During this Covid-19 pandemic, we’ve all been thinking more about viruses and, like Anika Chebrulo, how to stop them. Learning about Anika’s work and Covid-19 (Link to Worst Pandemic Ever?) made me realize I wanted to know more about viruses and how they worked.
If you classify viruses as alive, then viruses are the most abundant life form on Earth. Biologists estimate that the number of individual viral particles, called virons, on our planet as a 10 followed by 31 zeros, 10 nonillions. This is 10 times the estimated number of individual bacteria on earth and more that the number of stars in the universe, according to a National Geographic estimate.
What is a virus? Simply put, a virus is an infectious microbe containing a segment of DNA or RNA covered with a protective protein coating. Some familiar examples of virus are viruses that cause AIDS, COVID-19, measles, or smallpox.
Viruses infect living cells in order to reproduce. Reproduction seems to be the main thing and perhaps the only thing a virus does.
A virus cannot reproduce by itself outside of a living cell. It must infect a cell in order to reproduce. Once it enters the cell, it takes over the cell’s machinery for this purpose. Once the virus has multiplied itself enough times, it breaks out of the cell, often causing severe damage or killing the cell. These incubated viral particles go on to infect other cells in this host and then jump to other hosts. A virus infection and the resulting damage to many cells can injure the entire organism.
Viruses infect humans, but they also affect many other living organisms. Both animals and plants can get viruses. When a virus jumps from animals to humans, as with Ebola, MERS, and COVID-19, the resulting disease can be among the most deadly. Viruses even infect fungi and bacteria.
A virus consists of two or three parts. First, a virus needs a segment of genomic sequence, either a segment of DNA or RNA. The COVID-19 virus uses RNA for its genetic material. Some examples of viruses using DNA are smallpox, hepatitis B, and warts. The second part is a protein coating called a capsid, which protects the genetic material when the virus is outside of a living cell. Some viruses, like COVID, influenza, HIV, and hepatitis C, also have an oily (lipid) envelope as protection. That’s all. A virus is a very simple entity.
Is a virus alive?
At the start of this post, I said that viruses are the most abundant life form on earth if you classify them as living. Are they alive? Biologists use several criteria to decide if an entity is alive, including: 1) do they have cells, 2) do they use energy, 3) do they respond to their environment, and 4) are they able to reproduce.
As different biologists look at each of these criteria, they find it difficult to agree on whether a virus qualifies as living. For example, viruses do not have cells. That is pretty clear. But they do have containers called capsids, which serve the protective function of cell walls.
Do they use energy? Viruses are only active inside a host cell and they use the energy of that cell. For this reason, some biologists say no, they don’t use their own energy. Other biologists say they are using energy, although it is not energy they produce. The whole category of parasites complicates this criterion. For example, there are bacteria which use energy from their host cells, called obligate intracellular parasites. Biologists consider these bacteria to be living organisms. There is an argument that viruses are using energy in the same way, so they pass this criterion just as the parasitic bacteria do.
Do they respond to their environment? Again, biologists don’t agree. When they are dormant, there is no interaction with anything, but most individual virons are not outside a living host for long. They either find a new host within a few minutes, a few hours, or, at most, a couple of days under ideal conditions, or they “die.” Virons “die” when disruptions of the structures of their key proteins, nucleic acids, or fatty membrane prevent them from being able to infect cells. The ultraviolet rays in sunlight, a swipe with a bleach wipe, or handwashing are all able to neutralize a viron. Once inside a host cell, however, they interact with the cell and its component machinery. And clearly viruses like COVID mutate and evolve continuously. That is why there are so many COVID variants and we see natural selection at work as one variant drives out a predecessor.
Are they able to reproduce? Some biologists say no because viruses cannot reproduce on their own, since they require a host cell. On the other hand, it is very clear that once inside a host cell, viruses can reproduce rapidly with devastating effect. A complicating factor is the discovery of a new type of virus called a mimivirus that has the tools to copy its own DNA.
Will we need to expand the definition of what is alive, as we did when we discovered extremophiles? Before then, biologists accepted as axiomatic that no living thing could exist in conditions of extreme temperature, acidity, alkalinity, or chemical concentration. Then we found some.
Is a virus a living thing? Maybe. Sometimes. It depends on if you look at it outside a cell, which is an unstable transition for a viron, or inside a host cell where it is dramatically active.
Maybe for now, life is a spectrum with degrees of living along a scale. Perhaps someday soon, there will be a new synthesis that redefines life in a way that explains all the evidence we have. This ongoing discussion and new research are the basic processes of science.
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