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published on 1 October 2005 in life

Virus

A microscopic mystery
Virus is an ancient Latin word that means poison. It sounds scary for such a small thing. The bigger ones measure 400 nanometres, instead the smaller ones measure 20 nanometres. A nanometre equals to a billionth part of a metre which means a millionth part of a millimetre. So if we queue up 2500 big viruses or 50.000 small ones it would be one millimetre long.
We know very little about them; scientist are still discussing whether they are alive or inanimate material 100 years later  they have been discovered.
So what are viruses? Let’s start with some history…
Dimitri Ivanovsky, a Russian botanic discovered in 1892 the existence of small pathogenic agents (which can be substances or organisms that cause diseases). The Russian scientist was studding a tobacco plant disease and he was trying to isolate the pathogenic agent. Initially he thought  they were bacteria so he treated the infected juice extracted from the ill plant  by using a filter with a very fine mesh that was able to capture it. Surprised he saw the liquid pass through the filter and made the plant ill again: whatever caused the disease was smaller than a bacterium. Years later, researchers discovered that many other pathologies were caused by those small and mysterious creatures. A very important virus characteristic was discovered a bit before it was possible to observe one directly: they could be transformed into crystals like sugar, proteins and salt. The electronic microscope was discovered later and scientist were able to see that virus were morphologically different from other organisms.

What is a virus made of
A virus doesn’t have cells and they aren’t able to do the fundamental life activities such as respiration, photosynthesis, fermentation.
We must see virus as a small box in which there is an instruction-book. This small box is the outer part we see made of proteins and lipids. The instruction book is a DNA or RNA molecular. They aren’t able to reproduce themselves alone so they depend on other organisms’ cells such as bacteria, plants and animals.
How do they reproduce
Viruses get eaten after they come in contact and recognize the right cell. The cell digests them and destroys the proteic box and sets free the genetic content. At this point the viral DNA or RNA teaches the biosynthesis systems of the host cell to produce new proteins for the “container” and the new viral genetic material. These components assemble automatically making new viruses ready to infect  other cells.
Anatomy
The proteic small box is called capsid. The capsid’s shape changes from a virus type to another. The tobacco virus is a long spring, the flu ones (adenovirus) is an icosahedra (a 20 faced solid), the Ebola looks like a tadpole, instead the virus that infects bacteria (also said bacteriofagi) seem like space jets with small paws for landing. Every single virus that is placed outside the host cell is called virion  and its genetic content is called viroid.

The infection
The first step for a viral infection is the recognition between virus and the host cell. In other words the virus needs to “know” if the cell in contact is the right one for an infection, which means if it is able to help the virus reproduce itself, or not. How can a virus recognize the right cell? It is very easy: thanks to chemistry. Viruses that hit bacteria and plants have proteins in their capsid that, thanks to their structure, they can grip  the cell’s surface they want to infect. Viruses that hit animals use a more ingenious stratagem: they dress up like cells. When the new viruses are mature and are ready to leave from the host cell, they disguise themselves with a piece of membrane (made of fat substances: lipids) that covers up the cell; this way the recognition between  virus and new infected cells will be possible because the viral membrane will blend with the cell’s membrane.
The infection means: how do viruses move?
Viruses can’t move independently. So how can they reach the cells they wants to infect? We mustn’t forget that viruses are extremely light and small: some flute in the air and are breathed in by the host. Flu, cold, smallpox and poliomyelitis viruses use this kind of technique
Others pass from one organism to another  by exchanging organic liquid, such as blood: an example is the HIV-I, the AIDS virus. There are organisms that are transported by insects such as the yellow fever virus that spreads with mosquito bites. Viruses that infect plants have to pass through a thick cuticle that covers the vegetal cells, this is the main motivation some are transported by insects that eat nymph and so they have modified mouth apparatuses so they can drill the cells and aspire liquids. Others spread through alimentation: this is an example of those who provoke hepatitis A and B.

Vaccine
Viruses are responsible for serious diseases that strike men plants and animals. For many years medical science has been studding many ways to combat infections and save millions of lives. The principal weapon invented to prevent viral diseases is the vaccine. To understand how vaccines work we must resume the part we wrote on the recognition between virus and the host cells. Our immune system, which is a complex system made of specialized cells and chemical substances that defend our organism from pathogenic agents, is able to destroy viruses by using antibodies. The antibodies are proteins and they keep viruses from growing. They are produced by blood specialized cells called lymphocytes.
When a virus enters an organism, the lymphocytes recognize the presence of an alien element so they react by producing  antibodies that are able to attack the specific viruses. The virus recognition and the antibodies production are complex operations and aren’t immediate: sometimes unfortunately the immune system’s answer isn’t very efficient so the virus can cause the disease. The vaccine teaches the immune system to recognize pathogenic agents and to produce specific antibodies to attack them. In fact in  a vaccine there are some pathogenic agents that aren’t dangerous and makes our body kill them. If a natural virus enters a vaccinated body the immune system would be ready to attack them immediately.

Friendly virus
Not all viruses are dangerous or harmless: some are precious for us for different and important human activities. In the past years, researchers are studding them for the biological struggle against plant parasites. The mechanism is simple: to attack animals or fungus that are noxious for our agriculture such as insects that eat leaves and have viruses that kill the plant. The important thing is to be sure these viruses attack only noxious organisms.
Another important field where these friendly viruses are presented is in the biotechnologies .By using the interaction between viruses and bacteria, they can produce useful substances that cure many diseases. With a very complicated systems the responsible genes that produce the useful substance get inserted into the virus so they can insert it once again in the bacteria cell host with its own genetic material. This way  both bacteria and proteins will constitute new viruses and will produce a new medical substance that can be taken and used.

Written by Andrea Bellati

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
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