Hi everyone,
I’m not very knowledgeable about science, so I’m struggling to understand the notion that viruses aren’t “alive”, and the robot analogies people use. I understand that they don’t have some of the characteristics (cells, ability to reproduce), but my mind can’t wrap itself around the notion that they’re like objects. Can you please give some examples that could explain this in a way that is accessible to someone who isn’t very advanced in the subject?
Thanks
EDIT: wow thanks so much guys for so many amazing replies!!!
In: 29
It depends on your définition of alive.
Viruses are as much alive as any piece of DNA that reproduces like genes for example. Even if they are RNA viruses, their RNA is reproducing and evolving, so it’s alive in the same sense.
However, you can define alive do that only cells are alive. Then viruses aren’t alive.
There’s a little bit of a divide in biology on what the definition of life is. Traditionally, living organisms need to fulfil all of the following traits:
Homeostasis (regulation of internal environment)
Organisation (composed of cell(s))
Metabolism (transforming energy to cell components)
Growth
Adaptation (evolutionary change)
Response to stimuli
Reproduction (create offspring)
A virus is dependent on its host for many of these processes and as such not really considered life. The problem with the above definition is that it doesn’t 100% fit biological life all the time. Think for example about a mule, they aren’t able to reproduce, but still living.
Even though a virus doesn’t have its own metabolism, it does adapt. It does reproduce, but only through a host. There are biologist that consider a virus alive (I belong to this camp). There is another definition that I feel better captures life and that is that life is capable of darwinian evolution. This includes viruses, but also includes some other things that either really are on the boundary of life (self replicating proteins) or beyond (ideas can be argued to be able to evolve and selected on; some ideas stick around and adapt while others go extinct, sort of similar to a virus).
In conclusion, no definition of life is perfect and once you come at the boundary of life some sort of arbitrary line is drawn. Whether that line is put before or after a virus, self replicating proteins or ideas has interesting philosophical implications, but is not that relevant for the rest of biology.
Viruses are like toxins, or poison. The only difference between a poison and a virus is that when cells are ‘poisoned’ by a virus, they make more virus.
Obviously this is a simplification/exaggeration.
But why would you class a virus as alive?
– they can’t move or interact with thier environment
– they don’t need or use energy, food etc
– they don’t make things, or metabolise anything
– they can’t reproduce
The only thing remotely alive about them is that they are made of ‘biological’ material like DNA, RNA, and proteins.
Biologist here
This is a pretty interesting topic, and I’m afraid it’s going to take a longish answer to really get into it.
I think the best way to do this is to look at the spectrum of things from nonlife to life, and then you can decide what you think viruses are.
Everyone thinks **free living organisms** like you, or an oak tree, or a soil bacteria is alive. You take in raw materials from your environment, break them down, and use them to reproduce and repair. There’s a clear distinction between what’s in you and what’s out of you.
There are also some bacteria that are **intracellular parasites**, like _Rickettsia_. They live inside of other cells, and can’t make it on their own. But everyone agrees they are alive too. While they rely on the cell to keep them alive, they rely on it like we rely on our environment. They take resources from the cell around them, and use those resources inside of themselves to replicate and maintain their cells. There’s that same distinction between “inside” and “outside” of them. But these bacteria often can’t do as much themselves as normal living things can, because they are mooching off the host cells for various things instead of making them themselves.
Then you’ve got **viruses**. Viruses really come in two stages of their life cycle…there’s the virion stage (the virus particle floating around outside the cell) and the virocell stage (the infected cell). When a virion encounters a cell, its genetic material goes into the cell. But unlike our previous examples, that genetic material is copied by the cell’s own machinery instead of taking resources from the host cell and using those to replicate.
An intracellular parasite keeps a distinction between it and the cell around it, that distinction isn’t really strong with viruses. Viruses do form viroplasms (little virus factories in the cell). Sometimes these are pretty complex and have some distinction from the cell around them, sometimes they are very simple. But in all cases, the virus is reliant on the cell itself to make more viruses, rather than the virus doing the replicating.
Additionally, the virion _itself_ (the virus particles outside the cell) are simply genetic material encased in protein (and maybe some membranes). Cells can make new protein or make energy, they can move around and respond to stimuli. Virus particles are shaped so they can inject their genetic material, but that’s it. I’ve seen a decent argument that you can call the virocell (virus infected cell) alive, but not the virion.
Moving down, we have **Viroids**, Viroids are weird plant infections that are basically what you would get if you had a virus, but without the protein coat. They are just naked bits of RNA that cells copy, if they get inside the cell. They can transmit between cells and between plants if the leaves touch or sometimes through insect bites.
Similarly, we have **Plasmids**. These are loops of DNA (or RNA), common in bacteria and other microbes, that are separate from the main DNA of the microbe. They replicate in the cell using the cell’s machinery, and often they are transmitted between cells. Sometimes they hold useful genes (for the bacteria) like antibiotic resistance.
Next we have **Transposons/Jumping genes** These are bits of DNA in the chromosomes of various organisms that have genes which allow them to be cut or copied to other parts of the genome. They don’t spread _between_ cells, but they can multiply _within_ cells.
Next we have **regular genes**. Yeah, regular old bits of normal DNA. They replicate when the cell replicates, and get carried along to the new cell. They don’t multiply within cells like the transposons, but they are still multiplying using the resources of the cell they exist in.
Finally we have **prions**. Normal PrP protein is one shape. It’s just one of a great many proteins used by our cells. But a misfolded PrP protein can cause other PrP proteins to misfold. It’s kind of like if you had a messed up origami crane that you could bump into other origami cranes and cause them to mess up in the same way. It’s also kind of like dominoes knocking each other over…one falling domino turns another standing domino into a falling domino. Get infected by these, and eventually a bunch of your PrP proteins get messed up, clump together, and kill your brain cells. bleh. Pretty much nobody thinks prions are alive, any more than falling dominoes.
**Let’s sum it up**
So where’s the line for life? The reason people like to put it between “intracellular microbes” and “viruses” is because there’s a more distinct gap there. Intracellular microbes are a lot like free living microbes, just a bit more simplified and using the insides of other cells as an environment. Viruses, especially simple ones, are a lot like virions, which are just bits of loose RNA. Which are in turn a lot like plasmids and jumping genes, just able to get out of a cell more easily. Should we count transposons as alive? What about regular old genes? It sort of follows, and some people do think that makes sense (search for “selfish gene” if you want to know more).
But I still like putting the dividing line between viruses and cells. I think the distinction between things that take resources from their environment and use those internally to do stuff, vs things that have the right code to cause their “environment” (aka host cell) to make more of them. It’s the difference between a copy machine that can build more copy machines out of spare parts, vs a sheet of paper with “copy me” printed on it and left in the copy room.
Viruses don’t strike me as really being alive apart from the life provided by the host cell, although I do kind of like the argument that the virocell (virus + host cell) maybe counts as its own kind of living thing, even if the virus alone doesn’t. And not all viruses are equal. A giant virus with a huge amount of genes that basically remakes the host cell into something new is different from a little bitty virus with a handful of genes that get copied a bunch by the cell to produce a simple viral particle.
I think one of the hardest things to comprehend is that, despite not being alive, it can evolve. We usually think of evolution as something that applies only to living organisms, to help them survive as a species.
But evolution can happen with random chance. In fact, that’s kind of the definition as different genes change outcomes which changed based on chance.
Viruses tend to have a lot of mutations, and some of these mutations will result in producing more of itself. It’s the shotgun effect: throw enough things at a cell, eventually something will get through and reproduce.
Look up prions, too, if you really want a bizarre non-living/alive-ish protein.
I was listening to a podcast a couple of weeks ago where some scientists were debating what ‘life’ actually was and how you would define it. They both agreed that there was no universally accepted definition of what life actually was. They used the virus v bacteria debate as an example.
A virus particle on its own doesn’t have a metabolism. It doesn’t take in food, break it down for energy, and excrete waste. It just drifts until it bumps into a cell, then mechanically grabs onto that cell and injects its genetic material, which reprograms the cell to use up its own energy to make more virus particles.
A virus on it’s own does absolutely nothing, it’s just an inanimate object. The interesting thing happens when it interacts with a living cell. The cell absorbs it like it absorbs many similar things. Interacting with insides of the cell it breaks apart and then there are loose strands of RNA inside a cell. The cell does with them what it does with all loose strands of RNA and translates it into active proteins that start doing things inside a cell, there is no check to make sure the RNA originated from cells own genome.
That’s why a virus is not a living thing, virus itself doesn’t do any of the things a living thing does, everything that happens is done by the host cell up to and including making more viruses.
The definition of life is kinda blurry. There are plenty of definite yeses, tons of definite nos, and a fair number of “depends on how you wrote your definition”. Our current definition of life includes the following pieces, which must all be met:
Growth
Reproduction
Homeostasis (does it self-regulate its internal environment)
Metabolism (does it use materials and energy from its environment)
Ability to react to the environment
Complex organization
Adaptation over generations
All of these are required for something to be defined as living.
Let’s start with something that doesn’t have any of these characteristics, then look at some things that have some of the characteristics but don’t quite make the cut.
A rock is not living. It does not grow, reproduce, self-regulate, use energy or material from its environment, react to the environment, adapt, or have a complex organization.
Fire is not alive. Fire grows, reproduces, and uses energy and material from its environment, but it does not adapt, respond to environment, or have any complex organization.
Computers are not alive. Computers have complex organization, self-regulate somewhat (with fans, low battery mode, automatic virus scans, etc.), they take energy from the environment (though not new material), they respond to the environment (when you click on stuff and cat pictures appear), and it has a pretty complex organization, but it does not grow, reproduce, or adapt on its own.
Transposons are genes which copy themselves and insert the copy into a new spot in a cell’s genome. They are not alive. They reproduce and they adapt, but they do not have complex organization, grow (rather, they are made by parts of the cell), or self-regulate and saying that the cellular machinery copying and inserting the transposon is the transposon’s metabolism would be a stretch.
Viruses are the same as transposons except for a protein coating that lets viruses leave the cell they started in. They are not very complex and they don’t really self-regulate unless you count the fact that the protein shell keeps the DNA or RNA separated from the outside environment until it hits a suitable cell membrane. It does not use energy or material from the environment—all energy needed comes from the cell when the cell makes the pieces of the virus.
Because viruses are basically DNA capsules that are just clumps of proteins. They don’t have cells in the sense that unicellular or multicellular organisms do. Another thing about viruses is their method of reproduction. They basically require a host cell to hijack which they inject RNA into and the host cell makes copies of that virus. Not exactly how majority of living organisms reproduce.
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