Essential Logic and Facts Behind the Expansion of the Genetic Code: A Critical Assessment

The genetic code, long viewed as a frozen relic of early evolution, is now being systematically reprogrammed. This article examines the chemical, evolutionary, and technological foundations of its expansion, focusing on plasticity, codon architecture, and amino acid selection. The triplet codon structure reflects an evolutionary compromise between metabolic cost and information capacity, while proteins - the essential catalytic polymers - are built from α-L-amino acids derived from L-alanine. Introducing non-triplet codons or non-α amino acids would demand new metabolic pathways to supply precursors and energy for synthetic life. Current methods for incorporating noncanonical amino acids - stop codon suppression, sense codon reassignment, and quadruplet recoding - are transient and inefficient, consistent with the “ambiguous intermediate” model. Achieving stable expansion requires redesigning tRNA identity sets, improving ribosomal fidelity, rewiring metabolism, and potentially creating new foldamer scaffolds. Orthogonal translation systems and codon box deconstruction have become key tools, while permanent codon reassignment and engineered orthogonal systems mark the likely future. Core technologies include genome-scale codon swapping, directed evolution, cellular compartmentalization, and metabolic integration. Framed by evolutionary models such as Wong's coevolution theory, the WesthoffGrosjean model, and the Alanine World hypothesis, genetic code expansion emerges as a radical extension of life's chemical vocabulary.