The translation apparatus reads the next codon in the mRNA and attaches the specified amino acid onto methionine through a peptide bond. In most cases, this linear process of moving to the next codon and attaching the corresponding amino acid continues until one of the translation stop codons is encountered. Meanwhile, the nascent polypeptide chain folds by itself, or with the assistance of proteins called chaperones, into the functional protein. The biochemical rules that govern protein folding are often referred to as the second genetic code. In addition, some eukaryotic and prokaryotic proteins undergo protein splicing, which removes internal polypeptide segments called inteins. Some RNA transcripts are not translated; instead, they are cut and processed to form structural RNA molecules, such as tRNA and the three large RNA molecules associated with proteins in the ribosomes.