Country: United States
Date: August 2007
Why is phosphate within the DNA structure? Why not
something else such as a sulfate group?
This is a very complicated question indeed! The short answer is that the
biological 'machinery' of life would simply not work with sulfur, nor would
it with any other molecule. Yep, the phosphorus is that important. For more
details (or to answer 'why not?'), keep reading.
Before I begin, many scientists balk at questions that start with 'why'. We
are very good at answering questions like 'what' and 'how', but 'why' is
much tougher. 'Why' implies some kind of rationale. Finding a rationale for
natural phenomenon is not easy to do with science -- that's more the realm
of philosophy or theology rather than science. Sometimes it's just chance
that causes things.
There are three areas I hope to address for you. First, I'm going to try to
address how DNA would be different if it had sulfur instead of phosphorus,
and what impacts that might have on modern organisms. There are very good
reasons why you couldn't try to 'switch' to sulfur-DNA today. Second,
assuming life could work with sulfur-DNA, I will try to compare sulfur DNA
with phosphorus DNA to decide if one is better than the other. Third, I'm
going to try to answer 'why' DNA has phosphorus instead of sulfur. This is
the hardest question of all because answering it involves several important
assumptions and can involve theological considerations ( e.g. religion, God).
I will try to answer in purely scientific terms, but please understand that
many people answer this question using religious or other considerations.
Let's first ask how DNA would be different if we had sulfur-diester bonds
instead of phosphodiester bonds. From the periodic table, we see that Sulfur
has six electrons in its outer shell, while phosphorus has five. This means
that sulfate has a minus-2 charge, while phosphate has a minus-3 charge. In
DNA, where two free oxygens are required for the two ester linkages, in
phosphate there remains one double-bonded oxygen and one negatively charged,
while sulfate would leave two neutral double bonded oxygens. This means that
normal DNA is negatively charged, but sulfur-DNA would be neutral.
This difference in charge turns out to make a huge difference in the
structure of DNA. The two strands of DNA would not bind to each other the
same way, and the sulfur-DNA would be much less water soluble. Since cells
are basically sacks of water, this is a big problem. Many molecules require
the negative charge to bind to the DNA. In humans, DNA is tightly wrapped
around proteins called histones. If DNA were neutral instead of negatively
charged, most histones and other proteins would not be able to bind to the
DNA, and therefore not be able to function. You might know that
double-stranded DNA forms a double-helix shape. In between the strands of
the double helix there are two grooves. Many molecules recognize these
grooves and bind to them. If the DNA were made from sulfate, the DNA would
take on a different shape and the size and shape of the grooves would
change. Because the negative charges repel each other, sulfur-DNA might just
crumple up on itself and not even form a double helix. Many of the enzymes
and other substances that interact with DNA would no longer work.
Researchers have added a sulfur atom to the phosphate (phosphorothioate) and
found that few enzymes still work even with this minor change. And that's a
comparatively minor change. The result of a bigger change would almost
always be the death of the organism.
Because DNA stores genetic information, it's important that DNA be very
stable. DNA must also be able to unzip, be repaired, etc. If phosphate-DNA
were more stable than sulfate-DNA, that would be a good reason for using
phosphate. I do not know if this is true though. Sulfate and phosphate are
both very stable, so at first glance, sulfate might be equally as stable as
phosphate. I am not expert in this area of chemistry, so it's hard for me to
speculate on the stability of a sulfur-based backbone. Because sulfate-based
DNA is uncharged, the types of molecules that can chemically attack it are
certainly different, but how/if that would affect overall stability is
beyond my expertise. Perhaps also the energy or thermodynamics of attaching
sulfate instead of phosphorus are unfavorable -- maybe it's just harder to
make DNA from sulfur. Hopefully someone else has responded with information
Last, I want to talk about 'why' DNA is the way it is. Generally speaking,
every aspect of life is the way it is because it works that way. By 'it
works', I only mean that it is able to reproduce. Those things that are able
reproduce faster than they die are still around, those things that aren't
are no longer around. That's not to say that there isn't a 'better' way --
I'm not trying to make any value judgment.
It's possible also that there was simply more phosphorus around at the time
that life began to exist. The earth is constantly changing. For example, the
cell membranes of most cells are made of a compound called phospholipids.
It's basically a phosphate ion with a tail on the end. Researchers have
found that some organisms, a type of plankton for instance, can use sulfate
instead of phosphate in their cell membranes. The scientists who discovered
this suggested that oceans at certain times may have had more sulfur than
phosphorus, and therefore the plankton that used sulfur had an advantage of
those that didn't. Perhaps when DNA first came to be, sulfur was less
available, and therefore DNA (and cell membranes too) incorporated phosphate
instead. Because DNA is so critical to life, it's not something that changes
much even over very long periods of time. So once DNA developed with
phosphate, it was basically stuck. The conditions when life began certainly
influenced how life emerged, so perhaps if more sulfur was around then, DNA
might have incorporated DNA.
I want to emphasize that this answer is largely speculative. I am not aware
of any answer to your question that is definitive and widely accepted.
Either way, I hope this has helped.
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Update: June 2012