Aug 26

Aug 26 The future of DNA - Meet XNA

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Now it’s time to jump back in our time machine and come back to the present day, or even into the future…

For a long time, scientists thought that the genetic code was completely universal, with the same three letter DNA or RNA ‘words’ encoding the same amino acid building blocks of proteins. But recent research has shown there are rare deviations from this rule, known as non-canonical codes.

In one example reported in 2006, scientists at the University of Vigo used a computational method to search for deviations from the genetic code and found that arthropods use the code AGG for the amino acid lysine instead of serine, unlike the rest of life on earth.

Small deviations like this bring yet more questions, like whether the genetic code still evolving with a new, improved code poised to emerge?

Perhaps too impatient to wait millions or billions of years for evolution (and who can blame them?), some scientists have even begun to explore if we can improve upon nature’s solution and create alternative genetic codes.

The emerging field of xenobiology is a subfield of synthetic biology that involves using chemical substances not usually found in nature, for example, alternative DNA known as XNA, short for Xeno Nucleic Acid. Some experts in the field are creating XNA with completely new nucleotide bases beyond the standard A, C, T and G - in other words, they are adding new letters to the genetic alphabet.

You might wonder why you would want a genetic code with more letters. But an expanded code brings expanded possibilities for life... And us.

New genetic letters can encode more, different amino acids. With more amino acids come potentially new and exciting biological molecules and functions, which could be game-changers in the search for new drugs, particularly biopharmaceuticals, which are currently limited by the chemical properties of naturally occurring biological molecules.

But making new genetic letters is not easy because even small changes to the chemical structures of the naturally occurring nucleotides can throw off the double-helical structure of DNA.

Initially, scientists added two additional letters to DNA, but the latest iteration of XNA uses four synthetic nucleotides (B, P, S and Z) in addition to the Big Four of AGC and T. It’s called Hachimoji DNA, after the Japanese word meaning ‘eight letters’.

Researchers have even been able to adapt an enzyme to transcribe the Hachimoji DNA into Hachimoji RNA, suggesting that this novel system will be capable of sustaining life and could be used to produce novel proteins with new amino acids. However, they haven’t created a functioning Hachimoji gene just yet.

Before you start picturing new synthetic life forms escaping from laboratories, life based on Hachimoji DNA requires a ready supply of special B, P, S and Z nucleotides that are currently only found in the pages of chemical company catalogues rather than out in the world at large. So if they do escape, they will quickly die. Phew!

Although currently not much more than a laboratory curiosity, Hachimoji DNA demonstrates that nature’s nucleotides are not unique in their ability to store genetic information. In turn, this raises questions about whether DNA-based life on other planets might use a different genetic code entirely or whether a new and improved genetic code might evolve right here on earth in the next few billion years. Watch this space…

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