During elongation, ribosome moves in direction
Genetic information decoding demands that ribosomes traverse messenger RNA from 5' end toward 3' end, reading codons sequentially while synthesizing polypeptide amino to carboxyl directionally. During each elongation cycle, EF-G or eEF2 catalyzes translocation that advances the ribosome by three nucleotides in the 5' to 3' direction along mRNA, repositioning peptidyl-tRNA from A to P site and deacylated tRNA from P to E site, thereby exposing the next codon in A site for decoding. Reverse 3' to 5' movement, random walk, or bidirectional sliding would disrupt reading frame maintenance and cause
Ref: Lodish Molecular Cell Biology Fig 4-30; Alberts - ribosome moves 5'->3' on mRNA during elongation to preserve reading frame