The D arm is a feature in the tertiary structure of transfer RNA (tRNA). It is composed of the two D stems (four base pairs each; 10–13 and 22–25) and the D loop. The D loop contains the base dihydrouracil. The D loop's main function is that of recognition. It is widely believed that it will act as a recognition site for aminoacyl-tRNA synthetase, which is an enzyme involved in the aminoacylation of the tRNA molecule [1]. The D stem is also believed to have a recognition role although this is yet to be proved.
It is a highly variable region and is notable for its unusual conformation due to the over-crowding on one of the guanosine residues. It appears to play a large role in the stabilization of the tRNA's tertiary structure.