Every living cell on the Earth today uses DNA to store its genetic information and uses proteins to catalyse its metabolic reactions. DNA contains all the information needed to manufacture proteins, and some of these proteins (enzymes) catalyse the reactions that the cell needs to live and replicate. The one cannot operate without the other; DNA cannot naturally catalyse reactions and proteins cannot manufacture themselves. And therein lies the chicken-and-egg problem: if DNA and proteins cannot operate without each other, how did the system first evolve? One obvious answer is that early life used a single molecule for both information storage and catalysis, a form of self-replicating enzyme.
Exactly what form that self-replicating enzyme might have taken was first suggested 30 years ago, when RNA was put forward as the precursor to DNA and proteins in early lifeforms. In cells today, RNA is the go-between for DNA and proteins: a protein is manufactured from an RNA template, which has itself been created from a DNA template.
The idea remained purely speculative until the early 1980s when Thomas Cech at the University of Colorado and Sydney Altman at Yale University independently discovered RNA molecules with catalytic ability, now known as ribozymes. This discovery immediately put on a much firmer footing the idea that RNA could have been used for both storing information and catalysing reactions in early forms of life, and in 1986 the term 'RNA world' was coined.
Nevertheless, despite the fact that most scientists working in this field accept the validity of the idea, the RNA world hypothesis is still far from being proved. For one thing, in almost 20 years only seven types of natural ribozymes have been discovered: two remove introns (parts of RNA that don't code for proteins) from themselves; four cut themselves in two; and one trims off the end of an RNA precursor.
So, as David Bartel and Peter Unrau, researchers in the Whitehead Institute at Massachusetts Institute of Technology (MIT), Cambridge, US, say in a recent review (Trends Cell Biol., 1999, 9(12), M9): 'Although the reactions of natural ribozymes are fascinating and impressive, they do not approach the sophistication of the key reactions assumed by the RNA world hypothesis'. This is not exactly surprising; enzymes, being superior catalysts, would have usurped RNA's catalytic role as life evolved, and thus few examples of natural ribozymes remain. So Bartel, Unrau and other researchers are having to turn to a new experimental technique to determine what types of ribozymes might have been active in the RNA world.
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