1). (General) The essential ingredient, the foundation of a construction, bottom of a plant or any part of a structure and the part of an appendage nearest the body in arthropods. 2). Chemical group or substance having the property of accepting a proton (H+) from water to release OH-. 3). In DNA and RNA, the general term given to adenine (A), cytosine (C), guanine (G), uracil (U), and thymine (T), the five N-containing compounds released from nucleic acids by vigorous hydrolysis.