Something amazing happened early in the history of the Earth. Molecules were formed that had the unusual capacity to reproduce. And, not only that, but these molecules reproduced to the limit of the resources available and competed with other, similar, molecules to utilize every available reserve necessary to their growth and reproduction. These molecules were nucleic acids. The earliest nucleic acids were probably ribonucleic acids, strings of phosphate groups, sugars, and nitrogenous bases, known now by the acronym RNA [1]. Later, deoxyribonucleic acid (DNA) molecules came to share the available space with RNA. Each of these was similarly constructed as a long-chain molecule, made up of pyrimidine nucleic bases called cytosine (C) and thymine (T), or uracil (U), and the purines adenine (A), and guanine (G). RNA is made of uracil instead of the thymine in DNA. Some RNA and DNA molecules were single-stranded, and some formed as double strands. These molecules formed the genetic bases for all of life on Earth since then.
It is very difficult to define exactly what constitutes life. Most definitions of life start with the presence of cells, which contain DNA as a genetic blueprint for producing daughter cells that are formed with the potential to be identical to the parent cell. The daughter cell may not be exactly the same as its parent because of physical, chemical, and biological environmental influences on expression of the genetic material contained in the genes of the cell. The cell contains the mechanisms to translate the genetic blueprint into the biochemicals necessary for life, as we know it, to continue.
Viruses pose a difficult problem when trying to define what is life [3]. They contain either RNA or DNA molecules wrapped in a protective coating. They do not possess the ability to translate the genetic blueprint represented by either of these molecules into the necessary biochemical mix to sustain life. So, they are not cells, and not completely alive, as most people would agree.
Spores are alternate forms of living cells that are able to withstand harsh environments for extended periods of time. Some spores are reproductive, as are the spores from ferns and mushrooms. There are certain bacteria, notably Bacillus anthracis, the bacterium that causes anthrax, and Clostridium botulinum, the cause of deadly food poisoning, that form nonreproductive spores that can persist in the environment for many years. A spore has a hard casing that contains the genetic material of those bacteria or other microorganisms that form the structures. These physically and chemically resilient packages protect the genetic material during periods when the environmental conditions are so harsh that the growing (vegetative) form of the microbe would be killed.
Forterre [2] likens viruses to bacterial spores. Viruses, like spores, contain genetic material encased in a protective coating. Viruses are thus able to persist in the environment for long periods of time, even when the environment is so severe that a normal cell would not stand a chance of survival.
The vegetative form of a bacterium happens when the environment is favorable so that the bacterial spore can resume normal cellular activities of maintenance, growth, and reproduction. Forterre posits that the living form of a virus ensues when the virus enters a host living cell and usurps its genetic machinery to reproduce itself. This combination of virus and host cell, he calls a “virocell.”
One may question whether a combination of a virus and a living cell can form a single entity classified as a living organism. However, it is not unknown that fusion of cells from two or more individuals can coexist within a single individual. These combination individuals are called “chimera,” after a mythical creature having a lion’s head, goat’s body, and dragon’s tail. Chimera in mammals can be formed in the uterus during pregnancy with cells exchanged between twins, or between mother and fetus, or even during fertilization when genes from the male and female together form a zygote [1]. It is not such a leap of logic to suppose that a virocell is a chimeric form of life.
The conclusion that can be drawn from this is that viruses are, indeed, living, but in suspension until the proper environmental conditions present themselves, just as bacterial spores are living, but not actively functioning.
References
- A. T. Johnson, Biology for Engineers, 2nd ed., Boca Raton, FL, USA: Taylor & Francis/CRC Press, 2019.
- P. Forterre, “The virocell concept and environmental microbiology,” ISME J., vol. 7, no. 2, pp. 233–236, Feb. 2013, doi: 10.1038/ismej.2012.110. [Online]. Available: www.nature.com/articles/ismej2012110
- D. Quammen, “How viruses shape our world,” Nat. Geog., vol. 239, no. 2, pp. 40–67, Feb. 2021.