Puncturing the ego gene

The tricks cells use to generate complexity

By Jonathan Amos BBC News Online science staff

How big is yours - your genome that is?

The human species has had its ego bruised by the news that rice probably has more genes.

I liken it to a Swiss Army knife. You can do a lot of different things with one tool

Prof Gane Ka-Shu Wong

According to the research just published, each rice cell contains somewhere in the region of 40,000 to 60,000 genes, while each human cell has only about 30,000 to 40,000 genes.

But, hey, let's move the goalposts. We no longer judge the complexity of an organism by gene count alone. It's now about how an organism uses those genes.

Plants 'tinker'

"We all like to believe we as humans are better than anything else, and that we have more of everything.

"But if we pride ourselves on the number of genes in our genome, then we lose to a lowly rice plant," said Professor Gane Ka-Shu Wong, a Canadian scientist who worked on the DNA sequence of the rice subspecies indica.

"To salvage pride, we should look at protein count," he told BBC News Online.

The genes are the templates for the proteins, the sophisticated molecules that build and maintain an organism. In a complex animal like a human, a vast array of different proteins is required.

The trick is in generating that diversity. In plants, evolution has chosen duplication as the preferred route. "Plants copy their genes and tinker with them to do something new," said plant geneticist Professor Michael Bevan, from the John Innes Institute in Norwich, UK.

Bigger still

But that only gets you so far. "After a while, so many copies of genes that differ only so slightly from each other could give rise to confusion," Professor Wong said.

What you need, and what animals have got, is a system for generating a variety of different protein products from a limited number of genes. It's called alternative splicing.

Wheat: A monster genome

"I liken it to a Swiss Army knife," Professor Wong told BBC News Online. "You can do a lot of different things with one tool."

What is clear from the two DNA sequences of rice is that very little alternative splicing is going on. "This figure of 40-50,000 genes we found in rice is probably very close to the actual number of protein products. In humans - well, who knows? Some estimates put the number of protein products at well over 100,000," Professor Wong said.

Eventually, science might get around to sequencing the wheat genome. That has about 16 billion base pairs of DNA compared with the three billion found in humans. Size isn't everything.