Q.17 Co-translational translocation of proteins is observed in ________.
(A) endoplasmic reticulum
(B) Golgi complex
(C) mitochondria
(D) peroxisomes

Co-translational translocation occurs primarily at the endoplasmic reticulum, where proteins enter during synthesis. The correct answer to the query is option (A) endoplasmic reticulum.​​

Correct Answer

Co-translational translocation of proteins happens in the endoplasmic reticulum (ER). During translation, the signal recognition particle (SRP) binds nascent polypeptides with ER signal sequences on cytosolic ribosomes, directing the ribosome-nascent chain complex to the ER membrane’s Sec61 translocon for concurrent insertion into the ER lumen.​​

Option Explanations

Endoplasmic Reticulum (A) – Correct

Proteins destined for secretion or membranes are co-translationally translocated into the ER via SRP-mediated targeting to the translocon, enabling folding and glycosylation in the lumen.​

Golgi Complex (B) – Incorrect

The Golgi modifies and sorts proteins received post-translationally from the ER; it lacks ribosomes and does not support co-translational translocation.​

Mitochondria (C) – Incorrect

Mitochondrial protein import is mostly post-translational, using chaperones to deliver fully synthesized preproteins to TOM/TIM complexes, though some evidence suggests limited co-translational events.​

Peroxisomes (D) – Incorrect

Peroxisomal proteins are imported post-translationally from the cytosol via PTS receptors, without involvement of co-translational mechanisms or ribosomes.​