Question
Only 20 amino acids are considered as standard.

Answer

  1. It was believed that there are total 20 amino acids in the living world. But 21st amino acid called selenocysteine was discovered later.
  2. This amino acid is coded by UGA which is usually a termination codon.
  3. In both prokaryotic and eukaryotic cells polypeptide chains contain 100-300 amino acids and they are formed by specific arrangement of 21 amino acids.
  4. But formation of selenocysteine requires the availability of element selenium in the cells.
  5. Therefore, only 20 amino acids are considered as standard.

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