Which organelle is the site of protein synthesis?

• A. Nucleus
• B. Ribosomes
• C. Mitochondria
• D. Golgi apparatus

Answer: (B)

Protein synthesis occurs at ribosomes, the cellular machines that translate RNA into proteins. Ribosomes read messenger RNA and link together the corresponding amino acids to form a polypeptide chain. They can float freely in the cytoplasm or be attached to the rough endoplasmic reticulum; the attached ones typically handle proteins that will be secreted or inserted into membranes, while free ribosomes produce proteins that stay in the cytosol. The nucleus is where genetic information is stored and transcription happens, so it’s not the site where proteins are built. Mitochondria do have their own ribosomes for a small subset of proteins, but most cellular proteins are made on cytosolic ribosomes. The Golgi apparatus then modifies and ships these proteins after synthesis, rather than making them.