Decoding Proline Codon from In Vitro Translation Experiments

In vitro translation systems reveal genetic code assignments by synthesizing peptides from synthetic RNA templates across all reading frames. The given templates produce specific amino acid compositions, allowing deduction of proline’s codon among the options.

First Template Analysis

The RNA 5′-UCUCUCUC—(UC)ₙ—UCUCUCUC-3′ yields peptides with 50% serine and 50% leucine. Reading frames generate repeating codons UCU-CUC or CUC-UCU, confirming UCU codes for serine and CUC for leucine. Equal ratios match this binary incorporation, as other frames yield stop codons or non-matching peptides.​

Second Template Analysis

The RNA 5′-CCUCCUCCU—(CCU)ₙ—CCUCCU-3′ produces peptides with 33.3% serine, 33.3% leucine, and 33.3% proline. Possible reading frames include CCU-CCU-CCU (all proline), CUCCUC (CUC-serine/leucine frame), and UCC-UCC (serine frame), averaging one-third each. This ternary distribution uniquely fits CCU as proline.​

Why CCU is Proline Codon

CCU must encode proline to achieve 33.3% proline from the repeating CCU block, while flanking regions provide CUC (leucine) and UCC/UCU (serine). Standard genetic code confirms CCU, CCC, CCA, CCG as proline codons. Correct answer: (C) CCU.​

Option-by-Option Breakdown