l Impli ons of the Stru e of Deoxyribonu
Questions 1-10 are based on the following passage and
supplementary material. This passage is adapted from J. D.
Watson and F. H. C. Crick, “Genetical Implications of the
Structure of Deoxyribonucleic Acid.” ©1953 by Nature
Publishing Group. Watson and Crick deduced the structure
of DNA using evidence from Rosalind Franklin and R. G.
Gosling’s X-ray crystallography diagrams of DNA and
from Erwin Chargaff’s data on the base composition of
DNA.
The chemical formula of deoxyribonucleic acid
(DNA) 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 5
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, 10
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 15
there was only one chain in the chemical formula there
would only be one in the structural unit. However, the
density, taken with the X-ray evidence, suggests very
strongly that there are two.
The other biologically important feature is the 20
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 hydrogen-bonded to a single base from
the other. The important point is that only certain pairs 25
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. 30
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. 35
Adenine, for example, can occur on either chain; but
when it does, its partner on the other chain must always
be thymine.
The phosphate-sugar backbone of our model is
completely regular, but any sequence of the pairs of 40
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 45
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 50
duplicate itself.
