Most major biological molecules, including all proteins, DNA and RNA, point in one direction or another. In other words, they are chiral, or handed. Like how your left glove fits only your left hand and your right glove your right hand, chiral molecules can interact only with other molecules of compatible handedness.
Two chiralities are possible: left and right, formally called L for the Latin laevus and D for dexter. All life on Earth uses L proteins and D sugars. Even Archaea, a large group of microorganisms with unusual chemical compositions, stick to the program on the handedness of the main molecules they use.
