Structures composed primarily of RNA which are found primarily on the endoplasmic reticulum (site of protein synthesis).

• A. Endoplasmic reticulum
• B. Cytoplasm
• C. Ribosome
• D. Golgi apparatus

Answer: (C)

Ribosomes are the structures composed largely of RNA and proteins, with ribosomal RNA forming the core that catalyzes peptide bond formation during protein synthesis. Because of this RNA-rich composition, they are described as being primarily made of RNA. In many cells, ribosomes are attached to the rough endoplasmic reticulum, giving that membrane network a studded appearance and positioning the ribosomes where proteins destined for secretion or for membranes are synthesized. The endoplasmic reticulum itself is an organelle involved in protein folding and processing, not something composed mainly of RNA. The cytoplasm is the cellular fluid that houses ribosomes and other components but is not organized as RNA-rich structures. The Golgi apparatus processes and packages proteins, again not RNA-dominated. So the best match for “structures composed primarily of RNA and found on the endoplasmic reticulum at the site of protein synthesis” is the ribosome.