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Life need not be based on DNA. So say researchers who have created two new versions of the iconic molecule, which retain its double helix shape but are thinner or chunkier than the original.

鈥淭his is changing the rules of the game that every schoolchild learns,鈥� says Steven Benner of the Foundation for Applied Molecular Evolution in Alachua, Florida. It implies that extraterrestrial life might be based on alternative genetic molecules.

DNA carries our genes, which tell our bodies how to grow and are passed from parent to child. The structure of DNA was by James Watson and Francis Crick 鈥� with crucial help from Rosalind Franklin.

Watson and Crick realised that DNA is made up of two long chain-like molecules twisted around each other. The two chains are attached to each other via pairs of bases, one base on each chain. There are four types of base, and they only pair up in specific ways: adenine with thymine, and guanine with cytosine. The information in our genes is contained in the order of the bases.

Fat DNA

Benner and his colleagues made several alternative DNAs, in which they swapped out some of the standard bases for various combinations of eight similar molecules. Doing so made some of the resulting DNA-like molecules physically 鈥渟kinnier鈥� than standard DNA, while others were 鈥渇atter鈥�.

Nevertheless they all performed DNA鈥檚 crucial function: if two bases paired incorrectly, the misplaced one was swiftly ejected and replaced with the correct one. This is how DNA ensures our genes don鈥檛 become garbled, and the modified DNA did it just as well. 鈥淚 was quite surprised,鈥� says Benner.

It is not the first time modified DNAs have been found to work as well as the original. Philipp Holliger of the MRC Laboratory of Molecular Biology in Cambridge, UK has created 鈥�XNA鈥� in which the bases remain the same but the chains are altered. Genes can be copied from DNA to XNA and back.

Meanwhile Floyd Romesberg at the Scripps Research Institute in La Jolla, California has built DNA with six bases: the originals plus two artificial ones. In 2014 he got all six working in bacteria.

Rule breaker

However, the new DNA breaks what was thought to be a cardinal rule, established by Watson and Crick, which these previous modifications obeyed.

In normal DNA, when two bases pair up, one is large and the other small. For example, adenine is large while its partner thymine is small. Even when researchers have created , they have stuck to this rule.
But in the skinny DNA small pairs with small, while in the fat DNA large pairs with large. Both work.

鈥淭here鈥檚 no reason you had to develop a system where you have the pairing of the small with the large bases,鈥� says team member Millie Georgiadis of Indiana University in Indianapolis. It seems there are many ways to build a gene.

鈥淚t clearly raises the question of why we ended up with DNA,鈥� says Holliger. DNA might be subtly better than the alternatives, but he suspects it is simply an accident of chemistry: 鈥渨e are built from what we are built because that鈥檚 what was available.鈥�

That means the search for extraterrestrial life needs to look for more than just DNA. 鈥淲e currently have no good chemical reason to assume that, if we ever meet aliens, their genomes will be based on DNA,鈥� says Holliger.

Indeed, in 2008 Eiji Yashima of Nagoya University in Japan made a double helix out of entirely unrelated chemicals 鈥� and .