If you'd like a very brief explanation:
"10. Why does DNA have a double helix shape?
DNA has a double helix shape because of the difference in the number
of hydrogen bonds between the nitrogen bases."
source: Centennial College: DNA AND RNA EXERCISE
For a more satisfying explanation, though, I direct your attention to
James Watson's textbook, "Molecular Biology of the Gene, Fifth
Edition." The Addison-Wesley Publishing Company has very graciously
provided "Chapter 6. The Structures of DNA and RNA" as a sample of the
This chapter discusses this topic quite extensively, and it's pretty
clear that the double helix shape is indeed quite significant. Watson
explains that the double helix structure results from the way that
hydrogen bonds connect the base pairs of the each strand and because
of how base pairs "stack" on top of each other. Moreover, the twisty
double helix shape is essential for the stability of the DNA molecule.
Frankly, this subject is somewhat complex, so I recommend that you
read the whole chapter thoroughly, but this excerpt contains the basic
"The hydrogen bonds between complementary bases are a fundamental
feature of the double helix, contributing to the thermodynamic
stability of the helix and providing the information content and
specificity of base pairing. Hydrogen bonding might not at first
glance appear to contribute importantly to the stability of DNA for
the following reason. An organic molecule in aqueous solution has all
of its hydrogen bonding properties satisfied by water molecules that
come on and off very rapidly. As a result, for every hydrogen bond
that is made when a base pair forms, a hydrogen bond with water is
broken that was there before the base pair formed. Thus, the net
energetic contribution of hydrogen bonds to the stability of the
double helix would appear to be modest. However, when polynucleotide
strands are separate, water molecules are lined up on the bases. When
strands come together in the double helix, the water molecules are
displaced from the bases. This creates disorder and increases entropy,
thereby stabilizing the double helix. Hydrogen bonds are not the only
force that stabilizes the double helix. A second important
contribution comes from stacking interactions between the bases. The
bases are flat, relatively water-insoluble molecules, and they tend to
stack above each other roughly perpendicular to the direction of the
helical axis. Electron cloud interactions ( ) between bases in the
helical stacks contribute significantly to the stability of the double
James Watson, "Molecular Biology of the Gene, Fifth Edition," hosted
I also direct your attention to the section, "The Double Helix Exists
in Multiple Conformations." This section explains that the Double
Helix's shape is not constant. For one thing, the spiral shape is
affected by humidity. In high humidity, the Double Helix twists at a
rate of 10 bases per turn (i.e., complete 360 degree rotation). In low
humidity, though, the Double Helix twists more: 11 bases per turn.
Watson goes on to explain that the nature of the base pair stacking is
such that "two members of each base pair do not always lie exactly in
the same plane. Rather, they can display a 'propeller twist'
arrangement in which the two flat bases counter rotate relative to
each other along the long axis of the base pair, giving the base pair
a propeller-like character... Moreover, the precise rotation per base
pair is not a constant. As a result, the width of the major and minor
grooves varies locally. Thus, DNA molecules are never perfectly
regular double helices. Instead, their exact conformation depends on
which base pair (A:T, T:A, G:C, or C:G) is present at each position
along the double helix and on the identity of neighboring base pairs."
James Watson, "Molecular Biology of the Gene, Fifth Edition," hosted
Although the artists' models that you mention usually depict the
double helix in the same way, the fact is that the double helix shape
is not perfectly regular or even necessarily linear.
See these other key sections of the chapter for more information:
"Bases Can Flip Out from the Double Helix"
"DNA Is Usually a Right-Handed Double Helix"
"The Double Helix Has Minor and Major Grooves"
"DNA Can Sometimes Form a Left-Handed Helix"
"Some DNA Molecules Are Circles"
"double helix shape because"
"double helix" rotate "hydrogen bonds"
I hope this answers your questions. If not, please use the "request
clarification" feature. Thanks.
Request for Answer Clarification by
01 May 2003 16:52 PDT
Thank you for taking the time to put together this thorough reply. I
followed through and am reading Watson's own words for the first time,
and it's not at all bad.
I guess my question related more to the teaching and understanding of
science in the wider community. I don't question that the structure is
a double-helix, just as I don't question that E = mc2. (I also don't
doubt that if you had to construct something with this set of
nucleotides to perform this task, then you probably couldn't construct
anything but a double-helix.) But it seems to me that these elegant
edifices sometimes hinder further understanding. In school we all
solved a trite problem to calculate that one gram of coal ash could
release sufficient energy to power 15 houses for a year (or whatever).
(Remember: "Too cheap to meter!!".) Similarly we are taught that the
double-helix is the "secret of life"... and the lesson often stops
there. Or if it didn't stop there, it got a slight nudge 47 years
later in this Holy Grail quest - the "mapping of the human genome" -
and how THAT was supposed to solve everything.
I suppose to clarify my question, I would ask "Suppose the
fundamentals of the mechanics of DNA never mentioned a double-helix,
but merely two strands that paired, matched, split, synthesised and
replicated... how much understanding would be lost?" Nothing, a bit, a
lot, or everything? And how much artistic imagary/symbolism would be
Chapter 6 - DNA. It's a double helix.
Chapter 7 - Van Gogh. Cut off ear.
Thanks again for your reply.
Clarification of Answer by
01 May 2003 17:37 PDT
I see your point. My guess is that most of the population doesn't
really spend a lot of time thinking about the structure of DNA. As a
result, if DNA were (inaccurately) depicted as a "2 dimensional
ladder," relatively few people would be genuinely bothered.
However, I think the key point is that the Double Helix is not just a
theoretical model. As Watson's text explains, the DNA molecule is
literally shaped that way. Of course, in the real world, there are
probably no perfectly shaped double helices. Ultimately, though, there
is scientific evidence that the basic shape is accurate. Thus, I'd
say that, at least for people who concern themselves with molecular
biology, quite a bit of understanding would be lost if the DNA were
regularly depicted in an inaccurate, 2-dimensional way.
Even for the general population, though, I think it's important that
the model reflect reality. What would be the point of hiding the
reality of DNA's shape? To a certain extent, your question is
analogous to asking whether a globe is truly any more useful as a
model of the earth than a two-dimensional map. Doesn't a globe help
you to understand the earth in ways that a flat map cannot?