Why is DNA a Spiral Helix
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WHY are DNA strands twisted?
The reason for the double stranded DNA is neither obvious nor simple. The
reasons are a subtle interaction of the component nucleic acids, sugar,
phosphate components with each other and with water. To my knowledge it
defies a “one line” explanation. The website
provides a reasonably
accurate and comprehensible explanation.
We can only surmise, based on observation, but I'd offer that a first need
is that our cells (which contain DNA in nuclei and mitochondria) be small
....this due to the surface area requirement being optimized in small, not
large, cells. In order for the DNA to actually fit within the small size of
cells, there is the need for a compact structure, and one could argue that a
straight-line structure of DNA would necessitate very large cells. We would
probably not see organisms in their current form if cells needed to be that
As far as a structural reason for the coiling, one could surmise that due to
the hydrogen bonding between purines and pyrimidines, and their required
paring based on base chemical structure, the coiling is the only real way a
stable molecule could be built.
The short answer: compactness and a stable structure is accomplished by the
During replication, and the needed coupling of complimentary base pairs, the
coiling needs to be temporarily un-done. Once the replication is complete,
the coiling returns.
Thanks for using NEWTON!
The very short answer is that the twisted ladder is a low-energy state,
but without context, that answer is not a very good one. I'm not sure
of your grade level, so please respond if my more detailed answer
below is unclear or if you have more questions. And forgive me, as
it's quite long.
First, let me note that double-stranded DNA is what we're talking
about -- the structure of single-stranded DNA is a completely different
story (and one I'll save for a different day...).
Based on your question, I'm assuming you already know the primary
structure of DNA -- the bases, sugars, and phosphates. When you
combine two strands of DNA, the "rungs" of the ladder (the bases)
attract each other -- just like the opposite-pole magnets in the
example. This is known as DNA hybridization. You might also know that
the connections between the bases are very weak -- its easy to "melt"
DNA (cause the strands to separate). The reason DNA is anti-parallel
is because when the bases pair up, they bond more tightly (=lower
energy) when arranged in anti-parallel fashion than when parallel.
The base pair story is pretty famous, but for DNA shape, the
base-pairs are not the whole story. DNA also has a backbone made of
sugar (deoxyribose) and a very important Phosphate group (it's
important chemically because it's highly charged). You must remember
also that the the whole DNA molecule is surrounded by water. It turns
out, the sum of the DNA-DNA and DNA-water interactions determine the
shape of the DNA.
When DNA is in water, the twisted helix has lower energy than an
untwisted one. DNA "likes" itself, more than it "likes" water. By
twisting on itself, it makes more DNA-DNA connections, and fewer
DNA-water connections. The result is fewer repulsive interactions and
more attractive ones (and this means lower energy).
Specifically, the charged phosphates repulse each other and try to
space themselves as far apart as they can (with an untwisted ladder,
they are closer together), and the hydrophobic parts (meaning they
repel water) of the base pairs are more shielded from the water by the
backbone twisting around it.
If more water was touching more hydrophobic parts of the DNA, then the
DNA would be a higher energy state. The DNA, through random
fluctuations, ends up in a configuration to avoid those kinds of
interactions. When you add up all the small energy effects of the base
pairs, the backbone, and the water, it turns out that the twisted
helix is the favored (=low-energy) state.
It's interesting to note that DNA only forms a double-helix in water,
and only if it's the right balance of salt, and not too high a
temperature. If you have too much salt, or you put the DNA in ethanol
(e.g. not water), or if you heat the water too much, the DNA
"denatures" -- meaning it loses its twisted, helical shape. DNA is
*not* a double-helix all the time!
It's also interesting that DNA has at least three *different* stable
helical shapes (they're all twisted, but they're not all twisted the
same way). Although the most famous (beta-helix) is the most common,
other helices have been discovered in chemically-modified DNA
(changing the atoms changes the energy, which changes the shape) and
dehydrated DNA (less water means different energy which means
In terms of energy, this introduces a concept of "stability". A
"stable" molecule is one that tends to stay like it is -- whereas an
unstable molecule tends to change to something else. In terms of
energy, this means that the small vibrations of the molecule all
result in higher-energy states -- which means the vibrations tend to
lead back to the original configuration, or in other words it is
"stable". You can imagine a stable molecule as a marble inside a bowl.
If you nudge it around, regardless of which direction, it tends to
just end up back where it started. On the other hand, if you flip that
bowl over and place the marble on top of it, if you push the marble,
it's is going to roll off and end up somewhere else. That's like an
"unstable" molecule. In energy terms, an unstable molecule is one
where small vibrations lead to configurations that are lower energy,
which means the molecule will not tend to revert to its original
OK, George, if you're still with me, I commend you.
Now, this answer assumes you know what 'energy' means in the
context of molecular configuration. If not, here's an 'appendix' on that
The structure of DNA (and any molecule) really needs to be understood
in terms of energy. Chemical molecules are constantly vibrating and
moving around (thermal energy). A fundamental property of matter is
that it's constantly moving toward a lower-energy state (which is
related to entropy and the second law of thermodynamics). Very large
molecules like DNA or proteins have lots of possible shapes they might
be in -- but they tend toward the shape(s) that minimize their energy.
With this in mind, your question might be restated as, "why do twisted
strands of DNA have lower energy than untwisted ones?"
Let me start by explaining the concept of energy of a molecule. To
understand the "energy" of a molecule's configuration, imagine two
magnets. If you push opposite poles together, the magnets "stick". You
have to pull on them to get them apart. In other words, you have to do
work (pull them apart) to separate them. In contrast, if you push the
same poles together, you have to push to keep them close, and if you
let go, they separate. You have to do work (you have to expend energy)
to get them together.
You may know from basic physics that doing work on an object means you
are adding energy to it (if you push a ball, it starts rolling, it
gains kinetic energy, etc.). The same is true for the magnets -- by
doing work on the magnets, you are moving them into a higher energy
state. When you push two same-pole magnets together, the magnets are
at a higher energy state than if you have opposite-pole magnets
So how does this relate to molecules? A DNA molecule is made up of
lots of atoms, each with different connections (chemical bonds, or
just proximity) to other atoms. All of these atoms are vibrating
around. Just like magnets might push or pull on each other, so do the
molecules push and pull on each other. As the molecule is vibrating,
it tends to creep toward the configuration with the lowest energy
(recall the magnet example -- here, "lower energy" means more atoms
"pulling" on each other and fewer atoms "pushing" away).
It's commonly known that "oil and water don't mix" -- in molecules,
atoms have a cloud of electrical charge around them that depends on
the type of atom (element) and how it's bonded with other atoms near
it. With atoms, positive charges attract negative charges. But also,
uncharged atoms tend to attract other uncharged atoms (this is like
oil-oil mixing). A uncharged atom next to an charged atom also repels
(this is like the oil-water repulsion). So in addition to
positive-negative attraction, you could say that atoms with
"compatible" charge properties (oil-oil) tend to attract each other,
while atoms with "incompatible" charge properties tend repel each
I hope this is helpful,
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Update: June 2012