There are three types of RNA that have a direct role in protein synthesis—messenger RNA or mRNA, transfer RNA or tRNA, and ribosomal RNA or rRNA. mRNA is first transcribed from DNA by complementary base pairing. In an mRNA molecule, codons, groups of three nucleotides, specify the amino acid sequence and indicate where protein synthesis should start and stop. Translation occurs after the mRNA travels to the cytoplasm, where it is positioned on the small ribosomal subunit with the help of its rRNA. Next, a tRNA molecule, with a three-nucleotide anticodon sequence on one end and a specific amino acid on the other, binds to a complementary codon in the mRNA. The correct tRNAs bind to the mRNA sequentially, and the rRNA in the large ribosomal subunit catalyzes the formation of a peptide bond between two amino acids. The ribosome travels down the mRNA creating a polypeptide with a sequence of amino acids specified by the mRNA.