About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.
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I'm learning biology, and my textbook states clearly that prions, viroids, and virions (virus) are NOT alive because they need a host, and virions crystallize. However my tutor offered good arguments for them being alive since they feed and reproduce, plus viroids and virions have RNA.
So please give me your opinion, and your welcome to check out my "learn_biology" yahoo group. Thanks!
PS. You can check multiple items in the poll.
But has the scientific community generally agreed on a definition of life that it includes "cellular" and/or "metabolism"? Thanks for your opinions.
Definition of LIFE from my tutor:
A living organism on Earth is an entity containing protein and/or
nucleic acid, that imposes its form and function on another entity in
order to convert that entity into its own form and function, and does so
in order to persist and reproduce itself.
Definition of LIFE from Merriam-Webster:
an organismic state characterized by capacity for metabolism, growth, reaction to stimuli, and reproduction
Definition of life from Wikipedia:
Life is a condition that distinguishes organisms from inorganic objects, i.e. non-life, and dead organisms, being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally. A physical characteristic of life is that it feeds on negative entropy.
Interesting read on Mimivirus:
Mimivirus possesses many characteristics which place it at the boundary of living and non-living. It is as large as several bacterial species, such as Rickettsia conorii and Tropheryma whipplei, possesses a genome of comparable size to several bacteria, including those above, and codes for products previously not thought to be encoded by viruses. In addition, mimivirus possesses genes coding for nucleotide and amino acid synthesis, which even some small obligate intracellular bacteria lack. This means that unlike these bacteria, mimivirus is not dependent on the host cell genome for coding the metabolic pathways for these products. They do however, lack genes for ribosomal proteins, making mimivirus dependent for protein translation and energy metabolism. These factors combined have thrown scientists into debate over whether mimivirus is a distinct form of life, comparable on a domain scale to Eukarya, Archaea and Bacteria. Nevertheless, mimivirus does not exhibit the following characteristics, all of which are part of many conventional definitions of life: homeostasis, response to stimuli, growth in the normal sense of the term (instead replicating via self-assembly of individual components) or undergoing cellular division.
Perhaps you should analyze the underlying assumptions and decide why certain definitions would not be sufficient or too encompassing.
Living one day at a time;
Enjoying one moment at a time;
Accepting hardships as the pathway to peace;
I voted not alive. Yes they are complex things but it's just not the same as lets say a bacterium.
Virions, viruses and prions are just made up of some proteins and nucleic acid, which are equivalent to a small component of a cell. Their behaviour is far less complex than that of a bacterium, and same goes for the life cycles.
To me it's about complexity and these structures just don't compare to even the smallest bacterium (size not accounted).
Edit: Just noticed someone quoted Wikipedia. I'm sick of it being used to support an argument. I have never come across a page on the site without errors. The same goes for a lot of other websites, even if they seem legitimate. Want to provide evidence and be taken seriously? Use proper academic sources such as textbooks, articles from journals or websites belonging to research organisations such as the world health organisation. Not ones where anyone can write anything!
while wikipedia is not acceptable for academic work and always should be double-checked, it is a good starting point. Sometimes, even when i am doing academic work, I start with wikipedia and then check for its resources or if they aren't cited start searching for evidence.
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
I find most of the time the references have nothing to do with entry, and are just put there to make the information seem more legitimate. This site is a good starting point for A levels and first year undergrad, I advise people to avoid wikipedia completely. All the webpages I have come across on wikipedia are inaccurate.
I don't think prions can be considered to be alive by any means, they lack both nucleic acids and metabolism, save from being able to catalyse their own replication, so I voted 'no' there. (Surely all life should contain nucleic acids, at least until we create digital life or something)
Virions and viroids are sort of borderline cases like everybody knows. I usually like to think that viruses (here, virions) are 'false dead' when they are outside cells, and they 'steal' life from cells when they enter. After all, they do have metabolisim (the cell's...), their own genetic material, proteins, they reproduce, evolve, respond to their environment (this they do even outside the cells, when they contact their target receptor) - so usually think they're alive, after all (it is more fun that way )
However, my way of looking at things has its shortcomings when we go further and look at viroids, for example: they have nothing but nucleic acids :/ This being said, they (or some of them) have possibly evolved from more complex viruses, so if those were alive, why wouldn't viroids be - life probably cannot evolve towards death? But since I can't say viroids are alive, I probably cannot claim that viruses/virions are either...
Not to mention transposons, which are even more simplistic parasites: they're just dna segments that use the cellulary machinery to copy themselves and then integrate themselves to the genome. They're probably as simplistic parasites as anything can be. And surely _they_ cannot be said to be alive.
So, if transposons aren't alive, viroids cannot be either. And if virions only have a protein or protein+lipid "coat" and maybe some enzymes within it, how could that make them alive. So, after all this thinking I'm afraid I have to declare also virions dead. How sad...
Maybe it all comes down to metabolism: viroids, virions and transposons do pretty much all the stuff mentioned e.g. in Merriam-Webster or that Wikipedia article, save from their own metabolism. But even metabolism is a shaky criterion: unless other things than Jesus can rise from the dead, then all frozen microbes and even some multi-cellular organisms are alive even without any metabolism
Btw, are mitochondria "alive"? And if so, how about the nucleus or a plasmid..?
Ok, those plasmids, nuclei and transposons were mostly just a joke (although they have part in demonstarting how difficult it is to define 'life'), but mitochondria are an interesting case along viruses - they're not parasites, but they could be thought of as very specialised symbiots - much more so than algae and fungi in lichens. And I think they fulfill pretty much all criteria that can be required from alive beings.
Btw, does anybody know if mitochondria can be grown (or kept alive for prolonged times) in a culture. Maybe if not in practice, how about in theory? If you toss them the helper proteins and ingredients they require in a cell, could they live there happily ever after?
What about chloroplasts?
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