Amino acyl tRNA synthetases expend (the equivalent of) 2 ATP per
amino acid to link amino acid to tRNA (more if proofreading has to
remove mismatches). Elongation requires two GTP per tRNA. This reduces
the error rate of tRNA/mRNA mismatch to 10−4. In
eukaryotes, faulty mRNA never exits the nucleus; in prokaryotes, tmRNA
ensures destruction of proteins partly synthesised from broken
mRNAs.
Synthesis of polypeptide is directed along the mRNA from 5' to 3'.
Directedness implies increased order, and increasing order (decreasing
local entropy) requires an energy input.