Credit: AFP
It didn’t just change human biology. DNA is DNA: the same for E coli as for an elephant.
So the DNA sequencing machines that now read human genomes for a few thousand dollars in the space of a week (it took the original effort 14 years and US$3 billion dollars) are also decrypting the genomes of everything from water fleas to wheat. The changes that have gushed in are profound and not what we expected.
How many genes to make a human?
For starters we’ve had a huge lesson in humility. Early bets for the number of genes to make a human had it at around 300,000.
Surely we had to have 100 times more genes than the single-celled bacterium Haemophilus influenzae with its set of 2000 or so. In 1995 it was the first creature to have its genome read as a test run for the human tome.
The human genome sequence may have been published in 2001 but it was to take a couple more years till computer hacks could find all the genes buried in the vast stretches of so-called ‘junk DNA’.
Dismayingly the estimates kept plummeting. Finally we sounded out at 23,000 genes, a mere 10 times more than a bacterium and a trifle more than the roundworm, Caenorhabditis elegans.
The key to being complex is the junk
Another surprise has been discovering that we were wrong about the junk. Since the 1970s we’ve known that most of our DNA cannot be decrypted into what we traditionally recognize as genes: recipes for the proteins which do all the work of building cells and running life’s chemistry. In fact 98.5% of our three billion letters of DNA falls into the category of ‘junk’.
Now the cheap sequencing machines are revealing that junk DNA actually produces small strings of RNA – mirror image copies of bits of the DNA code. And that RNA is doing things that we never dreamed of.
Like computer software, it controls what the protein recipes are doing. It protects against the predations of RNA viruses and it keeps anarchistic jumping genes under control.
It may also explain how different genes are turned on and off in response to environmental signals – an exploding new field that goes by the name of epigenetics. For John Mattick (see "The trouble with genes."), this RNA made off the junk DNA template is the key to explaining why we are more complex than worms despite having around the same number and types of genes.
