Molecule that could explain life on Earth
It is easy to forget how improbable you are. Four billion years ago, Earth was a barren planet. Its oceans contained simple carbon-based chemicals, but nothing that could grow or adapt.
Then chemistry performed a remarkable trick: a molecule began to make copies of itself. The copies were occasionally imperfect and in those imperfections lay the possibility of change.
From that came evolution and everything alive today. For decades scientists have suspected that this ancestral molecule was a strand of ribonucleic acid (RNA). Now, in a Cambridge laboratory, researchers have provided perhaps the clearest glimpse yet of what the first spark of life looked like.
Unlike DNA, which stores genetic information, or proteins, RNA can do the work of DNA and proteins. It can store information in its sequence and fold into shapes that carry out chemical reactions. In theory, life may have begun when a single RNA molecule acquired the ability to replicate.
The idea has proved difficult to demonstrate. The RNA molecules known to copy other RNA strands have been large and intricate, consisting of 150 to 300 building blocks, or nucleotides. That poses a problem. Molecules that large are hard to copy, even in laboratories.
Now a team led by Philipp Holliger at the MRC Laboratory of Molecular Biology have identified a smaller candidate: an RNA enzyme of 45 nucleotides — “Quite Tiny 45”, or QT45. A molecule that small narrows the gap between chemistry and biology in a way that origin-of-life scientists have been chasing.
“It is the first RNA molecule that can, in a true sense, make copies of itself,” said Holliger. “It was critical to demonstrate this because this is really the cornerstone of what had become known as the RNA world hypothesis, the idea that life really starts when RNA begins to make copies of itself.”
QT45 works best in mildly alkaline, icy conditions, which was probable on early Earth. As water freezes, channels of chemical-rich brine form between ice crystals, where QT45 thrives, acting like a molecular welder. It scavenges scraps of RNA and pieces them together to build a new RNA strand. It can carry out both steps needed for selfcopying. First, it can make a mirror image of its nucleotide sequence. Using it as a template, it can assemble a copy.
Reactions are slow and inefficient, but the loop is there. It is also chemically flexible. This is crucial because early Earth was home to a primordial soup. For life to start, a molecule needed to be able to grab whatever was available.
The implications are far-reaching. Was life on Earth a fluke or an outcome likely to occur elsewhere? If the step from molecules to self-copying systems can be taken by something as small as QT45, the transition may not be quite as rare as once imagined.
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