According to the RNA World Hypothesis, life initiated with RNA molecules that evolved to replicate themselves. Recent discoveries reveal an RNA molecule capable of this self-replication, executing essential processes, though not simultaneously.

“It’s been a long quest to reach a point where we confidently state RNA can replicate itself under the right conditions, showcasing its potential,” says Philip Holliger at the MRC Molecular Biology Laboratory, Cambridge, UK.

In living organisms, proteins are pivotal, catalyzing chemical reactions while their synthesis instructions are encoded in double-stranded DNA. RNA, existing typically as a single strand, serves as a chemical analog of DNA.

While RNA is not as reliable for information storage due to its instability, it exhibits a unique capability: folding into protein-like enzymes that catalyze chemical reactions. This dual function of RNA as both storage and catalyst led to the hypothesis in the 1960s that the genesis of life may have hinged on self-catalyzing RNA molecules.

However, identifying such self-replicating molecules has proved exceptionally challenging. It was previously assumed that self-replicating RNA would be relatively large and complex, yet large RNAs are cumbersome to spread and duplicate.

Furthermore, while shorter RNA molecules have been known to form spontaneously under suitable conditions, the likelihood of larger molecules doing the same remains low.

“This insight led us to reconsider; perhaps something simpler and smaller could efficiently complete this process,” Holliger explains. “That search yielded QT45.”

RNA comprises nucleotide building blocks. The research team initiated the process by generating 1 trillion random sequences, each 20, 30, or 40 nucleotides long. They selected three capable of binding nucleotides and combined them for several rounds of evolution, introducing random mutations to enhance performance.

The resultant molecule, QT45, is composed of just 45 nucleotides. In alkaline, near-freezing water, single-stranded RNA can serve as a template to join short strands of two or three nucleotides, creating complementary strands, including those that mirror itself. “Although the process is currently slow with low yields, this is expected,” notes Holliger.

QT45 can also replicate itself using its complementary strands. “This is the first instance of RNA that can generate itself and its coding strand, representing the two core reactions of self-replication,” states Holliger. However, the team has yet to achieve both reactions occurring within the same container. Future efforts will focus on further evolving the molecule and experimenting with conditions like freeze-thaw cycles to see if simultaneous reactions are possible.

“The most fascinating aspect is that once the system begins self-replication, it also starts self-optimization,” Holliger adds, as the error-prone process generates various variants, some potentially more effective at replication.

“The findings from the Holliger lab represent a vital step toward fully self-replicating RNA.” asserts Sabine Muller from the University of Greifswald, Germany.

“A key takeaway from this discovery is the identification of intermediate-sized RNA oligomers capable of self-synthesizing,” remarks Zachary Adam at the University of Wisconsin-Madison.

The vast number of possible 45-nucleotide-long RNA sequences is “inconceivably large,” Adam notes, making the team’s discovery of QT45 from an initial batch of 1 trillion sequences mind-boggling.

In early Earth’s environment, a molecule akin to QT45 might have successfully replicated itself amidst conditions similar to those in modern-day Iceland, combining ice with hydrothermal activity that creates freeze-thaw cycles and pH gradients. Holliger believes compartmentalization is essential to segregate key components, with numerous possibilities for this occurrence, from pockets of meltwater in ice to cellular vesicles spontaneously formed from fatty acids.