Answer: The number of codons in the sequence is 21.
Explanation: A codon is a triplet of nucleotide which codes for a specific amino acid. Each amino acid has at least one codon that codes for it. Three nucleotide residues is required to form a codon and to code an amino acid. Since there are 63 nucleotides in the sequence, the number of codons present in the sequence is 63 divided by 3. 63/3 = 21