Ribosomal RNA (rRNA or ribosomes) and transfer RNA (tRNA) molecules combine to constitute the molecular machinery that links amino acids together into polypeptide chains. Each tRNA has a clover-like structure with an amino acid binding site at one end (the 3' end) and a region in the middle called the anticodon site. The anticodon sites on tRNA are complimentary to the specific codons on mRNA. As the ribosomal machinery moves along the mRNA chain, tRNA molecules with matching anticodons give up their amino acid ligands, adding the appropriate amino acid to the growing polymer. Since the ribosome component has catalytic activity like an enzyme, it is also called a ribozyme. The ribozyme hydrolyzes the phosphodiester bond between tRNA and amino acids, and also catalyzes the aminotransferase reaction, in which the amino acid is transferred to the end of the polypeptide.