Figure 11 is a diagram showing the essential structure of the large DNA
molecule. According to the Watson-Crick model,[7] the molecule consists
of two strands of smaller molecules twisted around each other to form a
double helix. Each strand consists of a sequence of the smaller
molecules linked linearly to each other. These smaller molecules are
called nucleotides, and each consists of three still smaller molecules,
a sugar (deoxyribose), phosphoric acid, and a nitrogen base. Each
nucleotide and its nearest neighbor are linked together (between the
sugar of one and the phosphoric acid of the neighbor). This leaves the
nitrogen base free to attach itself, through hydrogen bonding, to
another nitrogen base in the opposite strand of the helix.
In the DNA of higher organisms, there are only four types of nitrogen
bases: adenine, guanine, thymine, and cytosine. Adenine in either strand
of the helix pairs only with thymine in the opposite strand, and vice
versa, and guanine pairs only with cytosine, and vice versa, so that
each strand is complementary in structure to the other strand (see
Figure 12). The full structure resembles a long twisted ladder, with the
sugar and phosphate molecules of the nucleotides forming the uprights
and the linked nitrogen bases forming the rungs. Each upright strand is
essentially a mirror image of the other, although the two ends of any
one rung are dissimilar.
[Illustration: Figure 11 _Diagrammatic structure of the DNA molecule as
proposed by the Watson-Crick model._]
DNA: Diagrammatic single-strand chemical formula
DNA: Structural arrangement
When DNA is replicated, or copied, as the organism grows, the two
nucleotide strands separate from each other by disjoining the rungs at
the point where the bases meet, and each strand then makes a new and
similarly complementary strand. The result is two double-stranded DNA
molecules, each of which is identical to the parent molecule and
contains the same genetic material. When the cell divides, each of the
two daughter cells gets one of the new double strands; each new cell
thus always has the same amount of DNA and the same genetic material as
the parent cell.
(All that has been said so far about DNA replication depends upon an
assumption that the DNA molecule is in some way untwisted to allow
separation of two helical strands, but there is no compelling reason to
believe that such an untwisting does indeed take place, nor do we know,
if the untwisting does take place, how it is accomplished. Much that has
been said in the last few paragraphs is therefore purely speculative. It
is, however, based on sound observation and is a more logical
explanation than others that have been advanced.)
[Illustration: Figure 12 _The pairing of the nucleotide bases that make
up DNA._]
[Illustration: Figure 13 _The DNA molecule and how it replicates. (a)
The constituent submolecules. (b) Assembly of subunits in complete DNA
molecule. (c) “Unzipping” of the double nucleotide strand. (d) and (e)
The forming of a new strand by each individual strand. (f) DNA molecule
in twisted double-strand configuration._
Adapted from _Viruses and the Nature of Life_, Wendell M. Stanley and
Evans C. Valens, E. P. Dutton & Co., Inc., 1961, with permission.]