What Do We Think of Viruses?

Coronavirus (Source: CDC)

It has been a while since I have written anything on this blog, and I missed that. But sometimes you need to take care of things that simply can’t wait. For example, taking some time off to take care of your sick father. Under those circumstances, even when you are not actually looking after someone you love in person, it’s the constant worry that gets you. At the very least, that worry got me, and that’s something that has been in my mind all the time for the last few months. But now that I don’t have to worry about that anymore, I can focus on this blog again. I hope to write regularly from now on.

There is something else that most of us are worried about now though: coronavirus disease 2019 or COVID19 (to learn about the most recent developments on COVID19, please visit CDC’s website). I intend to write a few essays on viruses, and this would be the first one. Here, I am going to focus on a question that might seem strange without any biology background: are viruses alive? As cities, states and countries around the world shut down to prevent the spread of COVID19, the answer might seem obvious, but in reality, there is no clear-cut answer to this seemingly simple question.

Structure of a Cytomegalovirus (Courtesy: Wikipedia)

Let’s start with a rather “flippant” definition of a virus given by Dr. Nigel Brown. He defines a virus as “gift-wrapped nucleic acid”; the nucleic acid could be a single- or double-stranded DNA or RNA molecule, the genetic material. From that definition, we can see that viruses have a rather simple, acellular organization: their genetic material is enclosed inside something. And that something is a viral protein shell or capsid, something that is encoded in their genetic material.

Capsids of some viruses can contain lipids from their hosts as well, and we will see shortly, when we talk about viruses, we can’t get away without mentioning hosts, the specific organisms that a certain type of viruses can infect. This is why it is difficult to think of viruses as living entities because they can’t do anything outside their host. They can’t replicate because they don’t have the cellular machinery to make new copies of themselves. They simply have to wait to find a suitable host so that they can use their hosts’ cellular machinery to replicate their genetic materials and make their capsids. The DNA or RNA of some viruses also encode necessary enzymes for replication, and they do need their hosts’ cellular machinery to do that task. Since viruses can’t replicate with the help of their hosts, they are obligate intracellular parasites. Now, if you encounter a virus outside its host, would you think it is alive? Since it won’t be doing anything, would it be very different from a rock?

Things, however, are not that simple. Other obligate parasites also need help from their hosts to reproduce. Some types of bacteria, for example, are obligate parasites. Bacteria, however, belongs to the tree of life while viruses do not. One crucial difference, in this case, is that bacteria usually would have own cellular machinery to reproduce—they just need some help from their hosts. Viruses, on the other hand, are solely dependent on their hosts for replication. Similarly, the other obligate parasites can metabolize—they can breakdown food to generate energy, for example, but viruses can’t (the viruses don’t need to though since they can get everything from their hosts).

A tree of life showing three branches of living organisms (Courtesy: Wikipedia)

I mentioned the tree of life earlier, and here, I want to elaborate on that because one particular set of arguments on this topic comes from evolutionary perspectives that the tree of life aims to model. One can trace a cellular organisms’ lineage by looking at the tree of life—that is there is a continuous chain of “life” or trickle of genetic material from its ancient ancestors. Viruses, however, lack a comparable history.

On the other hand, scientists who consider viruses to be alive point out that we only are aware of a tiny fraction of all the viruses that exist today, and hence, our knowledge of the genetic diversity of the viruses is incomplete. Furthermore, in 2015, Arshan Nasir and Gustavo Caetano-Anollés investigated the evolution and origin of viruses. They looked at the genetic codes for capsid proteins and identified a subset of proteins that are unique to viruses. The authors concluded that viruses may have originated from early RNA-containing cells, which means that the viral ancestry can be traced back just like other cellular organisms. Yet if we simply focus on lineage or genetic material to consider something living, what can we say about the DNA molecules in our cells? Should we also consider DNA to be a living entity?

Long after we get through the COVID19 crisis—and I am sure we will—this debate would continue, especially among microbiologists. I, however, don’t think every debate needs to resolve with a clear-cut answer; rather, I believe that knowing different sides of an argument is what expands our knowledge. Now that many of us are stuck at home (and possibly bored), musing on debates like this might allow us to learn about something new and stay alert. If you do want to read more about this debate, please read Dr. Nigel Brown and Dr. David Bhella's commentary on the topic. In the meantime, please follow the instructions from the health authorities to stay safe and sound.