Where does protein synthesis occur?

• A. Nucleus
• B. Ribosome
• C. Mitochondrion
• D. Endoplasmic Reticulum

Answer: (B)

Protein synthesis, the process of building a polypeptide from an mRNA blueprint, happens at the ribosome. The ribosome is the molecular machine that reads the mRNA and, with tRNAs delivering specific amino acids, links them together to form the protein. In eukaryotic cells, ribosomes can be free in the cytosol or attached to the rough endoplasmic reticulum; proteins made on ribosomes attached to the rough ER are typically destined for secretion or for membranes and enter the ER lumen for processing. The nucleus is where transcription occurs to make mRNA, not where translation happens. Mitochondria do have their own ribosomes and can synthesize a subset of proteins encoded by mitochondrial DNA, but the bulk of cellular protein synthesis takes place on cytosolic ribosomes.