A synthetic repeating RNA of sequence (UCAG)₅ is translated. Considering the reading frames generated by this periodic tetranucleotide, what is the maximum number of different amino acids that can appear in the polypeptide?

Introduction / Context:
Synthetic polyribonucleotides with repeating patterns were historically used to decipher the genetic code. A repeating tetranucleotide (UCAG) repeated five times produces a periodic mRNA. The question asks how many different codons—and thus amino acids—can be produced from this periodic sequence during translation.

Given Data / Assumptions:

• RNA sequence: (UCAG) repeated, i.e., UCAGUCAGUCAG…
• Reading frames start at the first base and proceed triplets at a time.
• Standard genetic code; no frameshifts.

Concept / Approach:

For a strictly periodic tetranucleotide, the set of possible codons depends on the reading frame relative to the 4-base repeat. Because 3 does not evenly divide 4, different frames across the repeat yield a small set of distinct triplets that recur cyclically. We list the distinct triplets encountered in each frame of the repeating UCAG pattern and then map them to amino acids.

Step-by-Step Solution:

Verification / Alternative check:

Starting from any base within the period just permutes the same set {UCA, CAG, AGU, GUC}. No other codons appear because the sequence strictly repeats with period 4. Therefore, only 3 unique amino acids are encoded (Ser appears from two codons).

Why Other Options Are Wrong:

• One or Two: underestimate; we clearly have Ser, Val, and Gln.
• Four: we see four codons but only three distinct amino acids because two codons (UCA and AGU) both encode Ser.

Common Pitfalls:

• Counting distinct codons instead of distinct amino acids.
• Assuming the presence of all 20 amino acids in a random RNA; periodic repeats produce limited codon sets.

Final Answer:

Three