Monday, August 15, 2011

First life: The search for the first replicator - life - 15 August 2011 - New Scientist

First life: The search for the first replicator - life - 15 August 2011 - New Scientist

4 BILLION years before present: the surface of a newly formed planet around a medium-sized star is beginning to cool down. It's a violent place, bombarded by meteorites and riven by volcanic eruptions, with an atmosphere full of toxic gases. But almost as soon as water begins to form pools and oceans on its surface, something extraordinary happens. A molecule, or perhaps a set of molecules, capable of replicating itself arises.

This was the dawn of evolution. Once the first self-replicating entities appeared, natural selection kicked in, favouring any offspring with variations that made them better at replicating themselves. Soon the first simple cells appeared. The rest is prehistory.

Billions of years later, some of the descendants of those first cells evolved into organisms intelligent enough to wonder what their very earliest ancestor was like. What molecule started it all?

As far back as the 1960s, a few of those intelligent organisms began to suspect that the first self-replicating molecules were made of RNA, a close cousin of DNA. This idea has always had a huge problem, though - there was no known way by which RNA molecules could have formed on the primordial Earth. And if RNA molecules couldn't form spontaneously, how could self-replicating RNA molecules arise? Did some other replicator come first? If so, what was it? The answer is finally beginning to emerge.

When biologists first started to ponder how life arose, the question seemed baffling. In all organisms alive today, the hard work is done by proteins. Proteins can twist and fold into a wild diversity of shapes, so they can do just about anything, including acting as enzymes, substances that catalyse a huge range of chemical reactions. However, the information needed to make proteins is stored in DNA molecules. You can't make new proteins without DNA, and you can't make new DNA without proteins. So which came first, proteins or DNA?

The discovery in the 1960s that RNA could fold like a protein, albeit not into such complex structures, suggested an answer. If RNA could catalyse reactions as well as storing information, some RNA molecules might be capable of making more RNA molecules. And if that was the case, RNA replicators would have had no need for proteins. They could do everything themselves.

It was an appealing idea, but at the time it was complete speculation. No one had shown that RNA could catalyse reactions like protein enzymes. It was not until 1982, after decades of searching, that an RNA enzyme was finally discovered. Thomas Cech of the University of Colorado in Boulder found it in Tetrahymena thermophila, a bizarre single-celled animal with seven sexes (Science, vol 231, p 4737).

After that the floodgates opened. People discovered ever more RNA enzymes in living organisms and created new ones in their labs. RNA might be not be as good for storing information as DNA, being less stable, nor as versatile as proteins, but it was turning out to be a molecular jack of all trades. This was a huge boost to the idea that the first life consisted of RNA molecules that catalysed the production of more RNA molecules - "the RNA world", as Harvard chemist Walter Gilbert dubbed it 25 years ago (Nature, vol 319, p 618).

These RNA replicators may even have had sex. The RNA enzyme Cech discovered did not just catalyse any old reaction. It was a short section of RNA that could cut itself out of a longer chain. Reversing the reaction would add RNA to chains, meaning RNA replicators might have been able to swap bits with other RNA molecules. This ability would greatly accelerate evolution, because innovations made by separate lineages of replicators could be brought together in one lineage.

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