The Science of Discworld Revised Edition

now-discredited ‘vitalist’ theory – but because their matter is organized in an exceedingly intricate fashion. DNA does a lot of the routine ‘bookkeeping’ that
keeps
living creatures organized. Every cell of (nearly) every living organism contains its genome – a kind of code message written in DNA, which gives that organism a lot of hints about how to behave at the molecular level. (Exceptions are various viruses, on the boundary between life and non-life, which use a slightly different code.)
    This is why it was possible to clone Dolly the Sheep – to take an ordinary cell from an adult sheep and make it grow into another sheep. The trick actually requires
three
adult sheep. First, there’s the one from which you take the cell: call her ‘Dolly’s Mum’. Then you persuade the cell’s nucleus to forget that it came from an adult and to think that it’s back in the egg, and then you implant it into an egg from a second sheep (‘Egg Donor’). Then you put the egg into the uterus of the third sheep (‘Surrogate Mum’) so that it can grow into a normal lamb.
    Dolly is often said to have been a perfect copy of Dolly’s Mum, but that’s not completely true. For a start, certain parts of Dolly’s DNA came not from Dolly’s Mum, but from Egg Donor. And even if that slight difference had been fixed, Dolly could still differ in many ways from her ‘mother’, because sheep DNA is
not
a complete list of instructions for ‘how to build a sheep’. DNA is more like a recipe – and it assumes you already know how to set up your kitchen. So the recipe doesn’t say ‘put the mixture in a greased pan and place in an oven set to 400°F,’ for instance: it says ‘put the mixture in the oven’ and
assumes
that you know it needs to go in a pan and that the oven should be set to a standard temperature. In particular, sheep DNA leaves out the vital instruction ‘put the mixture inside a sheep’, but that’s the only place (as yet) where you can turn a fertilized sheep egg into a lamb. So even Surrogate Mum played a considerable role in determining what happened when the DNA recipe for Dolly was ‘obeyed’.
    Many biologists think that this is a minor objection – after all, Egg Donor and Surrogate Mum work the way they do because
their
DNA contains the information that makes them do it. But things that aren’t in
any
organism’s DNA may be essential for the reproductive cycle. A good example occurs in yeast, a plant that can turn sugar into alcohol and give off carbon dioxide. The entire DNA code for one species of yeast is now known. Thousands of experimentalists have played genetic games with yeast, then spun the beasties in a centrifuge to separate the DNA, from which they can work out the code. When you do this, you leave a scummy residue in the bottom of the test tube, but since it’s not DNA, you know it can’t be important for genetics, and you throw it away. And so they all did, until in 1997 one geneticist asked a stupid question. If it’s not DNA, what’s it
for
? What’s in that scummy residue, anyway?
    The answer was simple, and baffling. Prions. Lots and lots of them .
    A prion is a smallish protein molecule that can act as a catalyst for the formation of more protein molecules just like itself. Unlike DNA, it doesn’t do this by replication. Instead, it needs a supply of proteins that are
almost
like itself, but not quite – the right atoms, in the right order, but folded into the wrong shape. The prion attaches itself to such a protein, jiggles it around a bit, and nudges it into the same shape as the prion. So now you’ve got
more
prions, and the process speeds up.
    Prions are molecular preachers: they make more of themselves by converting the heathen, not by splitting into identical twins. The most notorious prion is the one that is believed to be the cause of BSE, ‘mad cow disease’. The protein that gets converted happens to be a key component of the cow’s brain, which is why infected cows lose coordination, stagger around, foam at the mouth, and look crazy. What does yeast want prions for? Without prions, yeast can’t reproduce. The protein-making instructions in its DNA sometimes make a protein that is folded into the wrong shape. When a yeast cell divides, it copies its DNA to each half, but it shares the prions (which can be topped up by converting other proteins). So here’s a case where, even on the molecular level, an organism’s DNA does
not
specify