Steven Projan wrote:
>> This is a frequently asked question and debated issue. Just because
> virus are parasites requiring a host cell in which to replicate does
> not make them non-living as there are many examples for bacteria
> (which most of us would consider alive) which are obligate
> intracellualr pathogens. But if we consider viruses as living
> entities than where do we draw the line? What about plasmids, which
> are composed solely of DNA and replicate inside cells (usually
> bacteria) but do not become encapsidated live viruses? In 1980
> Richard Novick wrote an article for Scientific American in which he
> referred to plasmids as the smallest of organisms. If we accept
> plasmids as organisms then where do we put prions? And what about
> those degenerate bacteria, the mitochondria and cholorplasts, do we
> consider these the formerly living, the undead, really nice
> parasites, symbionts?
>> Perhaps we should just consider that there is a continuum between
> what is the biochemical and what is the living. Perhaps life is like
> pornography - I believe it was Supreme Court Justice Hugo Black who
> said that he don't know how to define pornography but he knew it when
> he saw it.
>> Steve Projan
> Wyeth-Ayerst Research
But you can't define what you don't delimit - if there is a chemical
process or set of processes which can be identified as "life" there must
be distinguishing characteristics. If life is simply the ability to
replicate macromolecular structures from a pre-existing template then
crystals are alive. Since one essential characteristic of life is a local
entropy reduction crystallisation should qualify. Some theories of prion
formation are not really that far from this idea.
Personally I go for a definition which includes the presence of nucleic
acid (RNA, DNA or both). You still have a problem - is a 'flu virus an
organism or a community? Ditto for diploid and polyploid cells...
Graham Shepherd
microHero at compuserve.com
