Viruses are fundamentally different from any other creature on our planet. So far in our understanding of life, there are several characteristics which every living being possesses – they move, make use of energy, respond to changes in their environment, they incorporate new materials, use them to repair, grow and reproduce and throw away the unnecessary bi-products. From the smallest single-cell organism to the largest, most complex creature, these traits are undoubtably there. However, viruses do not work this way. They have no metabolism of their own and can only function whenever they inhabit a foreign cell, called a host. Until then the virus, known at this stage as a virion, is completely inert and does not show any signs of life at all. Which poses an interesting question – when exactly do we start to consider something to be alive? Viruses do exhibit some of the characteristics of living beings. They are made out of organic material and contain genetic information – a virus is actually just strains of DNA or RNA, surrounded by a protein coating. However, when they infect a foreign cell, they expose these genes, inducing the host metabolism to replicate their genetic code and proteins, which assemble into new viruses. So, by hijacking external cells, they are able to reproduce. Some viruses can replicate at alarmingly high rates, while others may lay dormant for long periods of time, making slowly new copies and remaining undetected.
This uncertainty over the nature of viruses has been bothering scientists ever since they were discovered. At the end of the 19th century some biologists noticed that not all diseases were caused by bacteria. They developed special filtration methods, which could remove all cells from a solution, but no matter how often they performed the procedure, the solutions remained infectious. So, they deducted that there must be some other particle, much smaller and undetectable through microscopes, which was the cause of the infection. Slowly the virus theory took shape, and, in the beginning, they were thought to be the simplest of gene-bearing life-forms. The breakthrough came in 1935 when Wendell Stanley, an American biochemist, managed to crystalize a tobacco mosaic virus. This made possible for its structure to be examined in detail, discovering that it lacked the essential systems for metabolism. For his research in 1946 he was awarded the Nobel Prize in Chemistry. During the latter half of the 20th century thousands of viruses were discovered, laying the foundation of modern molecular biology. By analyzing how these incomplete cells are able to force others into copying them, scientists have gained valuable insight into the way nucleic acids code proteins to record genetic information.
Another interesting topic concerns the virus’ origin and how they fit within the theory of evolution. Today scientists believe that they came into being as a part of a host gene, which was somehow separated and got a protein coating. This would explain why they lack so many essential components, but still have the necessary information to make copies of themselves. As for evolution, they may have played a much larger role than previously thought. There are viruses, which can permanently add their genes to a host’s lineage, essentially becoming a part of him. During analysis of the gene sequences of different creatures, scientists noticed that there are genes which are present in both simple bacteria, as well as humans, but are missing in intermediate species. This would mean that they cannot be an evolutionary trait, passed on by the generations. One explanation is that these genes came from viruses, which permanently embedded themselves in distant ancestors. In fact, viruses may account for as much as 8% of our total genetic code.