How many naturally occurring amino acids are incorporated into proteins during translation in the standard genetic code?

• A. 18
• B. 22
• C. 21
• D. 20

Answer: (D)

Verified step by step guidance

1. Understand the context: Translation is the process by which ribosomes synthesize proteins using mRNA as a template. The standard genetic code specifies which amino acids are incorporated into proteins.
2. Recall that the standard genetic code includes codons that correspond to amino acids. Each codon is a sequence of three nucleotides in mRNA that specifies a particular amino acid.
3. Identify the naturally occurring amino acids: There are 20 standard amino acids that are encoded by the genetic code and incorporated into proteins during translation. These amino acids include glycine, alanine, valine, leucine, isoleucine, serine, threonine, cysteine, methionine, proline, phenylalanine, tyrosine, tryptophan, histidine, lysine, arginine, aspartic acid, glutamic acid, asparagine, and glutamine.
4. Clarify exceptions: While there are other amino acids like selenocysteine and pyrrolysine that can be incorporated into proteins, they are not part of the standard genetic code and require special mechanisms for incorporation.
5. Conclude that the correct answer is based on the standard genetic code, which specifies 20 amino acids that are incorporated into proteins during translation.