Try shaking a colleague’s left hand with your right hand. It just doesn’t work, does it? Your right palm and her or his left palm cannot mesh comfortably because hands are chiral objects, having non-superimposable mirror images.
All objects have a mirror image (with the exception of vampires), but only objects that are not superimposable on their mirror image are chiral. So when we say that an object and its mirror image are superimposable, we mean that if we were to bring the image from behind the mirror, it could be made to coincide exactly with the object.
So a three-dimensional object is either chiral, or it is not.
Life’s Building Blocks Are Chiral
Molecules are tiny, three-dimensional objects too, and many of them are chiral. Louis Pasteur discovered this in 1848. A chiral molecule and its mirror image are called a pair of enantiomers. In the non-living universe, enantiomers of chiral molecules are expected to occur in equal parts, called racemic mixtures.
Some of the most important molecules of life, such as the nucleotides that make up the polymeric nucleic acids DNA and RNA, exist in principle as pairs of enantiomers known as D and L, or “left-handed” and “right-handed” forms.
A fact that has puzzled scientists for generations is that living organisms contain only D nucleotides! In other words, life is homochiral.
In itself, the homochirality of life is unremarkable. Scientists have shown in the lab that heterochiral DNA and RNA cannot function, or even form. But the big questions is: why is life as we know it D rather than L?