How life evolved from a mix of chemicals on the young planet Earth is one of science’s most enduring mysteries, which biochemists are attempting to solve by recreating the earliest building blocks of life in the laboratory.
Earth’s biology is based on DNA, which carries all an organism’s genetic information in a molecule that takes the shape of a spiraling ladder. RNA, the molecule that facilitates protein manufacturing, has a simpler shape–it’s a single strand, as opposed to DNA’s double strand–leading some biologists to propose the RNA world hypothesis in which RNA evolved first and eventually gave rise to DNA. But trying to imagine the assembly of RNA from its chemical components poses its own problems. How could RNA, which encodes proteins, first form, when proteins are needed for [its] synthesis? Now, scientists report that they’ve cooked up molecular hybrids of proteins and nucleic acids that skirt the dreaded paradox [ScienceNOW Daily News].
The hybrids they created could resemble the precursors to RNA, researchers report in Science. “It’s the pre-RNA world. There’s a hypothesis that says RNA is so complicated, it couldn’t have arisen de novo” — from scratch — “on early Earth,” said study co-author Luke Leman…. “So you need some more primitive genetic system that nature fiddled around with and finally decided to evolve into RNA” [Wired.com].
DNA and RNA sport a backbone of sugar and phosphate groups linked to the nucleotide bases that spell out the genetic code…. In hopes of finding something simpler, [the researchers] did away with the sugar-phosphate backbones altogether. Instead, they turned to amino acids, protein building blocks that have been shown to assemble under prebiotic conditions [ScienceNOW Daily News]. Researchers found that the nucleotide bases (which can be pictured as the rungs in a DNA ladder) automatically attached to the amino acid structure in a loose fashion, detaching and attaching themselves until stable. When mixed with single strands of DNA or RNA in water at room temperature, the [new hybrid] molecules arranged themselves in complementary strands, perhaps echoing the eventual ability of those genetic materials to duplicate themselves [Wired.com].
The next step is to see if the hybrid molecules are able to replicate themselves, says study coauthor Reza Ghadiri, but he says the study took an important stem in creating chemical sequences that formed on their own, without the presence of enzymes that wouldn’t have existed on early Earth. “We satisfy some of the requirements of the long-term goal of having a purely chemical system that is capable of undergoing Darwinian evolution” [Wired.com], he says.
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Image: Science / AAAS. The hybrid molecule links with an RNA molecule.