The chemical formula of deoxyribonucleic acid (D N A) is now well established.
The molecule is a very long chain, the backbone of which consists
of a regular alternation of sugar and phosphate groups.
To each sugar is attached a nitrogenous base, which can be of four different types.
Two of the possible bases—adenine and guanine—are purines,
and the other two—thymine and cytosine—are pyrimidines.
So far as is known, the sequence of bases along the chain is irregular.
The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide.
The first feature of our structure which is of
biological interest is that it consists not of one chain, but of two.
These two chains are both coiled around a common fiber axis.
It has often been assumed that since there was only one chain
in the chemical formula there would only be one in the structural unit.
However, the density, taken with the Xray evidence, suggests very strongly that there are two.
The other biologically important feature is the manner in which the two chains are held together.
This is done by hydrogen bonds between the bases.
The bases are joined together in pairs, a single base from one
chain being hydrogenbonded to a single base from the other.
The important point is that only certain pairs of bases will fit into the structure.
One member of a pair must be a purine and the other
a pyrimidine in order to bridge between the two chains.
If a pair consisted of two purines, for example, there would not be room for it.
We believe that the bases will be present almost entirely in their most probable forms.
If this is true, the conditions for forming hydrogen bonds are more restrictive, and
the only pairs of bases possible are: adenine with thymine, and guanine with cytosine.
Adenine, for example, can occur on either chain; but when it does,
its partner on the other chain must always be thymine.
The phosphatesugar backbone of our model is completely regular,
but any sequence of the pairs of bases can fit into the structure.
It follows that in a long molecule many different permutations are possible,
and it therefore seems likely that the precise sequence of bases
is the code which carries the genetical information.
If the actual order of the bases on one of the pair of chains were given,
one could write down the exact order of the bases on the other one,
because of the specific pairing.
Thus one chain is, as it were, the complement of the other,
and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself.