As science continues to progress, our understanding of life is evolving, thanks in part to the advances in molecular biology. Scientists can now take a closer look at organisms than ever before. Not only has the Linnaean classification system (Kingdom, Phylum, Class, Order, Family, Genus, Species) gone out of favor with many scientists due to the molecular unraveling of evolutionary histories of the species, but also the idea of what constitutes a living organism has been put on trial.
Living organisms grow, reproduce, and carry on biological processes within their cells. From the more complex organisms, such as mammals, to the one celled paramecium, life processes are conducted throughout the growing species’ lifetime. Viruses represent that unique biological entity bordering on the threshold of life. It has long been established that viruses exhibit none of the characteristics mentioned above, and the molecular study of them has begged the question are viruses alive?
In order to address this question, let’s first take a look at the structure of a virus. All viruses contain two things: a genome and a capsid (the protein coat that surrounds the genome). The genome can be DNA or RNA, single-stranded or double-stranded. The capsid varies in shape from rod-shaped to a more complex icosahedron. Although a few viruses contain unique enzymes, most viruses contain nothing else. Some viruses are surrounded by a lipid bilayer containing glycoproteins. These glycoproteins bind to specific receptors on the surface of the host cell, making the virus cell specific. Viruses that do not contain a lipid bilayer may have glycoproteins attached to the capsid. The binding of the virus via its glycoprotein to the host cell’s receptor initiates entry of the virus into the cell.
Viruses are obligate intracellular parasites, that is they can only survive within the cell. They have no cytoplasm, no nucleus, no mitochondria or other cellular organelles. They are incapable of making their own energy or manufacturing proteins. The virus relies on the host cell’s proteins/enzymes, ribosomes, and energy to replicate its DNA/RNA and to manufacture its capsid proteins, which are translated from its own genome. Viruses are assembled within the host cell from preformed components. They require no nutrition. Because of this, the virus exhibits no change during its life time. From the moment it forms and buds from the host cell, to the time of its “uncoating” in the next cell it infects, it is exactly the same.
One may wonder where these submicroscopic entities come from in the first place? There is speculation that they arose from life forms that have lost cellular functions. Another theory, and the one I support, is that viruses evolved as a result of the macromolecule (DNA or RNA) escaping the confines of the cell. Viruses range in size from 20nm – several thousand nm. Their genome ranges from ~6kb (about 10 genes) to ~1.2 Mbp as seen in the Mimivirus (possibly >900 genes). This is small compared to the genome of living organisms (E. coli >5,000 genes, humans ~21,000 genes). The Mimivirus is unique in that many scientists consider it a bridge between the nonliving virus and living organisms.
The question whether the virus is living or not will be under debate for some time. There is still much to learn about the submicroscopic organisms, and until we unravel the mysteries, these small, seemingly non-living particles will continue to infect and destroy their hosts. Many of the nucleic acids found in viruses have the propensity to integrate into the hosts’ genomes and are responsible for the onset of many cancers, a topic for another day. Make no mistakes, the virus is the perfect vehicle for transporting unwanted DNA or, as in the case of the HIV virus, RNA throughout the body.