Every DNA Molecule from a DNA Molecule. All of
this changed in the decade following 1944, when it
became evident that the physical material of heredity
is not, as had been generally supposed, protein but
instead is deoxyribonucleic acid (DNA). The problem
of replication again became real when J. D. Watson
and F. H. C. Crick in 1953 proposed, on the basis of
chemical considerations and X-ray diffraction data, that
the DNA molecule is a double helix. Its two strands
have -sugar-phosphate-sugar-phosphate- backbones
from which paired purine and pyrimidine bases extend
inward toward the axis of the helix and are held to-
gether by hydrogen bonds that regularly match adenine
with thymine and guanine with cytosine. The basic
problem of genetic continuity was at once recognized
to be the nature of the mechanism whereby the DNA
molecule replicates itself (Figure 1).
Several aspects of the model of DNA and its replica-
tion need some emphasis. First, the two strands of the
double helix are in every detail complementary. They
are not identical. A portion of the sequence that in
one strand might run -CATCATCAT- in the other
would run -GTAGTAGTA- and read in either direction
would be quite different from the first. The equivalence
in amount of adenine with thymine and of guanine
with cytosine, so characteristic of all DNA's no matter
what their AT:GC ratios may be, is a property of the
double helix and not of single-stranded DNA. Similarly,
the equality of purine bases to pyrimidine bases is a
property of the double helix, not of the single strand.
Replication itself is not a process that can be performed
in a single step by single-stranded DNA. The single
strand makes, or is a template for, a
complementary
strand, with a polarity that runs in the opposite direc-
tion along the molecule, since the 3′-5′ phosphate ester
linkages are reversed in direction in the two strands.
Properly considered,
replication is thus performed only
by the double helix, which upon separation and forma-
tion of two complementary strands generates two dou-
ble helices.
Watson and Crick found that certain evidence ex
cluded the possibility that the two polynucleotide
chains of a DNA molecule are paranemically coiled,
that is, are so coiled that they can simply slip into and
out of each other. Instead, they were plectonemically
coiled, like strands of a twisted rope, and therefore,
in order to come apart, they must in fact untwist. Since
they could not very well be conceived to replicate
while bound in the double helix, when every base is
paired and held by hydrogen bonds to its partner, it
seemed that replication must require a prior untwisting
and separation of the strands. As a most important part
of their theory of DNA structure, Watson and Crick
therefore postulated that replication is preceded by
uncoiling of the strands, after which each strand could
attract free nucleotides from the metabolic pool. These
nucleotides could then become united by phosphate
ester linkages so as to form a new strand that would,
with the original strand, twist into the double helix
again. Thus each of the separated strands of the original
double helix would serve as a template, and two double
helices would be produced from one.
Step by step, evidence has been found to support
the validity of this hypothesis. An important early
piece of evidence was Arthur Kornberg's discovery
(1956) that for the synthesis of DNA in vitro one must
supply a pool of nucleotide triphosphates, that is,
nucleotides which are already provided with the
high-energy phosphate bonds that enable formation of
the phosphate ester linkages to proceed. Calculations
by Cyrus Levinthal and H. R. Crane, and by others,
showed that the energy required to spin a very long
DNA molecule so as to untwist it is not inordinately
great but is only a small part of the available nucleotide
triphosphate energy of the cell, while the time required
would be brief. A model was proposed that envisaged
the progressive uncoiling of the double helix from one
end and the beginning of replication of the separated
strands while the remainder of the double helix was
still intact. It was found independently by Paul Doty
and J. Marmur that exposure of DNA to critical high
temperatures would lead to dissociation of the strands
of the double helix and that if cooling thereafter was
sufficiently gradual, the strands would in fact reassoci-
ate, or “anneal.” In this way it was possible to produce
certain kinds of hybrid DNA artificially, by bringing
about the association, while cooling, of single strands
from different sources.
A celebrated experiment of M. Meselson and F. W.
Stahl in 1958 provided very convincing evidence of
the correctness of the Watson-Crick hypothesis. A
culture of the bacterium Escherichia coli was first
grown in a medium containing heavy nitrogen (N15),
until all the DNA was labeled with this isotope. The
cells were then transferred to a medium containing
ordinary nitrogen (N14) for periods equal to one cell
generation and two cell generations. DNA was ex-
tracted from a sample of the original N15-labeled cells
and subjected to ultracentrifugation in a cesium chlo-
ride density gradient, which differentiates molecules
by weight. The DNA formed a single band at a charac-
teristic place. DNA from the sample taken after one
cell division formed a single band at a different place,
while DNA from the sample taken after two divisions
revealed two bands, one of them at the same place
as in the DNA from cells after the first replication,
the other a new band still further displaced from the
band characteristic of N15. The interpretation seems
clear. When the N15-labeled double helical DNA mol-
ecule replicates in medium containing only N14, each
new duplex will contain one strand labeled with heavy
and one labeled with ordinary nitrogen. There will
therefore be only a single band, but it should lie—as
indeed it does—just midway between the positions
occupied by pure N15-labeled and pure N14-labeled
DNA. After a second division, the separation of the
double helices will provide in each case one heavy and
one ordinary strand as templates. Hence, after replica-
tion, duplexes will be formed half of which will contain
one heavy and one ordinary strand and half of which
will contain two ordinary strands. The latter will form
a band at the position characteristic of DNA in which
all replication has occurred in medium with ordinary
nitrogen.
This experiment not only neatly confirms the Wat-
son-Crick hypothesis of replication, but shows that the
process is “semi-conservative,” which may be defined
in the following way. Once replication has taken place
in heavy nitrogen, there will always be some double
helices containing one heavy and one ordinary strand
when replicating in ordinary medium, but the propor-
tion should decline from one hundred per cent in the
first daughter generation to one-half in the next, one-
fourth in the third, one-eighth in the fourth, etc. If
this is so, the original strand that serves as a template
remains intact. Yet we know from Herbert Taylor's
studies that the original chromosome does not always
remain intact. It may undergo exchange at one or more
points with the new sister-chromatid that is formed.
There is clearly a discrepancy here; but it serves mainly
to emphasize the tremendous shift in dimensions when
a DNA double helix is compared with a chromosome.
The DNA double helix has a diameter of 20 Å, the
completely uncoiled chromosome one of at least 0.2
microns (or 2000 Å), one hundred times greater. Until
we know much more about the internal construction
of the chromosomes and the exact arrangement of the
DNA molecules in them, this hundred-fold difference
in dimensions (two orders of magnitude) leaves plenty
of scope both for molecules and for imagination. Nu-
merous models have been proposed to explain how the
replication of DNA can be semi-conservative while
that of the chromosome is not.
In this essay we have sought for the meaning of
reproduction in its broadest terms. We began with the
idea: all life from life of its own species. We have ended
with the replication of the DNA molecule. Reversing
the direction of our discourse, we see that the whole
of genetic continuity really lies here. Because each
DNA molecule can replicate itself, each gene and
chromosome undergoes replication. Because, whenever
the chromosomes divide, the sister chromatids separate
and move by means of the spindle mechanism into the
daughter cells, it follows that every cell comes from
a cell and contains within it the same genetic heritage.
Because cells arise mitotically from parent cells and
because each individual must originate as a single cell
or a cluster of cells derived either from two parent
organisms or from one, all life comes from life of its
own kind.
It remains for us to place this concept, genetic con-
tinuity, within a social context. Scientific ideas may
lead to technological applications which increase
human power and alter the course of civilization. In
time, the concept that genetic continuity resides ulti-
mately in the replicating strands of the DNA double
helix may assist in the techniques of genetic surgery
and manipulation whereby man will some day acquire
total control over the evolutionary process and alter
hereditary characteristics in selected directions. That
time is not yet. On the other hand, the greatest influ-
ence of scientific concepts may lie not in the field of
technological applications, but rather in the profound
alterations of man's philosophical views of nature, life,
and man himself. The ultimate concern of man finds
voice in the age-old cry: Whence? And whither?
In man's construction of his world view, the refine-
ment of the idea of genetic continuity to a point where
it is shown clearly to reside in the replications of a
remarkable sort of molecule represents the final step
in the validation of J. O. de La Mettrie's “L'Homme
Machine.” It is the ultimate reduction of life, symbo-
lized by its most unique and characteristic property,
reproduction, to the physical and chemical behavior
of molecules. All the genetically transmitted charac-
teristics and potentialities, both those defining the spe-
cies and those distinguishing the individual, are coded
in the sequence of nucleotides in the DNA molecule
and are produced during development through its
chemical control over the synthesis of a thousand—ten
thousand—proteins in the cells of the growing body.
Like the Theory of Organic Evolution, like the Theory
of Natural Selection, the full explication of Genetic
Continuity is destined to affect most profoundly man's
view of man, man's view of life.
Yet the mystery is not quite destroyed, not fully
replaced by “L'Homme Machine.” One must remem-
ber that the DNA double helix cannot replicate outside
of its most complex living surroundings. It cannot
replicate outside a system that includes not only neces-
sary components such as trinucleotides and necessary
sources of energy, such as adenosine triphosphate
(ATP); it also cannot replicate without the assistance
of a specific enzyme, itself a protein synthesized under
the directions of some part of the DNA molecules
present in the cell. Life, reproduction, the replicating
molecules are after all parts of an integral, complex
system; and it is the system that lives, reproduces itself,
and replicates its genetic code. There is mystery
enough here to satisfy anyone who persists in asking:
Whence? And whither?
The Principle of Genetic Continuity, in its final
refinement, would of itself produce a world of species
inalterable, of populations composed only of identical
individuals, because the DNA double helix replicates
itself so precisely. Yet the actual world is full of differ-
ent species, and populations belonging to the same
species exhibit incessant variety. The representatives
of the species Man differ almost as much as they re-
semble one another. Our world view must therefore
accommodate the existence of novelty and change in
hereditary characteristics, and the mutations of genes
and chromosomes which can be observed must have
their final locus in some change of a component of
the DNA, some error occurring in the process of exact
replication. Admit these alterations of the code, and
at once natural selection is supplied the material to
play upon. Thus, in our final view, Genetic Continuity
and Evolution are the two great themes of life, and
are linked through mutation and natural selection.
Genetic continuity implies the replication of chance
errors as well as the persistence in the main of the
old, tried and tested, reasonably successful attributes.
Genetic continuity is both the stable element in the
nature of man (and all other living species), and also
the basis for our hope that change may continue, that
new adaptations may be realized, and that a more
prescient creature may succeed us in the end.
