What is the name of the cellular structure that is the site of protein synthesis in the cytoplasm and is composed of rRNA and protein forming two subunits?

• A. Ribosomes
• B. Nucleolus
• C. Endoplasmic Reticulum
• D. Golgi

Answer: (A)

Protein synthesis occurs on ribosomes, RNA–protein machines that come together in two subunits to translate mRNA into a polypeptide. In the cytoplasm, ribosomes can be free to make cytosolic proteins or attached to the rough endoplasmic reticulum to synthesize proteins destined for secretion or membranes. Each ribosome is composed of ribosomal RNA and proteins, and the two subunits join during translation. The nucleolus is the nuclear site where rRNA is made and initial ribosome assembly begins, not where cytoplasmic protein synthesis occurs. The endoplasmic reticulum is a membrane network involved in processing and trafficking, while the Golgi modifies and sorts proteins after synthesis. Thus, the structure described is the ribosome.