Multiple choice

Which of the following is not a standard amino acid?

  1. Proline

  2. Tryptophan

  3. Selenocystine

  4. Methionine
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Selenocystine (or selenocysteine) is not one of the 20 standard amino acids. It's the 21st amino acid, incorporated co-translationally at UGA stop codons in a special mechanism. It contains selenium instead of sulfur and is called the '21st amino acid'.

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