Tuesday, 17 September 2013

A Brilliant New Update For The RNA World Hypothesis

Richard Feynman once said that forming life from organic molecules in the primordial soup would be like a hurricane passing through a scrap yard and building a jet aircraft in the process; in essence next to impossible. As a result, scientists have had to work hard to find ways of creating biological blocks from simple molecules to assemble into cell components. It has taken a long time, but we now have a plausible explanation as to how life could have formed.

The Miller Urey experiment sparked a new
wave of research into the origin of life
It begins with an experiment in 1953: the Miller Urey Experiment which demonstrated that from very simple gases, such as ammonia, water and carbon dioxide, amino acids (the building blocks of proteins) could be formed. Since then we have found ways of generating increasingly complex chemical systems all of which may have led to the first cells. Arguably, the most well-known of these is the RNA World Hypothesis: in place of the incredibly intricate DNA-centred world inside cells, the RNA world evolved first.

In this scenario, genetic information was exchanged using RNA driven by cyclical reactions which generated energy and organic molecules before giving rise to a more complex DNA- based system. It remains one of our best explanations for building cells.

A central tenet of the RNA World Hypothesis is that it used ribozymes, stretches of RNA, which catalyse the reactions needed to form proteins. Essentially they act as engineer and architect, able to encode biological information and then build it; important processes for building cells. Yet for the hypothesis to be correct, ancient RNA catalysts would have had to copy multiple sets of RNA blueprints as accurately as do modern-day enzymes.

It has been calculated that it would take much longer than the age of the universe for randomly generated RNA molecules to evolve sufficiently to achieve the modern level of sophistication. Given Earth's age of 4.5 billion years, living systems run entirely by RNA could not have reproduced and evolved either fast or accurately enough to give rise to the vast biological complexity on Earth today. What is more, we have never found any evidence of ancient ribozymes.

The molecular structure of the 'Urzyme'
A team of biochemists, however, led by Dr Charles Carter from the University of North Carolina have found an alternative solution to the RNA World Hypothesis.

Our genetic code is transcribed by two families of enzymes. By superimposing 3D molecular images of different enzymes from each group, Carter and his team identified a central core of amino acids common to all. Upon extracting these cores, the team found something remarkable.

It turned out that like ribozymes these enzymes, which Carter named 'Urzymes' (Ur meaning first), have the ability to catalyse reactions involving the creation of proteins, greatly accelerating the rate of construction. What is more, they are far simper than the modern day enzymes used in transcribing genetic information into proteins, making them a more likely candidate as the starter motor for the RNA World than the more complex ribozymes.

'Our results suggest that there were very active protein enzymes very early in the generation of life, before there were organisms,' said Carter, 'and those enzymes were very much like the Urzymes we've made.' The team's findings suggest that Urzymes may have evolved from simple peptide precursors and then co-evolved with RNA to produce the enzymes used for DNA transcription in what Carter has dubbed the 'Peptide RNA World Hypothesis'.

'To think that these two Urzymes might have launched protein synthesis before there was life on Earth is totally electrifying,' said Carter. 'I can't imagine a much more exciting result to be working on, if one is interested in the origin of life.' The trick with finding ways of building cells is finding as many steps as possible, each simpler than the last. The Peptide RNA World Hypothesis is a much needed addition, as vital as the Miller Urey Experiment itself.