What are stop codons?

What are stop codons?

• A. UAA, UAG, UGA; terminate translation
• B. AUG, UUG, UUU; start translation
• C. UGA, UAA; terminate transcription
• D. UCA, UGA, UAA; code for amino acids

Answer: (A)

Stop codons are signals that end protein synthesis during translation. There are three in the standard genetic code: UAA, UAG, and UGA. They don’t code for any amino acids. Instead, when the ribosome encounters one, release factors promote release of the newly made polypeptide and the ribosome dissociates from the mRNA. That’s why these codons terminate translation rather than start it or encode amino acids. The idea that these codons start translation is incorrect (the start codon is AUG, with occasional alternative starts in some contexts), and stop codons do not signal transcription termination (that involves different cellular machinery). Also, some codons listed as coding for amino acids are wrong for stop codons (codes like phenylalanine come from other codons, not stop signals).

Stop codons are signals that end protein synthesis during translation. There are three in the standard genetic code: UAA, UAG, and UGA. They don’t code for any amino acids. Instead, when the ribosome encounters one, release factors promote release of the newly made polypeptide and the ribosome dissociates from the mRNA. That’s why these codons terminate translation rather than start it or encode amino acids.

The idea that these codons start translation is incorrect (the start codon is AUG, with occasional alternative starts in some contexts), and stop codons do not signal transcription termination (that involves different cellular machinery). Also, some codons listed as coding for amino acids are wrong for stop codons (codes like phenylalanine come from other codons, not stop signals).