UAG and UGA are stop codons. Termination of chain growth in protein synthesis is brought about by UAA, UAG, UGA. The three stop codons have been given names : UAG is amber, UGA is opal, and UAA is ochre. Stop codons are also called "termination" or "nonsense" codons. They signal release of the nascent polypeptide from the ribosome because there is no cognate tRNA that has anticodons complementary to these stop signals, and so a release factor binds to the ribosome instead.