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Name: Anonymous
Status: Student
Grade:  Other
Location: N/A
Country: United States
Date: August 2007


Question:
Why is phosphate within the DNA structure? Why not something else such as a sulfate group?



Replies:
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 on that.

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.

Burr



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