Which organelle assembles amino acids into proteins?

• A. Ribosome
• B. Mitochondria
• C. Chloroplast
• D. Nucleus

Answer: (A)

Proteins are built from amino acids through translation, which happens on ribosomes. A ribosome reads the instructions in messenger RNA and, with tRNA delivering amino acids, links them together to form a polypeptide chain. This is the main site where amino acids are assembled into proteins, whether the ribosome is free in the cytoplasm or attached to the rough endoplasmic reticulum. Mitochondria generate energy (ATP) and have their own ribosomes for a few proteins, but that’s not their primary role. Chloroplasts carry out photosynthesis and also contain some ribosomes, but again, the general protein-building function is the ribosome’s job. The nucleus houses DNA and is where transcription occurs, not where proteins are assembled. So the organelle that assembles amino acids into proteins is the ribosome.