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
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.
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