A friend and biomedical engineer once explained chirality to me in the context of drugs where you often strive to produce a one-handed compound in a chemical reaction because it results in a more effective drug. (I'm probably both misremembering and butchering this account, but they're called enantiomers.)
To understand mirror life, consider good old-fashioned life. The building blocks of life, like DNA and proteins, all have a property called chirality. Derived from the Greek word for “handedness,” chirality means that these fundamental biomolecules come in two varieties: with either a right-handed or left-handed orientation. DNA, for instance, is made up of a right-handed double helix of sugars, like a ladder twisted only in a certain direction. Proteins, by contrast, are made up of left-handed amino acids.
The profound consequences start at a molecular level. Right-handed amino acids seem quite similar to their left-handed counterparts. But in fact, they’re significantly harder to break down, because the enzymes in Earth’s life are built to degrade proteins with left-handed chirality.
“If you give therapeutics to a person, especially protein or nucleic acid therapeutics, digestive enzymes in the body break them down rapidly, sometimes within minutes,” Michael Kay, a biochemist at the University of Utah and a co-author of the warning in Science, says in a statement. “This can make it very challenging to treat chronic illnesses in a way that’s cost-effective and convenient.”
But this same property could also make the cells dangerous. In a 299-page technical report that accompanied the article in Science, the team highlighted how “sufficiently robust mirror bacteria could spread through the environment unchecked by natural biological controls.”