary history”, then it could be considered as a certainly living organism because implies the existing co-evolution between viruses and their hosts, over time.