Scientists are close to demonstrating that the building blocks of DNA can form spontaneously from chemicals thought to be present on the early Earth. If they succeed, their research implies that DNA could have predated the birth of life. “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,” Christopher Switzer of the University of California, Riverside said.
In 2009 researchers finally managed to generate RNA (image below) using chemicals that they believe existed on the early Earth. Matthew Powner, at University College London, and his colleagues synthesised two of the four nucleotides that make up RNA. Their achievement suggested that RNA may have formed spontaneously - powerful support for the idea that life began in an "RNA world".
Powner's most recent work suggests that DNA might have come first, attempting to create DNA nucleotides through similar methods to those he used to make RNA nucleotides in 2009. Prebiotic chemists have largely ignored DNA, because its complexity suggests it cannot possibly form spontaneously.
"Everybody and his brother has been saying 'RNA, RNA, RNA'," says Steven Benner of the Foundation for Applied Molecular Evolution in Gainesville, Florida in an interview with New Scientist.
Nucleotides consist of a sugar attached to a phosphate and a nitrogen-containing base molecule - these bases are the familiar letters of the genetic code. DNA nucleotides, which link together to form DNA, are harder to make than RNA nucleotides, because DNA uses a different sugar that is tougher to work with.
Starting with a mix of chemicals, many of them thought to have been present on the early Earth, Powner has now created a sugar like that in DNA, linked to a molecule called AICA, which is similar to a base (Journal of the American Chemical Society, doi.org/h6q).
There is plenty still to do. Powner needs to turn AICA into a base, and add the phosphate. His molecule also has an unwanted sulphur atom, which helped the reactions along but now must be removed. Nevertheless, a DNA nucleotide is just a few years away, says Christopher Switzer of the University of California, Riverside. "It's practically a fait accompli at this point.
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. The story makes more sense, Switzer adds, 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.*Powner concludes that 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 countered saying 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. But unless there's a major breakthrough in the science, there's nothing to tell us exactly how and when life first used DNA.
The Daily Galaxy via New Scientist