| MadSci Network: Evolution |
As with many biology questions there are several different answers!
The easiest explanation is a simple structural-chemical one; the nucleosides that are used to build DNA are not symmetric (they are 5' nucleoside triphosphates). This means that there is an asymmetry when linking them together. Presumably, if the nucleosides were the other way round (3' phosphates), then the fragments would be on the other strand. I'm not sure about that, though.
Anyway, this simple explanation is annoying as there is still the problem of asymmetry in the first place. Why use 5' NTP and not 3' NTP to make DNA? This problem seems to be similar to that of the origin of asymmetry in proteins; protein amino acids are L, not R. The reason for this is obscure, but it seems likely that a choice was made at random, and this choice (L rather than R), was fixed by later evolution.
Of course, there could be a more chemical explanation - 3' NTPs could be less stable, or harder to make, or toxic, etc. The simplest explanation is still that both forms may have existed in the past, but that the 5'->3' organisms outcompeted the 3'->5' ones.
Sorry that there are no references, but this is mostly speculation, after all.
Try the links in the MadSci Library for more information on Evolution.