Last time we pulled a thread from onion juice. I bet everybody here knows what that thread was - it was DNA, wasn't it? There it is on the left - and on the right's a thread of my DNA. They're almost exactly the same, those two threads, but the subtle differences between them mean that one makes an onion and the other makes a human being.

The DNA in this baby's cells - this is Max by the way - makes him grow and become a child, and that's what we're going to explore today. The DNA in Max's cells is buried deep in the heart of them, in the nucleus.

We need two volunteers here who are going to come and pull open the nucleus in a moment. Inside the nucleus are the chromosomes to make a human being. If you grab hold of it, I'll give you a count to open it. Get ready to pull when I say, so it's three, two, one and open.

There are the forty-six chromosomes of human DNA. If we pull this up a little out of the way - that's it - we then need to sort these out. They come in pairs because half of them come from the mother and half from the father. We have some people who are going to come down and help to sort them out, I think. And the idea is that twenty-three of you - including you two, you're part of the gang - are going to sort them out, pair them up by colour and see how you do. That's right, keep going. The thing is to grab one and then try and find its partner. Of course you might grab two pairs; you'll just have to be clever about that. Here on the screen, you can see a picture at very high resolution, scanning electromicroscopy of real chromosomes as they are, flying across the landscape with those real chromosomes.

How are you doing? Have you got those sorted yet? Right, has anybody not got a pair? There's one person who has an unpaired set of chromosomes; does anybody know why that is? Max is a boy - this is the X and the Y chromosome. A girl has two of these X chromosomes but a boy has only one and in addition he has this fiddling little Y chromosome - and worse than that, it's just one gene on there, basically, that makes him a boy. So a boy is a kind of a slightly modified girl. The girls may feel that actually it's not a very successful modification either!

Now what I want you to do is to get in a big horseshoe and face outwards and I want you all to throw your chromosomes up to the very back row of the lecture theatre. That's it, well done! And now the volunteers can go back to their seats.

Meanwhile I want you to pull out the DNA that's in the chromosomes and I want it everywhere. There's a lot of DNA - I want it everywhere and coming down towards the front. Every cell in the human body has two metres of DNA, literally two metres! You're scaled up a bit here with this stuff and in all the cells of the human body, there is enough DNA that if it was laid end to end, it could go five hundred times to the sun and back. And here we see on the screen, again the reality of DNA spilling out of a human chromosome - there's the tattered remains, it's been treated with detergent and the DNA, see the lines of DNA spilling out? You're creating that with your strands of DNA, pulling out from those chromosomes.

But first, we have to go back and think about what DNA really is like and it's like this, it's a spiral with two strands. If we straighten it out, it's easier to see, there's two backbones - one going one way and one the other - and in between, there are these pairs of bases and they come in four sorts.

You have thymine, you have cytosine, you have quanine and adenine and they can form pairs. In fact, they always form pairs when they're in the middle of the DNA and the pairs are like that and these white bits are where the backbones go up.

What was discovered - the key thing about the structure of DNA when it was discovered nearly fifty years ago - is that the two base pairs, two sorts of base pairs, fit just on top of each other. So when you make DNA, the base pairs fit and make this nice regular spiral. We don't say those long words all the time, we call them T, A, C, and G for shortness.

Now the clever bit about these bases is that this is the information to make life. We call it the sequence, and by that we mean the order and the sort of bases along one of the strands. And then, because of the base pairing in the middle, the other strand is defined always by the first strand. So we just think of the sequence as being the order of the bases along one of the strands.

We have a model here which shows how it really is. If I was an atom looking at a molecule of DNA and I could sort of feel at that atomic resolution what it was like, then this is what I would see, it's a more solid feel. You can't see the base pairs in there so that's why we have the models, so that we can see more clearly.

Now, what I want to do is to think about the sort of information that's being stored there. If I could get you to hold the end of that - put a hand top and bottom so we can see it and stretch it out so everybody can see - and perhaps Max can come back? How are you doing Max, can you bear it? There he is! What we're looking at is how Max was made.

Now one way of making Max would be to have a blueprint, that's what we've got here. There's architectural drawings of Max, you see: side, head, elevation, all that. In fact at one time people thought that was how it was done. They thought they could see a little man inside the sperm and that this would grow and become a human.

The other way is this, and that's what we've been looking at with the ladder; that's DNA sequence scrolling past there on that board. We're going to have a look at some other ways of coding. The blueprint way, we can say, is analogue. Analogue means that the way of coding it is rather like the thing itself. Digital, on the other hand, is not at all. It's a code and it's only by some prearranged understanding that you can see what the code is meant to be. So I think I better put Max back otherwise I can't show you the stuff, but just bear in mind that it's Max that we're making out of all of this.

Now you see, here's some other analogue things. Look at this chap, is he analogue or digital? Shout out!

Analogue, exactly. He's supposed to be looking like the person that he represents. How about this?

Analogue. What about this, is this analogue or digital?

Digital - not quite so sure about that were you - but it's the English language and it is digital. It's just characters, they're meaningless, they don't look in the least like what you're trying to represent but we understand, we have dictionaries to understand what we mean by them. And this? Is this analogue or digital?

Digital. It's a computer, it has its own code which is binary. It has two symbols, unlike DNA which has four, but it very much is the same and finally, these two?

Analogue yes. Now there's only a few people perhaps who know what that is. It is an older sort of gramophone record - we mostly have CDs, which are ...

Digital because they have the same computer language. There's the analogue gramophone record. The grooves on this represent and make the sound when it's played back in exactly the same way as the sound waves coming to the ear. So when the sound is recorded, you get waves like the waves in the ear, in the air. And when it's played back, the waves go out again. So there's the analogue and then the digital where the sound is analysed in terms of numbers, which are then printed on in a binary code to the CD. We'll take these off again, I think.

Now when it was found that this was a digital code, it was absolutely astonishing, because life after all is a sort of general squishy thing, very complex.

Until DNA was really firmly identified as being the source, people felt that the information to make it must be coded in some much more complex way. But this actually is really digital. It's exactly analogous to the sort of software that we put in a computer.

Does anybody know how to play "Tomb Raiders"? Come on then, come on you, that's good. So you play that and show us how it's done. In order to play "Tomb Raiders", you have to put in software to the computer. The software has a code, just like our DNA, but the clever and paradoxical thing about life is that the DNA does much more than that, it actually makes the computer itself. Now you'd be pretty surprised if you put in some computer software and you got a new computer! You know you have to go to a shop to buy a new one - but we're not like that. Our hardware, our cells, our body, our structure are built by the code, as well as the software part which runs us.

What we're going to do now is to go onto the most important thing. I think I'll take your blueprint back. Shall I get out of your way so you can see what's going on? The most important and central thing about DNA, apart from the storage of information, is that it has the ability to copy itself. I think we've got a pool of nucleotides here who are going to come down and stand in the middle, is that right? Are you ready? And we also have Sticky here, who as well as being our gallant floor manager is now going to act as DNA polymerase and you'll see how it works. If you stand up, we've got a DNA helix, a DNA double strand waiting to be replicated. Hang on there for a minute - now remember when you come down, you're going to hold onto the person in front of you. Even when the strands separate, you're going to hold onto that shoulder in front of you. We want you to stay together and we won't take it too fast. What's going to happen is Sticky is going to take control as the polymerase and he is going to beat time as you replicate.

And now as the strands separate around Sticky, because of the base pairing, they can each pick up - hold onto the shoulder in front, keep those strands together - they can each pick up the complementary base just according to the pattern on there. That's right, keep going, that's right, good, good, good, good. And there we are, we've replicated our DNA and as they march out of the door, you see life in action! That is the process, well done, you did a perfect job.

And thank you Sticky, you were a perfect polymerase. So this is how life soldiers on. The DNA is being replicated but remember, it's not just software, it's hardware, so the new DNA goes into new cells, the cell can divide and start to make another part of the organism. And so your application of DNA is the heart of life and this is how the code works to build us. In a moment, we'll see how we go further from that.