Where are proteins synthesized in the cell?

• A. Nucleus
• B. Endoplasmic reticulum
• C. Golgi apparatus
• D. Ribosomes

Answer: (D)

Protein synthesis is carried out by ribosomes, the cellular machines that translate mRNA into polypeptide chains. In eukaryotic cells, ribosomes can be free in the cytosol or attached to the rough endoplasmic reticulum. Free ribosomes typically make proteins that function in the cytosol, while ribosomes on the rough ER synthesize proteins destined for secretion, for membranes, or for lysosomes. The signal peptide on a nascent protein directs ribosomes with the help of the signal recognition particle to the ER for those secretory and membrane proteins. The nucleus, endoplasmic reticulum, and Golgi apparatus play roles in transcription, processing, and trafficking, but the actual synthesis of new protein chains occurs at the ribosome.