Which cell structure is the site of protein synthesis in the cytoplasm, composed of ribosomal RNA and proteins, and formed by two subunits?

• A. Ribosome
• B. Mitochondrion
• C. Golgi apparatus
• D. Nucleus

Answer: (A)

Proteins are built on ribosomes, the cellular machines made of ribosomal RNA and proteins that come in two subunits. These small and large subunits come together to translate mRNA into a polypeptide, which is the process of protein synthesis. In the cytoplasm, ribosomes can be free-standing or attached to the rough endoplasmic reticulum, but either way they are the site where amino acids are assembled into proteins. Mitochondria do have their own ribosomes, but the main idea in the prompt is cytoplasmic protein synthesis by ribosomes in two subunits; the Golgi apparatus modifies and ships proteins, and the nucleus houses genetic material and transcription. So the structure described is the ribosome.