Re: What exactly keeps DNA from dissociating??

That's a very good observation and question. The answer lies in whether water by itself has the strength to pull a DNA molecule apart.

The short answer is water just is not strong enough to break down a molecule of DNA.

The longer answer is below:

It is true that water can break and make hydrogen bonds, and this occurs at a very high frequency all the time. In most cases these are short lived, weak interactions that are essential for life. Only when the water molecules are organized nicely, like in ice, or in nearly freezing water, can the collective hydrogen bonds have such a strong effect. In fact this is how water crystals form to begin with - the more hydrogen bonds form the tighter the complex becomes. A water molecule imposing a new hydrogen bond by itself is rather weak in comparison to the billion, if not trillion, already organized by the DNA.

Assuming that we had a solution of DNA in water, the water would not have the collective strength to pull the DNA apart. The DNA has a much greater affinity (greater force) for itself since it is so large and those H-bonds are already formed. DNA can also package itself very nicely.

Because DNA has this property it also has a melting point. The melting point is the temperature needed to separate these strands. This would be similar to a boiling point in water where a given amount of energy is needed to break these H-bonds and separate the water which forms gas. In fact, at high enough temperatures, just like in water, the H-bonds in the DNA are less effective and two DNA strands separate from each other. But without this temperature change, DNA is very stable being together.

Also remember the negative phosphate backbone which would be busy making their own H-bonds with any water molecule that comes close to the DNA.

This is actually a nice thought, otherwise our whole body would be ripped apart everytime we took a shower! Yes, the H-bonds that water can form are essential for all life but the internal protein and DNA structures which also form H-bonds are much stronger to maintain itself than what water could do to it.

That is why some regions of both proteins and DNA are called hydrophobic (water fearing) or hydrophilic (water loving). We actually want this association with water.

Here's an excersize you can try:

Take a rubberband, hold the ends of twirl it in your fingers. You will see the two strands of the rubber band twist and twist and twist around each other. The more you twist the tighter those strands become and after a while you see the twists forming more twists. Now ask a friend to try to pull those strands apart - it won't be easy! DNA is often found like this, it is called supercoiling. In the cell DNA is packaged with hundreds of proteins and stored away as chromosomes. Either way, water just can't do the job by itself!

I hope this answered your question and good luck in your studies!

Matt

[Moderator's note: You can find much more information about hydrogen-bonding, water-molecules, and DNA complexes in a good college-level Biochemistry or Molecular Biology textbook. For example, Biochemistry, by L. Stryer, Molecular Biology of the Cell by Alberts, and Molecular Cell Biology, by Lodish are all excellent resources.]