Viruses - A Brief Introduction to Viruses
It was the Dutch microbiologist Martinus Beijerinck (1851-1931) who coined the term "virus" for these tiny, invisible materials. Beijerinck discovered what is now known as the tobacco masaic virus in 1898. Since that time, more than 5,000 types of viruses, with millions of sub-types, have been found and studied. Viruses have been discovered in almost all Earth ecosystems and are considered to be the most abundant biological form.
The word 'virus' is derived from the Latin word 'virus' which referred to poisons or other similar substances. The term 'virulent' is derived from the Latin 'virulentus' meaning poisonous, and came into use as early as 1400AD. Use of the term 'virus' to refer to an agent that causes infectious disease was first recorded in 1728, although the actual discovery of viruses themselves did not occur until 1892.
WHAT IS A VIRUS?
Viruses are interesting entities - considered neither living nor non-living by scientists. They are parasitical in nature, relying on host cells that they infect and take control over, causing the host cells to produce additional viral proteins and genetic material.
When a virus is active, it infects a host cell causing more viruses to be created. This results in the death of the host cell as the new viral material bursts through the cell walls and goes on to infect other host cells. Outside the cell, the infective particles are called virions.
Viruses are primarily classified depending upon the organisms they infect - animals, plants, or bacteria. Secondary classification into families and genera is based based on three structural considerations: 1) the type and size of their nucleic acid (RNA or DNA), 2) the size and shape of the capsid, and 3) whether they have a lipid envelope surrounding the nucleocapsid (the capsid enclosed nucleic acid). Two shapes dominate viral forms. There are rods, sometimes called filaments, and spheres.
PARTS OF A VIRUS
There are three primary parts to a virus:
Nucleic Acid - In prokaryotic and eukaryotic cells, genetic material is encoded in double-stranded DNA. Viruses contain nucleic acid which is held within DNA or within a single strand of RNA (but never both). This nucleic acid holds the genetic information for synthesis of virus-specific proteins.
Capsid - Sometimes called the nucleocapsid, it is a protein shell that encloses the nucleic acid. The capsid has three functions, which are to (1) protect the nucleic acide from digestion by enzymes, (2) provide proteins that enable the virus to penetrate the host cell membrane, and (3) contain special sites on its surface that allow the virion to attach to a host cell.
Envelope - The envelope is made of glycoproteins and surrounds the capsid. It consists of two lipid layers that are interspersed with protein molecules. The envelope may contain materials from both the membrane of the host cell and that of the virus. In some cases, spikes made of glycoproteins are also present. These spikes help the virus attach to the surface of the host cell.
HOW VIRUSES AFFECT HUMANS
Diseases in humans caused by viruses include influenza, SARS, the common cold, chickenpox, shingles, smallpox, AIDS, herpes, polio, rabies and Ebola - just to name a few! Even some cancers have now been determined to be caused by viruses. An example of virus-caused cancer is the the human papillomavirus (HPV), which has been found to cause cervical and vulvar cancer.
Virologists are presently investigating the relationship between viruses and many other diseases. For example, a possible connection between the human herpes virus six (HHV6) and certain neurological diseseases (like multiple
sclerosis and chronic fatigue syndrome) is being studied. Also under debate is whether a virus which causes neurological disease in horses, the borna virus, might also be a causative factor in certain human psychiatric illnesses.
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