text: Many of the essential molecules for life on Earth can exist in two mirror-image forms, referred to as "left-handed" and "right-handed" where handed refers to direction in which polarized light skews when beamed through a pure solution of the molecule, but living organisms do not use both. Proteins are exclusively composed of left-handed amino acids; RNA and DNA contain only right-handed sugars. This phenomenon is known as homochirality. It is not known whether homochirality emerged before or after life, whether the building blocks of life must have this particular chirality, or indeed whether life needs to be homochiral. Protein chains built from amino acids of mixed chirality tend not to fold or function as catalysts, but mirror-image proteins have been constructed that work the same but on substrates of opposite handedness.