Re: what causes a strand of dna to twist giving it a double helix shape

I've found a very clear explanation here, http://www.thetech.org/genetics/ask.php?id=109, that explains the shape in terms of the need to fill 'holes' in DNA.

A short summary of Dr Balakrishnan's explanation is this:

Imagine a simple wooden ladder, with two upright poles to represent the DNA backbone, and short sideways rungs to represent the base-pairs.

Of course, I should mention that this ladder represents TWO strands of DNA, one going 'forwards' (or up the ladder) and the other 'backwards' (or down the ladder).

Now, hold the bottom of the ladder still, and twist the top to the right. This should alter the shape until it is a double helix. As it twists, the distance between the rungs of the ladder gets shorter - these are the 'holes' that are being filled in the explanation I link to above.

Naturally, if you could do this with a real ladder, it would probably break. Luckily, DNA is more flexible, and can twist and untwist like this. Interestingly, you can get DNA twisted into a right-handed helix (called 'B' DNA) and a rarer left-handed version (called 'Z' DNA).

There is a short summary article here: http://pubs.acs.org/cen/news/83/i43/8343notw4.html [If this link doesn't seem to work properly, I found that hitting the 'refresh' button on my browser fixed the problem. -- Moderator]

That is quite advanced, but still clear if you ignore some of the details. It is talking about a short piece of DNA that is somehow twisted into both B (right) and Z (left), with a bit in the middle where a base has 'flipped out'. The scientists who made the structure think that real DNA in our cells does this too.