Of the 64 possible codons present in the genetic code, three encode stop signals that terminate translation and one encodes both a start signal that initiates translation and the amino acid methionine.
61 of the possible 64 codons encode twenty different amino acids. Most of these amino acids can be encoded by more than one codon. Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called stop codons: UGA, UAG, and UAA. In general, these do not encode amino acids. However, the stop codon UGA is sometimes used to encode a 21st amino acid called selenocysteine (Sec), but only if the mRNA additionally contains a specific sequence of nucleotides called a selenocysteine insertion sequence (SECIS). The stop codon UAG is also sometimes used by a few species of microorganisms to encode a 22nd amino acid called pyrrolysine (Pyl).
One of the 61 codons that encode amino acids, AUG, has a special function. In addition to specifying the amino acid methionine, it also serves as the start codon to initiate translation. The reading frame for translation is set by the AUG start codon.
Practice Questions
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Key Points
• Three codons (UAA, UAG, and UGA) are known as stop codons and they signal the termination of translation. They do not encode amino acids.
• In special cases, the stop codons UGA and UAG can encode special 21st and 22nd amino acids.
• One codon (AUG) is known as a start codon and initiates the process of translation. It also encodes the amino acid methionine.
Key Terms
Codon: A sequence of three adjacent nucleotides that encode a specific amino acid or stop signal during protein synthesis (translation).
Translation: The process where RNA is used to create a new polypeptide chain (protein).
Stop codon: One of three codons (UAA, UAG, and UGA) that signals the end of translation.
Start codon: The codon AUG, which both signals the start of translation and encodes the amino acid methionine.
Reading frame: The way a sequence of genetic code (DNA or RNA) is split into groups of three nucleotides (codons) from the beginning of the sequence; the reading frame for translation is set by the start codon AUG.