The Vital Question: Why is life the way it is?

by Nick Lane

The trouble is that spliceosomes are slow. Even today, after nearly 2 billion years of evolutionary refinement, they take several minutes to cut out a single intron. In contrast, ribosomes work at a furious pace – up to 10 amino acids per second. It takes barely half a minute to make a standard bacterial protein, about 250 amino acids in length. Even if the spliceosome could gain access to RNA (which is not easy as RNA is often encrusted in multiple ribosomes) it could not stop the formation of a large number of useless proteins, with their introns incorporated intact. How could an error catastrophe be averted? Simply by inserting a barrier in the way, according to Martin and Koonin. The nuclear membrane is a barrier separating transcription from translation – inside the nucleus, genes are transcribed into RNA codescripts; outside the nucleus, the RNAs are translated into proteins on the ribosomes. Crucially, the slow process of splicing takes place inside the nucleus, before the ribosomes can get anywhere near the RNA. That is the whole point of the nucleus: to keep ribosomes at bay. This explains why eukaryotes need a nucleus but prokaryotes don’t – prokaryotes don’t have an intron problem.