An extremely long macromolecule that is the main component of chromosomes and is the material that transfers genetic characteristics in all life forms, constructed of two nucleotide strands coiled around each other in a ladderlike arrangement with the sidepieces composed of alternating phosphate and deoxyribose units and the rungs composed of the purine and pyrimidine bases adenine, guanine, cytosine, and thymine: the genetic information of DNA is encoded in the sequence of the bases and is transcribed as the strands unwind and replicate.

The one of a pair of alternative alleles that masks the effect of the other when both are present in the same cell or organism.

Situated on or toward the upper side of the body, equivalent to the back, or posterior, in humans.

The first segment of the small intestine; the contents of the stomach and the pancreatic and bile ducts empty into it. Site of digestion and some absorption.