Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne DNA Structure

Name: Marty
Status: other
Grade: other
Location: IN
Country: USA
Date: Winter 2013-14


Question: 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.



Replies:
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 you suggest.

I hope this has helped, Burr



Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 223
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: November 2011
Weclome To Newton

Argonne National Laboratory