Messenger RNA is crucial to the working of the genome (Image: Laguna Design / SPL)

ONCE, it all seemed so beautifully simple. Our DNA, we thought, consisted of a set of recipes, or genes, for making proteins, and once we had identified them all and worked out what they do, we would be a long way towards understanding what makes us what we are.

If only. One of the big shocks that emerged from the human genome project was that we have only around 23,500 genes – barely more than a nematode worm. But in many other ways our genome is turning out to be dizzyingly complex (see diagram).

“It is very difficult to wrap your head around how big the genome is and how complicated,” says Ewan Birney of the European Bioinformatics Institute near Cambridge, UK, who is part of a major project to uncover the workings of the genome. “It’s very confusing and intimidating.”

For starters, rather than each gene coding for one protein, they often code for many. The coding parts of genes come in pieces, like beads on a string, and by splicing out different beads, or exons, after RNA copies are made, a single gene can potentially code for tens of thousands of different proteins, although the average is about five. Recent studies suggest up to 95 per cent of our genes may be alternatively spliced in this way. Even more astonishingly, in at least one case in humans, RNA copies of different genes are spliced together. If this is commonplace, it would vastly multiply the potential number of different proteins.

Another recent discovery …