When Watson and Crick met Haribo

Every March, Cambridge hosts a Science Festival - two weeks of events, all free, covering everything from maths to astronomy, physics to veterinary science. Departments open their doors and invite the public in, and if you miss it, you really do miss out.

Most of the events happen on the first Saturday, and that happened to be yesterday. There's far more on offer than we can possibly manage, so we are selective about the events we attend. This year - oh joy! - we managed to fit in a lot of cell biology, my very favourite subject. So, this week, in honour of the festival, I'm going to attempt to bring back Kiddie Science. Hurrah! For background on DNA structure, in case this makes no sense at all, this is a very good place to start.

One of the groups we visited came from the institute where I used to work, and they had a fun and simple table introducing the idea of DNA structure using gummy bears. We decided to take it one stage further. If you want to have a go too, you will need a packet of cocktail sticks and a LOT of jelly sweeties.

Simply put, the most common form of DNA is a bit like a ladder that is twisted around to form a double helix. In order to model a small part of it, first we built a pair of phosphate backbones - like the outside edges of the ladder - using two colours of jelly babies and some cocktail sticks. Observant biologists will notice that the two strands are running in opposite directions with the heads of the red men pointing away from the heads of the yellow men.

The part of DNA that does the clever genetic coding are the 4 nucleotides, or bases, that fit between the long backbones like the rungs of the ladder.

These bases are of two types - the larger purines adenine and guanine (A and G), and the smaller pyrimidines thymine and cytosine (T and C). One of the crucial things to realise is that there is only a limited amount of space in between the backbones, so in order to keep your ladder legs parallel, you need to pair a large (purine) base with a small (pyrimidine) one - in this case our two choices for the large bases were red and yellow fruity bobbles, while the small bases were purple and orange gummy bears. The girls soon decided which sweets to pair together and our first molecule was made.

Now, as Watson and Crick very soon realised, this complementary base pairing gives a huge clue as to how DNA can be accurately replicated - essential if you're a cell that wants to divide properly! So, we split the two halves of the molecule apart:

and used each half as a template to build a second strand.

Now all we had to do was skewer our rungs with cocktail sticks,

Pick up the ladders and give them a twist:

to form a giant edible DNA double helix!

Apologies to 1. cell biologists; and 2. vegetarians. My DPhil supervisor would be so proud!