Mirror image

It is a question that has fascinated science fiction authors for decades: how would you react if you came face to face with an exact copy of yourself? Now, in a paper just published, scientists have announced they are a step closer to answering it.

At the heart of this breakthrough is chirality. An image is chiral when it cannot be exactly mapped onto its mirror image. Human hands are one of the most visible examples of chirality, which is why chiral objects are referred to as "left-handed" and "right-handed".

All of the basic building blocks of life - amino acids, RNA and DNA - are chiral. But we do not make use of both "hands". In living beings, almost all amino acids appear in their left-handed form, while RNA and DNA take their right-handed form.

Now scientists at Zhu Lab in Beijing have announced that for the first time, they have managed to synthesise a left-handed DNA polymerase - an enzyme that creates new DNA.

This is an extraordinary achievement. DNA is unimaginably complex, because to make enzymes it has to be folded in a very specific way by other proteins. If this folding process goes wrong, the enzyme will break down. If it goes very wrong, it can turn into an enzyme that folds other enzymes incorrectly and cause severe medical problems.

It opens the door to creating a mirror image - or, more accurately, a chiral image - of any living thing, by synthesising each amino acid, DNA and RNA strand in its opposite chiral form. Some scientists have dubbed this "mirror life".

Ordinary molecules would not be able to interact with their mirror counterparts, including the enzymes that bacteria use to digest organic matter. So in theory, this could allow us to produce matter that cannot biodegrade.

This would have huge advantages for science. DNA can be used for information storage: in 2019, all English-language Wikipedia content was encoded on synthetic DNA. But while right-handed DNA breaks down quite easily, the new left-handed DNA will not interact with molecules around it, so the data will last for longer.

This discovery could also be quite dangerous. Left-handed and right-handed molecules can have very different effects. 

Sometimes these are benign: limonene, the substance that gives citrus fruits their smell and taste, tastes of lemons in its right-handed form and oranges in its left-handed form. But they can also have devastating unintended consequences. In its left-handed form, the drug thalidomide can be used to treat morning sickness; its right-handed form causes birth defects.

So if we start to produce too much mirror life, it could have unexpected results. Some bacteria are capable of digesting left-handed glucose, so creating a lot of it would give them a big advantage over bacteria that cannot. Those bacteria could then multiply and outcompete the bacteria that human beings and other life depend on. In other words, mirror life could end life on Earth as we know it.

Are scientists playing with fire?

Yes, say some. This kind of science could have any number of unforeseen consequences. One study on chirality has already been suspended because it created an enzyme that might have given someone brain damage. The ecosystem is so complex that the slightest change to it could bring everything tumbling down. In creating mirror life, we might end up destroying the original.

Not at all, say others. Mirror life could have any number of applications, from synthesising medicines, to mass-producing food, to prolonging human life. Given these benefits, it would be irresponsible not to do this kind of research. As long as we proceed with caution and take pains to eliminate the adverse effects, there is no reason to fear experiments with mirror life.