The D arm is a feature in the tertiary structure

Tertiary structure

In biochemistry and molecular biology, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...

 of transfer RNA

Transfer RNA

Transfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...

 (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

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 involved in the aminoacylation of the tRNA molecule . 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

Guanosine

Guanosine is a purine nucleoside comprising guanine attached to a ribose ring via a β-N9-glycosidic bond. Guanosine can be phosphorylated to become guanosine monophosphate , cyclic guanosine monophosphate , guanosine diphosphate , and guanosine triphosphate...

residues. It appears to play a large role in the stabilization of the tRNA's tertiary structure.