A type of RNA that brings the amino acids to the ribosomes to make proteins. There are 20 kinds of transfer RNA molecules, one for each of the 20 main amino acids. A transfer RNA molecule has an amino acid attached to it, and contains the anticodon corresponding to that amino acid in another part of its structure. In protein synthesis, each codon in the messenger RNA combines with the appropriate tRNA's anticodon, and the amino acids are arranged in order to make the protein.