Transfer RNA (tRNA) molecules are approximately 80 nucleotides in length. Their secondary structure includes four short double-helical elements and three loops (D, anti-codon, and T loops). Further hydrogen bonds mediate the characteristic L-shaped molecular structure. Transfer RNAs have two regions of fundamental functional importance: the anti-codon, which is responsible for specific mRNA codon recognition, and the 3' end, to which the tRNA's corresponding amino acid is attached (by aminoacyl-tRNA synthetases). Transfer RNAs cope with the degeneracy of the genetic code in two manners: having more than one tRNA (with a specific anti-codon) for a particular amino acid; and 'wobble' base-pairing, i.e. permitting non-standard base-pairing at the 3rd anti-codon position.