A SYNTHETIC molecule that performs an essential function of life 鈥� self-replication 鈥� could shed light on the origin of all living things. The lab-born strand of ribonucleic acid (RNA) can evolve in a test tube to double itself ever more swiftly.
This is the first time that an experiment has produced an RNA that can sustain its own replication.
, a biochemist at the Scripps Research Institute in La Jolla, California, and colleague Tracey Lincoln studied RNA because most researchers think early life stored information in this sister molecule to DNA. One of the reasons for this is that, unlike DNA, RNA molecules can catalyse chemical reactions. 鈥淲e鈥檙e trying to jump in at the last signpost we have back there in the early history of life,鈥� Joyce says.
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Joyce and Lincoln created their RNA enzyme, or ribozyme, called R3C, from scratch to perform a single function: stitching two shorter RNAs together to create a clone of itself.
Next, Lincoln redesigned R3C, making a sister RNA that could itself join two RNAs into a ribozyme. Rather than replicating themselves, each molecule was able to make a copy of its sister, a process called cross replication. The pairs double until there are no more starting bits of RNA left. 鈥淲e just let them amplify themselves silly,鈥� says Joyce.
The team then sought to 鈥渆volve鈥� their molecule. They added different versions of R3C ribozyme pairs to test tubes containing a wider range of RNA building blocks. Giving the ribozyme pairs an array of components meant they could concoct new ribozymes. Since each enzyme has slightly different properties, these new forms might be better or worse at replicating their sister ribozyme.
What came out bore an eerie resemblance to natural selection : a few sequences proved to be winners, but most lost out (Science, ). The victors emerged because they could replicate faster in the face of competition, Joyce says.
鈥淎 few RNA sequences proved to be winners. These were the ones that could replicate faster in the face of competition鈥�
鈥淚 wouldn鈥檛 call these molecules alive,鈥� he cautions, since life is not simply the ability to replicate like crazy. The molecules must also gain new functions without laboratory tinkering 鈥� something that Joyce says he has no idea how to create yet.
A life-mimicking molecule will also need to assemble itself from simpler components than two halves, says , a biochemist at the University of California, Santa Cruz.
DNA and RNA normally replicate with the help of a protein enzyme that joins individual nucleotide 鈥渓etters鈥� together. Early life may have had an enzyme that did the same, or this enzyme could have joined short stretches of RNA, Robertson says.
The true story of the origin of life will always remain elusive, however. 鈥淸It] is a historical problem that we鈥檙e never going to be able to verify,鈥� Robertson says.