Where are proteins synthesized in the cell?

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

Answer: (B)

Proteins are built by ribosomes, the cellular machines that translate mRNA into a chain of amino acids. Ribosomes can float freely in the cytosol or be attached to the rough endoplasmic reticulum; this placement helps determine where the new protein will go next. Proteins made by free ribosomes usually remain in the cytosol or are targeted to organelles like the nucleus or mitochondria, while those made on the rough ER typically enter the secretory pathway, becoming secreted, membrane-bound, or lysosomal proteins. The nucleus, mitochondria, and Golgi have other roles—nucleus holds DNA and carries out transcription, mitochondria generate energy (and have a small set of their own ribosomes), and the Golgi modifies and sorts proteins after synthesis. But the synthesis itself happens at ribosomes.