Protein Synthesis—Primary Cellular Site After transcription, at which cellular site does translation (protein synthesis) actually occur?

Introduction / Context:
Translation is the process of decoding messenger RNA (mRNA) into a polypeptide chain. This reaction is catalyzed by ribosomes, ribonucleoprotein complexes found in all cells. The question focuses on identifying the actual catalytic site of protein synthesis rather than locations involved in trafficking or transcription.

Given Data / Assumptions:

• mRNA is synthesized in the nucleus (eukaryotes) and exported to the cytoplasm.
• Ribosomes can be free in the cytosol or bound to the rough endoplasmic reticulum (RER).
• Ribosomes catalyze peptide bond formation via peptidyl transferase activity (rRNA-based).

Concept / Approach:
Distinguish the site where the chemistry of peptide bond formation occurs from organelles that process or sort proteins. Regardless of whether a ribosome is cytosolic or attached to RER, translation occurs on the ribosome itself. Organelles such as the Golgi apparatus act later in modification/sorting and chromosomes carry DNA for transcription, not translation.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical reconstitution of translation in vitro requires ribosomes, tRNAs, mRNA, and factors—demonstrating that ribosomes are the active site of protein synthesis.

Why Other Options Are Wrong:

• Nucleus: transcription/processing, not translation.
• Endoplasmic reticulum: provides membrane surface; the ribosome performs catalysis.
• Chromosome: DNA storage; not the site of peptide bond formation.
• Golgi: post-translational processing and sorting.

Common Pitfalls:
Equating “on the ER” with “in the ER.” Translation occurs on ER-bound ribosomes, not inside the ER lumen (polypeptides are translocated during synthesis).

Final Answer:
Ribosome