Biomolecular alphabet sizes: Proteins are built from how many standard amino acids, and DNA/RNA are built from how many distinct nucleotides?

Introduction:
The information molecules of life use limited alphabets to encode vast complexity. Proteins are polymers of amino acids, while nucleic acids (DNA and RNA) are polymers of nucleotides. This question checks quantitative recall of the canonical building blocks used in the central dogma: twenty standard amino acids for proteins and four standard nucleotides for DNA and RNA (with uracil replacing thymine in RNA).

Given Data / Assumptions:

• Standard genetic code utilizes 20 common amino acids in protein synthesis.
• DNA uses A, T, G, C; RNA uses A, U, G, C.
• Rare exceptions (selenocysteine, pyrrolysine) exist but are non-canonical and specialized.

Concept / Approach:
While the genetic code has 64 codons (4^3) due to triplet combinations, these codons map redundantly to the same set of 20 standard amino acids plus stop signals. Therefore, the correct pairing is 20 standard amino acids and 4 nucleotides in the basic alphabet for DNA/RNA. Confusing codon count with amino acid count is a common error addressed here.

Step-by-Step Solution:

Verification / Alternative check:
Introductory biochemistry texts and genetic code tables show degeneracy: multiple codons per amino acid, confirming that the codon count exceeds the amino acid count without expanding the protein alphabet beyond 20 in standard contexts.

Why Other Options Are Wrong:

• 20,64: reverses the relationship; 64 codons are not nucleotides.
• 3,20 or 4,20: invert the mapping; nucleotides are not more numerous than amino acids in the standard set.
• 22,5: references rare amino acids and an incorrect nucleotide count.

Common Pitfalls:
Equating codon number to amino acid number and forgetting RNA substitutes uracil for thymine without increasing the nucleotide alphabet size.

Final Answer:
20,4