Naked Science Forum
Life Sciences => Cells, Microbes & Viruses => Topic started by: Zeig on 25/10/2006 23:12:09
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i know a virus is not considerd "alive" but if some one could give a non text book definition of the word that would be great. So, what would if be considerd if it still moves around and copying itself? nothing i know of such as a rock can do that...
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Technically, a virus cannot copy itself.
An ordinary cell contains DNA that is a blueprint of what the cell is to look like (the architects drawings in very crude terms) and the tools by which those drawings can be built as a new cell.
A virus contains only the DNA (or RNA – which can be turned into DNA by the cell, or can be used directly as if it was DNA within the cell) which contains the architects drawings of what the virus should look like, and then has to fool an ordinary cell into accepting the viruses DNA as being a valid part of the cells DNA, so that the ordinary cell will then use the tools it has for building its own cells also to build more virus particles.
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Most things which are obvious on one level suddenly becomes quite difficult when you look closely. I think the generally accepted view is that life is something that can reproduce itself and it's structure can be inherited, and this structure can be somehow mutated (basically the prerequisites of evolution)
This throws up the problem that something that being alive in one context - a virus in a cell, a computer virus on a computer, or a human on earth isn't in another - a virus out of a cell, a computer virus in a printout, or a human in a vacuum, or for that matter on their own.
I would have said that this just means you have to give context when you say something is alive.
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True Dave, but the major point which sets viruses apart from other forms of "life" is that they are incapable of autonomous replication. They are the ultimate parasite and consist of little more than an infectious bag of genes which is entirely dependent upon a host cell for replication.
A computer virus still fits this definition because without a computer it would be unable to replicate.
A more recent addition to this fold was the discovery of prion proteins, the agents thought to cause BSE and CJD. These are just misfolded protein sequences but they seem to be "infectious" and can catalyse their own production by coverting the brain's native (healthy) prion into the abnormal form. In this respect they're even more weird than a virus.
Chris
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It always annoys me when vaccines are said to be made from 'Killed' viruses when somehow the genetic materiel has been damaged and the casing is used to promote an immune response
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But without being in an ecosystem eg plants, various bacteria etc we are still unable to replicate. Giant Pandas can only replicate when surrounded with bamboo shoots. Surely you can say that a flu virus can only replicate in the environment of a human cell?
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But without being in an ecosystem eg plants, various bacteria etc we are still unable to replicate. Giant Pandas can only replicate when surrounded with bamboo shoots. Surely you can say that a flu virus can only replicate in the environment of a human cell?
Without totally disagreeing with you, I think there is a subtle distinction, albeit one might consider it to be an arbitrary one (and one that might cause problems when deciding whether prions are living).
With a virus, the process of creating a new virus is external to the virus itself, whereas with all cellular life, the processes that create new cells are internal to the cell itself.
Thus one might rightly say that a cell reproduces itself, whereas a virus merely causes itself to be reproduced.
The problem is that with a prion, there is no external mechanism (although a prion does not have an inside and outside, a new prion is created directly by the existing prion, and there is not an intermediate vector in the process).
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Personally I think that the processes of viral replication is very distinct from any other organism as its characteristics in terms of structure and mode of replication. They very host specific, it's not a choice, it is in their DNA that encodes the protein to recognise specific sturcture (antigen) on the host. I don't know if everyone is familiar with the method of replication in virus
There is also a group of bacteria that you might argue to be not bacteria but intra-celluar parasite the Chlamydiaceae which is also unable to self replicate and need to be inside human cell. but still in term of genome composition is very similar to bacteria as it codes for bacterial cell structure.
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Chlamydiae are rather peculiar by bacterial standards, but then if one goes down that road, how do you judge a distinction between an intracellular bacteria like chlamydiae and a fully captured organism like mitochondria?
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A basic difference between a virus and an obligate intracellular parasite is that a virus has no 'metabolism' of its own. It cannot make ATP. All living things can.
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A basic difference between a virus and an obligate intracellular parasite is that a virus has no 'metabolism' of its own. It cannot make ATP. All living things can.
Does this mean that you would choose to define life as that which contains ATP? Or is it more broadly, that life must consume/convert energy in some way?
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Yes, I think so.... or at least has the capacity to. I know seeds, or spores aren't, but they can. All a virus can do is pervert another cells metabolism.
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hmm are men sort of like a virus then....
the process of a man replicating itself is outside of the man, we just inject our DNA and then we move on
man is to woman as virus is to ___ whats the most accurate response? CELL!
of course there are differences, but idk seems interesting
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I would refer to a virus as a particle of nucleic acid (DNA or RNA) surrounded by a protein sheath and in some cases a membranous envelope which is highly infectious
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They are alive - though they don't fulfill all the criteria set out by Schrodinger in the 1930s. Here's a thought - they contain DNA, so can you imagine what the reaction would be if, say, a probe went to Mars or a comet or something and viral material was found there and it was reported "No we didn't find any life, only viruses"?