DNA’s molecular midwives

There are lots of unanswered questions about the how the first genetic polymers formed on the prebiotic .The researchers investigated how small nucleotide fragments might have joined together in long polymers - forming the first DNA or RNA strands.

A significant problem with examining how DNA first formed is that small chains of nucleotides bond with themselves. Therefore they favour intramolecular bonding, and tend to curl up and cyclise, forming small rings rather than a long chain.

The researchers found that the presence of certain compounds can stabilise a growing strand of DNA. The compounds are known as intercalators - large flat molecules that can slide between the bases in DNA holding the structure in place with non-covalent interactions. The compounds have been termed midwife molecules by the team, since once the polymer is formed they are no longer needed. 

Ethidium and Coralyne are both examples of midwife molecules, they are large, multiringed organic compounds, but it is unlikely that either of these was the very first midwife. According to the team, there might be been a planar midwife molecule involved in the very beginnings of life on Earth, but it remains unidentified, and may not even still be around.

One fascinating prospect is that the first midwife molecule was responsible for selecting the base-pairing combinations of adenine-thymine (A-T) and cytosine-guanine (C-G) that make up DNA. While ethidium was found to stabilise the traditional Watson-Crick pairing, the azacyanine aza3 was found to stabilise an alternative system where adenine binds to adenine (A-A). 

This study illustrates adequately that a combination of nucleic acids and structurally unrelated molecules is more likely to produce a self-replicating system - and thus life - than nucleic acids alone.