What composes a ribosome?

• A. A single large protein
• B. A lipid bilayer
• C. A large and a small subunit made of proteins and ribosomal RNA
• D. DNA-protein complex

Answer: (C)

Ribosomes are ribonucleoprotein machines built from two subunits, a large and a small one, that come together during translation. Each subunit contains ribosomal RNA and ribosomal proteins; the RNA not only helps hold the structure together but also provides the catalytic center for peptide bond formation, with proteins stabilizing and aiding assembly. In bacteria the full ribosome is made of 50S and 30S subunits (70S total); in eukaryotes it’s 60S and 40S (80S total). This composition is why a lipid bilayer, a DNA–protein complex, or a single large protein do not describe a ribosome.