Where is the site of protein synthesis located in a cell?

• A. Nucleus
• B. Ribosomes
• C. Mitochondria
• D. Endoplasmic Reticulum

Answer: (B)

Proteins are synthesized by ribosomes, the cell’s protein-synthesizing machines that read messenger RNA and assemble amino acids into polypeptide chains. Ribosomes can float freely in the cytoplasm to make cytosolic proteins or become attached to the rough endoplasmic reticulum to produce proteins destined for secretion or membranes. The nucleus houses DNA and is the site of transcription, not translation. Mitochondria do have their own ribosomes for a small set of proteins, but the main protein-building activity occurs on cytosolic ribosomes. The endoplasmic reticulum plays a key role in processing and trafficking certain proteins, but the translation itself is carried out by ribosomes.