I've not found a good clear answer to just exactly what the advantages
of this double helical structure are, either in magazines or on the
net.  I mean I gather the structure permits duplication of genetic
information that would NOT happen if the molecules or acids or bases
(whatever) were not so arranged.  BUT HOW?  I know no biochemistry to
speak of, but am hoping some answerer or 'commentor' will offer an
esplantion or refer me to sites that do.

the "heart" of the secret of DNA being "information for making a cell"
lies in the fact that the alphabetof this DNA language consists of 4
bases  that go into making a sequence on the DNA  - much like how the
alphabets in a language can be used to make words  and sentences that
carry information because of the sequence orders of the bases. Now  an
amazing property of the DNA bases is that each base can actually bind
to "a complementary structure" meaning that the base A will bind to C
and T will binds to G. that is what maintains its helical properties
(if you calculate the chemical composition of the DNA strand and its
repititive backbone - the double helix happens to be the structure
that the DNA sequence most favorably adopts).

so if you had a sequence that read ATCG - in the DNA - the cell
actually first gets you CGAT. (the complement) - and the complement of
the complement gives you back the starting base sequence.DNA
replicates by making a complementary set of DNA sequences - which get
read back to the original - through the process that DNA makes RNA
which then converts the sttarting DNA sequence into the corresponding
protein.

Technically the key requirements for the genetic material are three:
it must be able to carry information, the cell must be able to make
copies of it and (occasionally) the copies must be slightly wrong.
These could be accomplished in many ways: there is no *need* for the
molecule to be either double-stranded or helical. RNA acts as the
genetic material in some viruses and is often a single-stranded
molecule: there are even some viruses that containe single-stranded
DNA.
What is critical is that the structure of the molecule dictates to the
cell a way to create a duplicate of itself. In the normal process, as
described by gautamthor-ga, the sequence of bases on one strand
automatically determines the sequence on the other since each of the
four bases will only form a stable structure with one of the others.
In DNA this process is the result of weak bonds forming between
certain pairs of bases, but not others, meaning that there are only
two possible combinations that attract each other (G - C and A -T)
A simple analogy: imagine you have a  line of boys and girls, some
blue-eyed and some brown, in any combination waiting for partners to
dance with. If only blue-eyed will dance with blue and brown with
brown, then you can predict what will happen: If the first child is a
brown-eyed boy, then the partner will be a brown-eyed girl; a
blue-eyed girl will attract a blue-eyed boy, and so on down the line.
Once the complete double line is formed, you could separate them and
still know what the other line must look like.

I must disagree with the above comments. Complimentary base pairing is
a wonderful thing, to be sure, but that does not answer the question
of why the double helical structure of DNA is of any importance.
Indeed, "What is critical is that the structure of the molecule
dictates to the cell a way to create a duplicate of itself." is very
relevant to complimentary base pairing, but I would hesitate before
saying that the double helix is crucial in this role.

I would agrue that both the double stranded nature and the double
helix structure of DNA are quite important. Firstly, one needs to
understand that the primary differnce between DNA and RNA is the
presence of an oxygen atom in the phosphate-sugar backbone (i.e. DNA=
DIOXYribonucleic acid, RNA-Ribonucleic acid) This oxygen makes for
much more stable connections along the back bone that enable the
structure to form reliable double helix DNA molecules. Yes, RNA can
form double strands, but these are rare and are mostly in viruses,
which have a considerably high mutation rate, that would be
disadvantageous for diploid organisms. Furthermore, many transcription
factors (proteins that bind to DNA and promote transctiption or
repression of a gene) bind to specific DNA consensus sequences but are
also dependent on the presence of major and minor grooves found in
those regions that aid in DNA-protein interactions, think T-boxes
transctiption factors for instance (Muller and Herrmann, Nature, 389
p884). Thridly, I would argue that the double stranded nature of DNA
facilitates in the DNA-protein interactions necessary to compact these
massively large molecules into the nucleus. There are several histone
protein complexes that bind along the length of the DNA that
facilitate compactation, they may also be dependent on major and minor
grooves of the double helix. The structured compaction of DNA by
histones is necessary to extablishing tertiary organixation within
DNA. That is DNA is not linear as we think about it but in contact
with regions that may be quite some distance away. To find a
regulatory sequence 3000bp upstream of a gene may be indicative of a
close physical relationship between this DNA region and the gene of
interest in vivo, which is caused by this compaction by histones which
is a unique feature to DNA and not RNA.i

I've read all the foregoing answers; certainly I will peruse them some
more.  It is complex, I think.  And I surely appreciate them!!  But
allow me to follow up as below.  (As I write, my questions seem clear
to me but I often find what is clear to me is often not as clear to my
reader.  So if you find yourself feeling "I'm not sure what he's
driving at", please ask me for clarification.)

Would be fair to say the double helical structure provides for
duplication of a cell with considerable efficiency, simply because of
the intertwining spiral construction?

If that is the case,  would it suggest or mean that the components
(those bases) are so physically positioned that the duplication is
abetted by the physical positioning of the bases resulting from the
double helix?  I mean could things work as well if (as I think
rickyjoey hinted) if you had 2 strands side-by-side for a
structure--such as if 2 lines of x's were placed side by side and
practically touching.  Like this: (I could not get them to print
closer together here.)

xxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxx

Are there also non-structural factors in the double helix model that
also abet duplication?

Chromatin can condense and divide among daughter cells quite quickly,
but I would not say that this is simply because of the double helix
structure of DNA. Although it is important in DNA condensation, there
are lots of other components involved. Particularly, you may want to
look at components that are involved in attaching centromeres to
microtubules during mitosis. I don't think I was hinting that simply
having two strands of DNA close to each other is good enough for this.
Other components in the cytoplasm are distributed to the daughters by
other means, including other DNA and RNA species. These molecules can
be segregated randomly (i.e. just left to their own devices), if they
are not associated with some cellular components, or localized to
certain parts of the cells so that the two daughters are asymmetrical.

As for DNA synthesis, I don't recall any information stating that a
double helix is important for this process. Although someone may make
the argument that DNA polymerase interacts with DNA at specific major
and minor grooves, and as such you would expect that DNA synthesis to
be interrupted if the DNA secondary structure was not present. There
may be some papers on this but I am not that familiar with DNA
poymerase specifics.
i

Before Watson and Crick wrote their famous paper on the double helix
nature of DNA, other scientists, like Linus Pauling, felt that DNA was
a single helix.  There was initially no fundamental reason to suspect
that DNA had to be double stranded.  So, you might answer this
question by saying that it may simply be a result of evolutionary
fate.

But there may also be clear advantages in having duplicate
complementary copies of DNA.  If you think about it, the two strands
are sort of like 'backup' copies because damage on one strand can be
repaired by attaching a complementary base. It's always better to have
two copies than one. This sort of redundancy is present even in modern
human technology as a safety factor.

Another possible advantage, as some have mentioned already here, is
the simplicity of duplication. If the basic idea is to get two copies
out of one, then simply unwinding the strands and allowing
complementary base pairs to attach themselves is a cinch. -How might
this work with only one strand?- The complementary nature allows
identical copies to be made. (Of course, these copies are
semi-conservative: after duplication, each of the two copies has one
old and one new strand of DNA).

Just to clarify a comment by af40-ga, Linus Pauling actually proposed
a triple helix model of DNA, not a single helix.

Furthermore, the benifit of double helix DNA is the redundancy in
terms of preventing mutations.  The strand that is being copied can be
used as a proof against the strand that is being generated. This
semiconservative method of DNA replication is really quite cool, as if
improper base pairing occurs, binding does not occure (ie the purines
(A, G) will not bind to the pyrimidines (C, U, T), this allows for
efficient DNA repair mechanisms through the variety of proof reading
enzymes.  This allows for efficient replication in terms of minimizing
mutaions in the genentic material.  With regards to mikewa-ga's
comment, for evolution, yes, the material must be slightly wrong on
occassion, but the rate of errors must be reduced to an absolute
minimum.  We can see the problem with errors, as a single amino acid
substitution, which can arrise from a single base-pair change can
result in a lethal phenotype. The double stranded DNA does not
inherantly confer any efficiency simply due to its double-stranded,
alpha-helical configuration. It is the chemical properties that allow
such a structure which confer the benifit of double stranded DNA.

It is fascinating to read all these answers.

I'd like to ask now 2 questions prompted primarily but not entirely by
pforcelli-ga's comment.  They are:

1.  HOW do the chemical properties allow the double helical structure?

2.  Why, from an evolutionary standpoint, must there be some (tho'
rare) errors in the copies?

The simple answer to your second question is that evolution (as per
Darwin's theory of natural selection) can be most easily (or is that
parsimoniously) defined as "descent with modification". That
modification is manifest in mutations that affect some aspect of a
gene, e.g. expression levels, timing, patten, or the RNA and proteins
produced from that gene. the descent part meand that these changes
must be passed down to the next generation.

Umm - it is actually a whole lot simpler that has been portrayed here.
It comes down to a topological solution to two conflicting requirements.
First, DNA must be extremely stable, and second, the code sequence
must be easily available.

Here's how it is resolved.  The two strands are not chemically
connected!  They are held together by very weak polar interactions. 
That means that they are easily separated to allow the enzymes like
RNA and DNA polymerase to access the enclosed code.  BUT - because the
two strands are intertwined in a double helix, it is impossible for
them to fall apart!

The double-helix allows easy access to the coding bases by allowing
local unwinding, but it remains globally stable due to the topological
constraints.

To pforcelli-ga,

Sorry, triple. But as regards your comments:  "Furthermore, the
benifit of double helix DNA is the redundancy in terms of preventing
mutations."

there also seems to be a redundancy in our answers, if you scroll
above, where I say:

" If you think about it, the two strands are sort of like 'backup'
copies because damage on one strand can be repaired by attaching a
complementary base. It's always better to have two copies than one.
This sort of redundancy is present even in modern human technology as
a safety factor."

I will try and answer wumply-ga's questions:

1. Most structural conformations, in, say, proteins, or DNA, or any
organic compound really, are due to various chemical and bond
interactions. In DNA the base pairings are actually rather weak
hydrogen bonds that can be broken apart (denatured) at high
temperatures. I believe the G-C bonds are somewhat stronger than the
A-T bonds because the former are triple hydrogen bonded as opposed to
double, and this can actually be a useful tool in examining the nature
of strands of DNA.

2. You asked 'why' there must be mutations. First, let me state that a
mutation in an autosomal DNA will not have any effect upon evolution
because the DNA is not passed on to offspring.  Whatever happens-
whether the DNA is repaired- or whether the outcome is deleterious- or
if it is (rarely) advantageous- stays with the individual. On the
other hand, if the sex chromosomes are affected, and are passed on to
offspring, there is a very rare chance that such genetic information
provides some advantage to the offspring which may be passed on from
generation to generation.  In evolutionary terms, when we speak of an
evolutionary advantage, we really mean differential reproduction,
because it does not matter what the genetic effect is unless the end
result is more offspring.  This advantage may be the ability to gather
food more easily and thus it enhances the viability of the organism.
With greater viability, reproduction may also be enhanced because the
individual has more time to reproduce. Or, this selection can be based
on sexual phenotype alone. If you look at the male peacock, for
example, you may notice that it has a colorful set of feathers in its
tail fan.  It is clear that this very colorful set of feathers is not
directly advantageous in terms of survivability. But for some reason
female hens prefer more colorful feathers, and so the bigger and more
colorful males have more offspring. (Of course, some have argued that
the colorful tail is actually highly correlated with some other
desirable trait in the male peacock, and thus the colorful tail is
merely a signal of a quality male). Whatever the reason for female
preference in this case, the selection is sexual.

Okay. I don't know how many of the responders are geneticists, but
there is enough misleading information here to make this more
complicated than necessary.
DNA is deoxy, not dioxy. It has one less oxygen than RNA, not one more
DNA polymerase does not interact with the grooves, the two strands
have already separated by the time DNA polymerase attaches
It is complementary, not complimentary. 
Base pairing does not require the molecule to be double stranded,
except during replication. there are single-stranded DNA viruses that
work perfectly well, and can pack more DNA per unit volume than double
stranded

Still, to try and answer a couple of the follow-up questions. If there
is NO mutation, then all life would consist of many identical
individuals of a single species.
The helical nature of DNA is simply an inherent result of the
restrictions on bond angles and distances present in the molecule.
there is nothing magical about it being a helix, many biological
molecules form helices.

Bravo to mikewa for catching my spelling and structural errors.. how embarrassing.?

basically it's so it can be squeezed into a really small space coz if
it was layed out it would stretch from earth to the sun, and that's
just the dna in one human cell!!

Uh, no. A single cell contains about two metres of DNA

To give a really quick answer...
The double helix struture is thought to have come about due to
evolution, as it allows more information to be stored safely, whilst
remaining effective and efficient than other methods (e.g.,
Prokaryotes etc) . The double helix structure is not the only
struture/shape DNA can be stored in, but it does have its uses - esp
as a higher evolutionary organisms require much more DNA information
to be stored, and allows for genetic evolution to occur more quickly
and safely.

The double helix  gives more 'bang' for your buck.

Should like to ask the following of ufoolme-ga' particularly...but of
others too if anyone feel inclined to respond.

First, may I ask what prokaryotes are and how DNA is arranged in them?

Secondly, re the double helix structure.  What specifically IS it that
makes it SO efficient at making copies? Or HOW does it give more bang
for the buck?

I know no biochemistry; only the generalities in popular magazines and
TV/internet news, so if this is too complex a question, let it go.

I'm wondering, though, is the double helix more efficient because it
allows a part of one strand to be really physically close to a part on
the other strand so that the two parts can easily combine ehemically? 
(This is a 'feeling' of impression I've sort of sensed).  Or what?

The world is so INCREDIBLE!

Because when the DNA is copied in two, one of them is copied from the
first strand, and the other from the second. Hope this will clarify at
bit all the stuff they have sent you.

Hey ya, wumply-ga.
In prokaryotes (bacteria) most or all of an organism's genetic
information is stored in one long, circular DNA ring.

This circular shape is fine for simpler organisms, but for humans etc
- it wouldn't work. Nature is thought to have overcome this problem by
changing the shape to a more complex one that of a double helix
whereby transcriptions/reproduction can be maintained and monitored
allowing more information to be stored, and hence higher life forms
such as our selfs to exist! Double helix shapes come in three forms
alpha, beta, z(eta?) but the method for dna replication remains the
same. In fact it is very similar to that for the circular DNA, with
only a few minor changes - that of DNA repair, other kinds of
monitoring.

Currently theory also suggests that a more complex shape itself could
have futher effect, an example of this is with sex determination - but
most of this theory is too new to tell either way in my opinion. Tho
is kinda interesting to know.

By the way Biochemisty can never explain anything to anyone, unless
they are a chemist ;p

(sorry for any spelling/grammer/annoying errors in this, i'm just
writing it out quickly)

Hi ufoolme-ga:

Read your Sept. 26 answer but I find I would have still more questions
if we could meet and had a DNA model in front of us.  But we don't and
I see no way (not even email) that such would even be possible or that
you would be in a position to spend all that time.  But I do
apprecdiate the answer you gave me.

By the way, I saw no errors in your comment.

wumply