Date: Winter 2013-14
Why is all DNA of all living organisms a clockwise helix. Doesn't this mean that all life came from perhaps a single biochemical event? If there were even ten such events, some DNA would be clockwise and some counterclockwise. This seems to answer the question as to whether life is a freak event or common event on earth-like planets.
Marty, let me answer both your question, and also your supposition.
First, the chemical physics of the DNA molecule govern its shape. It
turns out, DNA can have multiple shapes that are energetically stable.
These are known as a-DNA, b-DNA, and z-DNA. Each is helical, but with
different types of 'twist'. In fact, z-DNA is a left-handed spiral
(the opposite of the more commonly known b-DNA). So I guess the first
comment is that it is not quite accurate to say "DNA of all living
organisms [is] a clockwise helix", as you suggest. In fact, DNA has
lots of different shapes.
Second, your argument implies that all "life" that might be spawned is
still around today. That's definitely not true -- it is quick common
knowledge that some organisms have become extinct. In fact, the life
that is around today not only had to spawn into existence, but it also
had to out-compete all the other life forms around it. Thus, it is
entirely possible, and perhaps likely, that many more organisms may
have lived and become extinct without us having direct evidence.
Although we don't know all the details of how life began, this same
line or reasoning would suggest that other biomolecules could have
spawned and formed different configurations of life that don't rely on
traditional double-helix DNA. So the question is... are there any such
things? The answer is YES!
The first item that comes to mind is viruses, which frequently use
single-stranged DNA, or even RNA to pass on their genetic code.
Organelles in the body, such as Mitochondria, are also suspected of
being formerly-independent organisms that were subsumed by larger
cells and put to work. In fact, Mitochondria have their own set of
genetic code, RNA that is distinct from the DNA in the nucleus.There
is even a hypothesis that life originally was built on RNA, which then
gave rise to DNA-based life. All of these ideas contradict the
single-biological event idea you suggest.
Last, I want to talk about the probabilities associated with very
large numbers. It's really unlikely for someone to be hit by
lightning, but people still do get hit. That's because there are so
many people and so much lighting. The large numbers of chances
outweigh the low likelihood of a single event. Alternatively, it's
really unlikely that a specific person wins the lottery, but it's
highly probable that someone wins. In terms of biology, the same
applies to the concept of 'freak events'. Because of the enormous
number of molecules we're talking about, amazingly, stunningly,
freakishly alarmingly unlikely things still occur -- and they occur a
lot! A given genetic mutation might be very unlikely, but with
billions and billions of organisms (or billions-times-billions
perhaps), it becomes very likely to occur. In short, freak events
*are* common, when dealing with such large numbers of chances.
Obviously I have not attempted to fully detail or discuss many of the
topics here. My goal was simply to outline a few ideas that contradict
the point of view you shared. In short, I think there is a lot more
complexity and richness to the story about the origins of life than
I hope this has helped,
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Update: November 2011