Chemists are close to showing that DNA could form spontaneously from chemicals which were present during primordial Earth. If they succeed, the work could imply that DNA could predate the birth of life as we know it.
The scientists published their findings in the Journal of the American Chemical Society. DNA is essential to all life on Earth, yet most biologists think that life began with RNA, not DNA. DNA stores genetic information, but RNA can fold itself into complex shapes that can clamp onto other molecules and speed up chemical reactions. RNA is structurally simpler than DNA, so it’s easier to make.
After decades of trying, researchers were finally able to generate RNA in 2009 using the chemicals that probably existed on early Earth. Matthew Powner, at University College London, and his colleagues synthesized two of the four nucleotides that make up RNA. This suggests that RNA might have formed spontaneously. His latest work includes trying to make DNA nucleotides through similar methods.
Nucleotides are made up a sugar attached to a phosphate and a nitrogen-containing base molecule. These bases form 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 and is tougher to work with.
The scientists started with a mix of chemicals, which were thought to have been present on early Earth. Powner has created a sugar like in DNA, linked to a molecule called AICA, which is similar to a base.
Now, Powner needs to turn AICA into a base and add the phosphate. His molecule also has an unwanted sulfur atom, which helped the reactions along but now must be removed. Powner states that a DNA nucleotide is only a few years away.
Prebiotic chemists have ignored DNA because of its inherent complexity, making it unlikely that it occurred spontaneously in nature. It also makes sense that RNA-based life eventually switched to DNA because it is better at storing information. If this was the case, it would be interesting to find out why and how did life make this switch.
Modern organisms can convert RNA nucleotides into DNA nucleotides using special enzymes that are costly to produce in terms of cellular energy and base materials. It makes more sense that DNA nucleotides were 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.
Early organisms must have scavenged for materials this way, trying to find DNA in their environment. Powner thinks that life might have begun with RNA and DNA, in which the two types of nucleotides were intermingled. Molecules containing a mix of DNA and RNA nucleotides can perform the functions of pure RNA. Powner thinks that life started out using these hybrid molecules before purifying them into DNA and RNA.
Reference: “Multicomponent Assembly of Proposed DNA Precursors in Water” by Matthew W. Powner, Shao-Liang Zheng and Jack W. Szostak, 27 July 2012, Journal of the American Chemical Society.