I was reading the “What does viridae mean” thread and someone made the statement that viruses are “not really alive”. I’ve read this many times before, but don’t understand it. How can they infect us and proliferate if they’re dead?
I tried to Google for information, but apparently can’t figure out the search terms.
So if anyone can explain it or provide a web site with a good explanation, I would appreciate it.
They are nucleoproteins – nucleic acid with a protective protein “shell.”
So are genes.
Are genes “alive”? They’re certainly essential parts of every living thing, but are they themselves alive?
Viruses function as parasites, entering into cells and encoding for reproduction of the virus, just as if they were a gene.
(Genetic therapy, in theory – and I’m not sure how far they’ve gotten towards this in practice – introduces “good” genes to replace defective, genetic-disease-causing genes, by tailoring virus-like transmitters in which the nucleic acid of the imitation virus is the good gene, along with “take-one-and-pass-it-on” coding to cause it to reproduce and spread.)
they are “half alive”. They lack most of the machinery for reproduction or growth, instead they use your cells energy and metabolism, usually until the cell dies. They are a bundle of genes wrapped up in a protein, and as such are not really alive. In that form they will stay forever quite happily until activated. However I prefer to say that the combination (your cells + virus) is a valid living system, perfectly capable of reproduction etc.
Think of them as small biological machines.
Their sole purpose is to get in to cells and deliver the genetic material that will persuade the cell to make more virus.
Outside the cell they are inert chunks of protein and nucleic acid.
Yeah, until you double-click on them, and then you’re in big trouble.
Viruses are like chain letters - they contain instructions that, when executed, result in the distribution of copies of the original, but they do not themselves contain the copying mechanism.
Whether they are alive is a matter of how you define alive (and most people define it to include attributes such as metabolism, so they don’t qualify)
Life is usually defined as consisting of two main elements:
[ul]
[li]Metabolism, or the ability to convert food into energy and new bits of yourself.[/li][li]Response to the environment through some kind of sensory apparatus. Even bacteria will flee an unpleasant chemical introduced into their environment, and the bodies of people in deep comas will respond to mechanical and chemical stimuli.[/li][/ul](Reproduction isn’t on this list, because there are plenty of living things that are born sterile. That is, admittedly, an aberration, but it’s hardly unknown.)
Viruses have neither attribute. They don’t metabolize anything and they have no sensory apparatus. They’re purely mechanical, in one sense, and pure information, in another: They only have enough physical complexity to attach to a cell wall and inject their genes, but their genes code for the creation of a whole batch of new viruses.
Viral genes are subject to all of the environmental stresses that modify the genes of living cells, and they evolve in the same patterns. In fact, they evolve much faster than the genes of living cells, as evidenced by the fact that you need a new flu vaccine every year, yet the same antibiotics will work against run-of-the-mill bacterial infections. (Antibiotics misuse has created resistant strains, however, and that is a huge threat to world health.) So there are strong parallels between viruses and living things, not least because they’re made out of the same stuff.
Viruses are worlds more complex than poisons, for example, but they aren’t quite complex enough to be called living beings in my eyes. They’re still endlessly fascinating.
Or just a series of complicated chemical reactions.
Not really a series of complicated chemical reactions – just a very large series of them. Most biological reactions are essentially quite simple – add a group here, take a group away there, change one there. Since they involve proteins, the mechanisms for some biological reactions are rather complicated, and probably the most complicated chemical reactions known. The complicated thing about biochemistry isn’t the complexity of the reactions, though – it’s the sheer number of different reactions that take place.
In the simplest sense, viruses are genetic material protected by a protein coat that allows them to travel within and between organisms. Viruses lack all or most of the machinery required to reproduce their genetic material, so they use another organism’s machinery to reproduce. In that regard, they’re simply parasites, and there are countless examples of organisms that use another organism to do some chemical reactions for them. This saves the organism the resources necessary to do these reactions. Even humans do this – the synthesis of vitamin B12 (and many other vitamins), for example, is difficult, so we get it from food. (There are bacteria in the human digestive tract that make B12, but it isn’t absorbed. IIRC the vitamin B12 we get from meat is also ultimately produced by bacteria.) Viruses have an absolutely minimal biochemistry, and they totally depend on the host organism to perform even the most basic tasks.
One common criterion for life is the ability to undergo mutation, which is one of the few criteria by which viruses can be considered alive. By most standards, viruses are ‘dead’, but clearly they’re more ‘alive’ than a rock or a glass of distilled water.
Thanks to everyone who posted. I think I’ve basically got it, but I’ll go through it again because I tend to drift while studying anything these days. Any word might send me off on an orgy of digression. But I di…oh, never mind; just thanks.
I have often heard biology called “the one science that can’t define its own subject matter” – i.e., there is no agreed upon definition for “life” among biologists. Life is like pornography, it is hard to pin down an exact definition, but “I know it when I see it.” Generally, all biologists have been able to come up with are working definitions for life. These often postulate that for something to be alive, it must possess qualities such as:
With other conditions sometimes thrown in. The problem is that none of these sets of conditions ever includes all the things we intuitively know are alive, while excluding those we know are not. For example, fire grows, consumes matter, transforms energy, creates waste products, moves, reproduces itself, and responds to stimuli. Crystals are also capable of most of the properties of life, yet most people would not consider them alive (interestingly, due to their simple, uniform structure, viruses are capable of crystallization). Artificial life – whether computerized or mechanical – can fulfill many of the definitions for life, though most people would exclude these forms from consideration. Mules, since they are incapable of reproduction, would not be considered alive by the above definition – and neither would a lone man on a desert island.
So what it boils down to is a matter of semantics. Another definition of life that is often used is that it requires water, and that it is composed of cells. This definition excludes viri, but whether or not we include them in our classifications is probably of very little concern to them.
Imagine a factory in which you have a whole load of machines. These machines have a hopper in the top. If you feed a card with instructions on it for manufacturing a particular thing into the hopper, the machine will begin making multiple things as instructed, and spit them out all over the place.
Imagine one card gets introduced into the system that has instructions on it for making a copy of itself. It goes into the hopper of one machine which will start spitting out more copies of that card, which will fall into the hoppers of other machines, which will spit out more copies of the card and so on.
That card with instructions on it is a virus.
It’s hard to think of it as alive. But it does happen to be of a character that results in its own perpetuation.