Answer
A stop codon is three base sequence in an mRNA chain that signals the end of the assembly of a polypeptide chain.
Work Step by Step
A codon is a three base sequence in a nucleic acid chain that species an amino acid. In the process of DNA translation, the ribosomes interpret the DNA message encoded in m-RNA. With the help of t-RNAs, the specified amino acids are assembled into a peptide chain based on the instructions in mRNA code.
Regular codons specify amino acids, but there are special codons that tell the peptide assembly machinery when to start, and when to stop the assembling process. The codon that initiates the the assembling is usually ( in higher organisms ) the base triplet AUG. There is also a codon that signals termination of the assembly of a polypeptide sequence; this is the stop codon. In fact, there are actually three stop codons in eukaryotes ; they are UAG, UGA and UAA.
