One molecule of GTP is used to provide energy for this step. The met-tRNA is attachedat the ribosome's P site. As this continues, 5. The shift in positions occurs again, and the rest of the steps are repeated as described above. Each elongation step requires energy from 2 GTP molecules.
Elongation continues until a stop codon is reached.
By the time the full polypeptide chain has been built, most polypeptides will have folded into their usual forms. Post-translational modifications are changes that happen after the sequence of amino acids is completely built and termination of translation has occurred. The first 20 or so amino acids joined to form the membrane polypeptide make up a signal peptide that is recognized by a signal recognition particle while the polypeptide is still being built by the ribosome. The SRP brings the growing polypeptide and the ribosome to the cytoplasmic surface of the endoplasmic reticulum and connects the ribosome to a protein in the ER membrane. The ribosome, now attached to the ER membrane, continues the synthesis of the polypeptide.