Living in the Universe

RNA World

April 4, 2008 on 8:00 am | In Life on Earth | Comments Off

 First Week of April 2008

This story for the first week of April 2008 is not a prank. Recent lab results have shed light on an era in the Earth’s history that’s been shrouded in darkness: the time, perhaps four billion years ago, when the motor of life first turned over. There’s essentially no physical evidence that comes down to us unaltered from four billion years ago, so we have to speculate on how life started.

The origin of life is the ultimate chicken and egg problem. On the one hand there’s DNA, the information storing molecule or the genetic code, and on the other hand there are the many proteins that facilitate life’s chemical reactions. The origin of life contains this enigma: How did the complex phenomenon of a working cell get started? Historically the explanation has revolved around DNA because that’s the molecule that serves as the pattern for building proteins. Proteins in turn can form enzymes, which catalyze or facilitate biochemical reactions including the crucial construction of DNA, and thereby is the paradox. Genes require enzymes, but enzymes require genes. Which came first?

Most scientists have focused on DNA, but other life scientists have focused on a concept called “RNA World” which postulates that life began with RNA. RNA, like DNA, is built of chains of molecules called nucleotides. Our understanding of RNA has come a long way since the 1960s when what is called the central dogma of molecular biology held that RNA was just a messenger boy that carried DNA’s information to ribosomes, the cellular factories where proteins get built. In the 1980s biologists realized that not only could RNA transfer information but like proteins it could also process chemicals; it could catalyze reactions. The ability to do both jobs suggested that RNA, and not DNA, could be the primary molecule of life. Much of this work was done by Thomas Cech of the University of Colorado, who won the Nobel Prize in chemistry in 1989 for these insights.

According to the lead scientist of the study under consideration, done by NASA and funded under the Exobiology and Evolutionary Biology program, DNA stores information like a computer hard drive. Niles Lehman, professor of chemistry at Portland State, says, “Beyond that, DNA doesn’t do anything. RNA on the other hand can fold into a 3-D structure that allows it to catalyze a chemical reaction.” Even if RNA can catalyze chemical reactions, in modern cells it gets information from DNA, so how could RNA have been assembled before DNA even existed?
The recent experiments by Lehman and others may have revealed the answer. Individual units or nucleotides of RNA can spontaneously self-assemble. Lehman and his colleagues started their experiments by removing from a bacterium an RNA molecule that works as a self-replicating enzyme. They cut it into chunks, each about fifty nucleotides long, and watched the chunks reassemble themselves into a working enzyme. He said, “We mix the fragments together in salt water at forty-eight degrees, have lunch, come back, and we have self-replicating RNAs in the test tube.” Obviously reassembling an enzyme you’ve stolen from bacteria and then sliced into pieces doesn’t prove that a working enzyme could have formed in the prebiotic world, but there was a method to the apparent madness of Lehman’s experiment. Fifty bases may be a magic number. Lehman quotes chemist James Ferris of Renssalaer Polytechnic Institute who’s been able to string together forty or fifty RNA nucleotides using clay as the catalyst. It’s conceivable that that could have happened in the prebiotic world too.

Summarizing the results these experiments, RNA World begins with three steps: prebiotic synthesis of the individual RNA nucleotides, assembly of intermediate chains, and then final assembly into longer chains. Ferris and Lehman between them have demonstrated steps two and three, but Ferris notes that nobody has yet demonstrated a prebiotic synthesis for individual nucleotide basis from which he constructs the RNA strands. Still, the new results are interesting enough to suggest that RNA can achieve enough complexity to transition from the chemical to the biological realms. The idea that RNA can begin to replicate itself from fragments is very exciting because it identifies the leap in complexity required to kick-start biology.

The astrobiological implications of this work are obvious. The raw materials, the chemical ingredients for life, are known to exist everywhere in the universe, and they will be present on the surface of planets, in many cases with the liquid medium of water available to dissolve them. Once you’ve gone up the first few steps to form fifty base nucleotides, nature and natural processes take over. Life will self-assembly and a replicating molecule will emerge from the chemical mix. If it happened on Earth four billion years ago, it probably could have happened on any similar location. Removing the mystery of the formation of life of Earth will give us a much clearer sense of how often the event can occur elsewhere in the universe.