For some time now, scientists have thought that RNA first arose naturally and then DNA developed from the RNA. Matthew Powner, one of those involved in the research into how RNA might have arisen spontaneously, now thinks that DNA might have arisen on its own without RNA.
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The focus on RNA is understandable because DNA is a lot more complex than RNA. Logically, it makes sense that RNA would develop first, with RNA-based organisms doing something to cause DNA to develop. At the same time, though, a world with only RNA is difficult to comprehend because it would be difficult for any organism to survive.
Conventional wisdom is that RNA-based life eventually switched to DNA because DNA is better at storing information. In other words, RNA organisms made the first DNA.
If that is true, how did life make the switch? Modern organisms can convert RNA nucleotides into DNA nucleotides, but only using special enzymes that are costly to produce in terms of energy and materials. "You have to know that DNA does something good for you before you invent something like that," Switzer says.
He says the story makes more sense if DNA nucleotides were naturally present in the environment. Organisms could have taken up and used them, later developing the tools to make their own DNA once it became clear how advantageous the molecule was - and once natural supplies began to run low.
Early organisms must have scavenged for materials in this way, says Matthew Levy of the Albert Einstein College of Medicine in New York City. "The early Earth was probably a bloody mess," he says, with all manner of rich pickings on offer.
Powner suggests another alternative. Life may have begun with an "RNA and DNA world", in which the two types of nucleotides were intermingled. Powner's co-author Jack Szostak, of the Harvard Medical School, has shown that "mongrel" molecules containing a mix of DNA and RNA nucleotides can perform some of the functions of pure RNA (Proceedings of the National Academy of Sciences, doi.org/bj8r97). Powner suggests that life started out using these hybrid molecules, gradually purifying them into DNA and RNA.
Benner says it makes more sense for the first life to have used pure DNA and RNA as early as possible. Both work better than the mongrel molecules.
Source: New Scientist, August 25, 2012
If it turns out that DNA can arise naturally without requiring RNA, or at least without requiring a world dominated by RNA, that will have significant implications for the origins and development of life. We are, however, a long way off before knowing whether or not this is really possible. I don't know if we'll see any definitive breakthroughs in our lifetime, but we can hope.