Are Viruses Alive?

Viruses are remarkably diverse. Some have RNA genomes, some have DNA genomes. Others have single-stranded genomes, and some have double-stranded genomes. But within this diversity, they still have common features. They are tiny, with a diameter less than 200 nanometers, hundreds of times smaller than most bacterium. This means they cannot be seen with a light microscope, but with an electron microscope. They can only replicate within a host cell. They also don’t have ribosomes. Ribosomes are an essential part of a cell in a living organism. They are a mixture of protein and RNA that starts being made in the nucleolus in a cell, and they perform RNA translation.

So then, if viruses are diverse and can reproduce, are they alive? Do they have a place in the Tree of Life?

There is no universally-agreed upon stance on this. It depends on who you ask. It also depends on how one defines life. The specifics vary, but most biologists understand life to exhibit these key features: the organism must be able to reproduce, make energy for itself, respond to stimuli, evolve, and be able to maintain a stable environment within its cells (homeostasis).

Viruses can do some of these things. They evolve. They also can reproduce, but not on their own. When a virus reproduces, it must gain access to a host’s cell where it essentially takes over to create a virus-making factory.

They also cannot carry out metabolic processes. They cannot produce energy or control internal environments. They also lack ribosomes and cannot independently form proteins from molecules of messenger RNA. So, by these definitions of life, viruses are not alive. They are often called “obligate intracellular parasites.”

However, some suggest they represent a different type of organism, a capsid-encoding organism (CEO), and should thus be included on the Tree of Life with all organisms. They would share the tree with ribosome-encoding organisms (REOs) which are all bacteria, eukarya, and archaea.

The idea of including viruses on the Tree of Life is fairly new, and started to come about with the 2003 discovery of a new and very large virus, Acanthamoeba polyphaga mimivirus. Mimiviruses sort of challenge the traditional definitions of viruses. They are larger, have more genes, including genes with the ability to make proteins, which viruses don’t normally do. They also can be parasitized by other viruses. However, they are still inert outside of a cell and do not have a single ancestral lineage.

So while determining if viruses are alive is definitely a complicated issue, what is certain is all of these perspectives will continue to change and grow over time.

In our next blog posting on viruses, we will take a look at some origins of the viruses that we deal with, and how they've made their mark in the fossil record.

SOURCES 

Abrahão, Jônatas and Rodrigues, Rodrigo. “Redefining Life.” The Biologist 63(2) p12-15

Edmonds, Molly. “What is a virus, and how does it become a danger to human life?” How Stuff Works. (accessed online: https://science.howstuffworks.com/life/cellular-microscopic/virus-danger.htm)

Wessner, D. R. (2010) The Origins of Viruses. Nature Education 3(9):37

“Where did viruses come from?” PBS Eons. (accessed online: https://www.youtube.com/watch?v=X31g5TB-MRo)