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[–]Scientwist 36 points37 points ago

Some of these answers are partially correct but I'd like to expand on them a little bit:

It all boils down to "How do we define life?" That's a tricky question, right? It seems obvious on the macroscopic level but once you go microscopic things get more muddled.

The most universal definition of life is not a true definition! Instead it is a checklist:

  1. Does the object undergo homeostasis (that's a big word for maintenance)?
  2. Does it metabolize chemicals to create energy for itself to utilize?
  3. Does it grow and adapt to changes in it's environment?
  4. Does it reproduce to create new objects?

If the answer to all of the above questions is yes, then (scientifically speaking) the object is alive.

Now to get to viruses. You probably know (from other responses or previous teachings) that a virus hi-jacks a host cell for it's own benefit. But I doubt you've ever been told why. Viruses do this because they are unable to reproduce (which they do in a unique and very complicated manner) on their own. They need to infect a different cell in order to reproduce.

If we follow the strict definition of life (the list above) then it appears that viruses fail to meet this requirement. This is where the canonical "viruses aren't alive" statement has it's roots. Without a cell to infect, the virus will eventually die off. Your point is well taken, however. This is all a semantic argument over definitions to non-biologists.

An aside; a few years back there was this huge discovery about viruses that has since led people to refute the claim that viruses aren't true lifeforms. I can go into detail about that if you'd like but hopefully this post answered your original question.

[–]NotAgain2011 14 points15 points ago

I want to know about this huge discovery.

[–]Scientwist 24 points25 points ago

In the most simple terms: Two virus were discovered and one of them that infected a the other virus. The second virus was found to have limited gene expression capabilities without a host cell and has a larger genome than most bacterium do!

Basically, this discovery makes it hard to continue categorizing viruses as non-living when at least one exists that is larger and more complex than bacterium (which we refer to as "alive") and is capable of transcription. The additional discovery of the virophage (fancy way of saying virus infecting other virus) only furthered complicates things because viruses are only supposed to infect things that are "alive". Since this discovery (2008 if I'm remembering correctly), the whole discussion on life vs non-life in viruses has been filled on it's head.

For more details check out this and this.

[–]NotAgain2011 0 points1 point ago

awesome, I've always had trouble accepting that they weren't alive maybe someday the accepted understanding will change.

[–]sendenten 3 points4 points ago

I'd like to hear about this discovery.

[–]Scientwist 4 points5 points ago

I replied above to NotAgain2011 but if you'd like more clarification feel free to ask!

[–]pynaple 1 point2 points ago

I'm curious as to the "unique and very complicated manner" of reproduction. Care to tell?

[–]Scientwist 1 point2 points ago

Oh I missed this. Sorry! It is a very complicated series of events that is sort of unique to each virus but the basics are:

  • Infect Host Cell
  • Hijack cellular replication/transcription/translational machinery
  • Produce as much viral DNA/RNA/Proteins as possible
  • Assemble Virions (functional virus molecules)
  • Lyse cell releasing virions to all nearby cells

Each step is WAY more complex than what I have put here and as I said above, is usually virus-specific. Then there is the whole issue of avoiding the host's immune response which makes things even more complex!

[–]pynaple 0 points1 point ago

woah... I'm scared of virus' now

[–]thediffrence 1 point2 points ago

Excellent answer. When you say that something needs to be able to reproduce to be considered alive, do you mean reproduce sexually? Or just replicating cells, DNA, etc?

[–]made_your_day 4 points5 points ago

Bacteria and some plants reproduce asexually. They are considered living.

[–]Scientwist 2 points3 points ago

The latter of the two. Sexual reproduction is not the only means for creating offspring as pointed out by made_your_day as well.

[–]NuttyMcPherson 0 points1 point ago

A human (and many many lifeforms) cannot reproduce on its own, so how is that different from a virus?

[–][deleted] 0 points1 point ago

Follow the checklist posted above.

[–]Scientwist 0 points1 point ago

Again be careful with your terms. To you (and 99% of the entire world) reproduction means sex. In biology this is not the case. Instead we refer to reproduction in the cellular sense. You can get a cut and heal from it can you not? That is due to cells reproducing. Cue jokes involving self-reproduction! :)

[–]SweetSweetSilence[S] 1 point2 points ago

I like this answer best. But how does a virus know when to mutate in order to make infection more effective? Or is it just always doing that? And what are they considered then?

[–]Scientwist 10 points11 points ago

You almost fell into the most common trap in understanding evolution. There is no "knowing when to mutate" viruses always mutate and they do so incredibly rapidly. This is why we constantly need new HIV drugs. The virus mutates in order to avoid the drugs effect.

Remember, evolution has no end goal. Selection itself is not an intelligent process. The viruses that mutate to be less efficient at infecting just don't spread as well and they eventually go "extinct" as the more fit viruses spread rapidly.

Additionally, viruses are their own class of life-form (even if they are not life). Don't think that their mutation has made them something other than a virus. A bacterial strain can mutate all it wants but without thousands of years of time, it will still be just a bacterium.

[–]SweetSweetSilence[S] 0 points1 point ago

Ah, I see. I get it now. The mutating thing was just the last thing on my mind about it. Thanks for your detailed answers! :)

[–][deleted] 0 points1 point ago

viruses always mutate and they do so incredibly rapidly.

The virus mutates in order to avoid the drugs effect.

Can viruses have their cake and eat them too?

[–]dddaaabbb 3 points4 points ago

And of course that's why that trap is so common: even a scientwist can mistake their wording.

The virus doesn't mutate in order to avoid the drug's effect. It mutates and only the mutations immune to the drug flourish. (Interestingly, in the absence of the drug, those mutations might have died out because they are otherwise inferior to the other mutations!)

[–]Scientwist 0 points1 point ago

Thanks for the help! and indeed d0g63rt caught me misusing my terms as well. I guess it just goes to show how easy it is to misuse the terms and cause confusion.

[–]madcaesar 0 points1 point ago

I'm RES tagging you as "Biologist"

[–]Scientwist 0 points1 point ago

Hahah many thanks but I'm merely a lowly grad student in biological sciences. I won't be a full grown biologist for a few years yet.

[–]madcaesar 0 points1 point ago

I'm getting in on you while the stock is low! Once you're a full blow biologist I'm going to cash in big time :) good luck with your studies!

[–]Scientwist 0 points1 point ago

Haha I can support that plan. Thanks!

[–]YeshkepSe 0 points1 point ago

There's also been some recent genetic analysis that suggests at least some viruses might actually have evolved from the same Last Universal Common Ancestor as all other lifeforms. See here for more.

http://blogs.discovermagazine.com/loom/2011/07/14/trouble-in-the-fourth-domain/

[–]Scientwist 0 points1 point ago

I had never seen this before, thanks! Although I wonder how relevant that is. Arguably everything involving DNA/RNA/Protein biological processes must have come from one single source. So, is it more useful to consider these things as related, or are we better served by our current taxonomic definitions? Thoughts?

[–]YeshkepSe 0 points1 point ago

I think this is just a case of science marching on. The case has yet to be seen as overwhelming by the majority of modern biologists, but I suspect it's only a matter of time.

And actually, life could have been generated multiple times -- it's just that everything currently considered to be unambiguously living, and even the things we're tentatively considering labelling as such, appear to trace back to a single common ancestor. Kind of like mitochondrial Eve -- it's not that there was only one woman, just that her descendents outbred literally everyone who wasn't related.

[–]Scientwist 0 points1 point ago

I totally get and agree with "M. Eve" out-breeding all other lifeforms being a possibility. I just personally think that it seems more likely that a single life-form arose and rapid mutations caused it to out-compete its sisters.

Sort of a moot point since we can't ever test it but still fun to hypothesize about!

[–]YeshkepSe 0 points1 point ago

Short of discovering a lifeform on Earth that's demonstrably not related to LUCA, yeah, it's hard to prove. Although if the raw materials were dispersed and self-assembled according to the RNA World model, multiple, diffusely-defined biogenerative events are actually implied.

[–]minasituation -4 points-3 points ago

Great post, just not very explained like I'm 5 :/

[–]KovaaK 0 points1 point ago

Toward the top right of this page, you can find this under guidelines:

Keep your answers simple! We're shooting for elementary-school age answers. But -- ** please, no arguments about what an "actual five year old" would know or ask!** We're all about simple answers to complicated questions. Use your best judgment and stay within the spirit of the subreddit.

All that matters is whether it is a simple, comprehensible answer.

[–]Scientwist 1 point2 points ago

I know. I tried very hard to create an acceptable metaphor for the process and I had a decent one involving zombies but then I thought that Zombies would probably be even worse for a 5 year-old than the complex answer above. In any event, I don't think you should be downvoted as you have a legitimate criticism of what I wrote. I'll try to be more ELI5 and less askscience next time!

[–]minasituation 0 points1 point ago

I appreciate you :)

[–]RussellG2000 11 points12 points ago

Life means you can get energy from eating something (plants, animals, sunlight, minerals, and so on), you can have babies of your own that grow up and have more babies, and you can adapt to your surroundings to survive. A virus on the other hand does not eat anything else to make energy to live. There is still a lot of debate over whether or not a virus is alive, but for now it isnt.

[–]NotAgain2011 9 points10 points ago

Upvote for most ELI5ish answer

[–]RussellG2000 3 points4 points ago

That is what this is about isn't it?

[–]NotAgain2011 2 points3 points ago

yeah, that gets lost sometimes

[–]Hindu_Wardrobe -2 points-1 points ago

to harvest energy

FTFY, energy is never created.

[–]RussellG2000 3 points4 points ago

Apart from theoretical physics, I never said they create energy. I said they "get" energy, or harvest if that is what you want to say. Going as elementary as possible.

[–]Hindu_Wardrobe -1 points0 points ago

I'm only nitpicking. :)

[–]Venia 2 points3 points ago

getting != create.

[–]Hindu_Wardrobe 0 points1 point ago

"Making energy" = "creating energy".

[–]Venia 0 points1 point ago

Point taken good sir. I missed that last sentence, read the first one, brain shut off.

[–]Hindu_Wardrobe -1 points0 points ago

That is 'good miss' to you, but indeed. Again, I'm only nitpicking. No hard feelings. :)

[–]ameoba 8 points9 points ago

When people were first discovering how viruses worked, a lot of scientists felt the same way as you. There was a lot of argument of what 'life' really was. In the end the finally decided that "life" needed to reproduce on its own and that was that.

[–]aresius 10 points11 points ago

So basically mules don't live?

[–]thediffrence 13 points14 points ago

It's not so much reproducing sexually, as it is the organism's cells/dna/parts replicating. Mule's cells certainly divide and grow on their own during animal development and cellular repair and whatnot. For a virus, it is not possible to reproduce any part of itself without a host cell or organism to provide the tools (enzymes, etc) and a suitable environment.

[–]DT19 4 points5 points ago

I think there is a line drawn between being alive and being life, but I won't swear to it. Either way, good point.

[–]ccai 3 points4 points ago

That's sterility, that's like saying people with reproductive problems aren't alive either. It's more of genetic incompatibility, it's a slosh of genetic incompatibility due to the reproduction of two different species (a donkey and horse). For the most part the genetic material is rendered dysfunctional due to the hybridization, but there have been cases where mules/other hybrid animals have had live young. Most of the time in these situations, the female is still fertile, but a VAST majority of the time the male is infertile.

Fun fact: Mules can see all four feet when they walk down slopped land, this why they are good for those tours going down the Grand Canyon. They watch every step they take to avoid slipping.

[–]BeyondSight 0 points1 point ago

it's kinda like a magnet.

When it gets near another certain thing, it attaches, combines, and self replicates.

"Mutation" doesn't require intelligence.

It's simply evolution in super speed.

Basically, you use a certain poision to kill all the ants... However a few ants won't be effected by the poison by some mutation. Those few ants will recolonize, and you wind up with an entire population of ants that are now immune to that poison.

Bam, evolution in super speed.

[–]made_your_day 2 points3 points ago

I'm going to have to disagree. There many parasitic organism that require a host for reproduction. Plants sometimes need animals for dispersal and sometimes even part of the priming for the reproduction (e.g. digesting a plant seed to induce germination).

I would replace the phrase "reproduce on its own" with "have a metabolism". This is something in common with all

[–]tadrinth 2 points3 points ago*

Metaquestion: Why is there confusion and disagreement about the definition of life and whether or not a virus fits that definition?

Living things have a number of properties in common, because all life has a common origin and has been strongly optimized for the ability to replicate.

Since living things have reliable properties and were important to the reproductive success of your ancestors, those who had an instinctive, hardcoded intuition for living things were more reproductively successful. Since living things have a lot of properties that aren't found in things like rocks or tools, the intuition is complicated (lots of factors involved).

So, modern humans have a very strong and very complicated intuition about life. However, that intuitition developed to recognize the living things our ancestors dealt with, not viruses. Evolution doesn't optimize for what it doesn't see. Your intuition delivers a very sharp signal for everything a cave man ever saw, but it doesn't deal so well with viruses.

So, when people try to define the word "life" what they actually do is try to convert their complex, multifactored intuition with fuzzy boundaries into a simple set of rules with sharp boundaries. You can imagine how well that usually works. Ideally, you have a set of rules which closely enough resembles the intuition that when someone hears your definition, they recognize it and map it onto their own intuition.

The problem with viruses is that they have some of the factors that our intuition maps onto 'life' but not all of them. That confuses the hell out of our intuition, so not everyone agrees as to their classification. No matter what definition you pick, someone will object that their intuition disagrees on edge cases like this, so there is no satisfying answer.

A much better option is to use the term 'replicator'. You can come up with a much simpler definition for it because it isn't trying to convey a massively complicated intuition. Replicators make more copies of themselves. You can get pretty close to a mathematical or technical description.

So, are viruses replicators? Yep. Sperm replicators? Yep.

Humans also tend to think of things in terms of a vital force (a mistake called vitalism) even when they should know better, because that's how our intuition is wired. Using 'replicator' instead of 'living' helps reduce that particular cognitive bias.

The best option is to be specific about what properties you care about, rather than using an unsatisfactory or confusing definitions.

[–]2dollarb 0 points1 point ago

Viruses are more similar to machines than to life.

  • They do not eat.
  • They do not sleep.
  • They do not respirate (breathe).
  • They cannot think or feel.
  • They cannot reproduce without a host.

Viruses are like a special mechanical key. The key fits into a special lock on the outside of special cells. Once the key is inserted the mechanics start to work. The Virus then runs through its program combining the DNA or RNA with its host cell, creating more of the virus keys to infect more cells.

[–]nolotusnotes 5 points6 points ago

They do not eat.
They do not sleep.
They do not respirate (breathe).
They cannot think or feel.
They cannot reproduce without a host.

...And they will not stop until they find and kill Sarah Connor.

[–]2dollarb 0 points1 point ago

Hah! I almost spit out my pudding. Thank you for the chuckle. =)

[–]angrymonkey 0 points1 point ago

Sleeping, respirating, thinking, and feeling are not prerequisites for life. And really, all life is like a machine.

[–]2dollarb -2 points-1 points ago

No, they are not. But, a 5 yr old wouldn't know that now, would they?

And even when you explain the actual prerequisites, they won't understand. This question really doesn't belong here, but hey, I gave it a shot. ...Unlike some other people I might mention.

[–]angrymonkey -1 points0 points ago

My complaint is that your answer is untrue. One can give simple answers that aren't false-- just look at Scientwist's explanation.

But a five yr old wouldn't know that now, would they?

So you think that's a good reason to lie to them (and more importantly, to the OP)? They don't know enough to detect your untruths? How are they supposed to learn, then?

A five year old would come away from your explanation with an incorrect/absent understanding of what separates viruses from other life forms, and life from non-life. I'd say no explanation is better than an incorrect one.

[–]2dollarb -1 points0 points ago*

That's excellently put. And I'm sure you've never told your children the lie about Santa, or the Tooth fairy, or the Easter bunny. Or a whole host of other things they were not capable of understanding.

I'm not saying any of the above is right or wrong. It just is. I apologize for misleading the 5yr olds...

Self righteous monkey is angry.

[–]angrymonkey 0 points1 point ago

And I'm sure you've never told your children the lie about Santa, or the Tooth fairy, or the Easter bunny.

And if you used those to explain OP's question about the nature of viruses, your answer would be just as unhelpful for the same reasons.

I apologize for misleading the 5yr olds...

It's not five year olds you're misleading. It's adults in search of a real answer explained in simple language.

Self righteous monkey is angry.

Read your comments, dude. One of us is self-righteous and angry, and it's not me.

[–]poopsmith1976 3 points4 points ago

Viruses need a host cell to reproduce

[–]sodhi 1 point2 points ago

I would expect this to be a novelty account only replying to andrewsmith comments, simply adding 'poop' to them.

[–]nvolker 3 points4 points ago

Homestarrunner, man. Look it up.

[–]miamiburn 1 point2 points ago*

First off, you must understand the two most defining observable features of a virus - the protein coat, and the DNA within this coat. If they have DNA, one would assume they are alive though, right? Wrong. Viruses do not eat, sleep, breathe, reproduce (without a host), have any sense of their surroundings, or grow. Furthermore, they are non-cellular. Hopefully, this clarifies things a little bit.

Source: I'm a Biosystems Engineer.

[–]hydroxy 0 points1 point ago

Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations

Viruses can do none of these on their own, without a host cell they are just protein/nucleotide structures floating around doing nothing

[–]blueskin 0 points1 point ago

  • They do not move autonomously, either consciously or reactively
  • They have no senses
  • They have no biological process such as respiration, and indeed, do not make use of energy or resources
  • They can not reproduce unaided

There is no strict definition of life, but a virus fails on the majority of commonly-accepted criteria.

[–]iamvillainmo 1 point2 points ago

Viruses lack most of the machinery needed to replicate independently of a host, which are defining characteristics of life.

FWIW, many scientists argue that viruses should be considered to be life but they are really just assemblages of protein and nucleic acid (DNA or RNA) that are inert until they infect a susceptible cell.

[–]FMERCURY 0 points1 point ago

Many obligate intracellular parasites lack this ability but are considered alive.

http://en.wikipedia.org/wiki/Chlamydia_trachomatis

[–]iamvillainmo 0 points1 point ago

This is correct but obligate parasites also have their own "houskeeping machinery". They have their own means of replicating their genetic material built into their DNA. Viruses are not afforded these genes and rely on their host cell genetic replicating machinery.

Edit: Even many obligate parasites are only obligate during certain phases of their life-cycle, btw.

[–]Hindu_Wardrobe -5 points-4 points ago

No cells, no life.