Structural Properties of the DNA Helix
Why is DNA a helix?
The tendency towards a helix comes from the stacking of
the individual bases on top of one another. Both the sugar
and phosphate which constitute the backbone are quite
soluble in water. However, the DNA bases which are in the
middle of the helix are relatively hydrophobic and
insoluble. For example, one could readily dissolve more
than 500 g of deoxyribose sugar in a litre of solution, and
more than 100 g of phosphate will also readily dissolve in
the same volume. Only half of a single gram of adenine will
dissolve in a litre of water. Since the bases are flat, they
stack on top of each other in order to form a more
hydrophobic ‘mini-environment’. The bases twist slightly
in order to maximize their hydrophobic interactions with
each other, and it is this twisting of the stacked bases that
gives rise to a helix. Free adenine molecules, on their own,
will spontaneously stack on top of themselves to form
single-stranded helices in solution! Thus the reason for a
helix in DNA is primarily due to the hydrophobic stacking
interactions of the bases.