Ribosomes are composed of two subunits called the large and the small subunits. Which pair best describes these subunits?

• A. Large and small subunits
• B. Ribosomal RNA
• C. Proteins and DNAs
• D. Nucleolar subunits

Answer: (A)

Ribosomes are built from two distinct parts that come together to carry out protein synthesis: a small subunit and a large subunit. This pairing is the correct description because the two pieces are literally named by their size and work together to form the functional ribosome. Each subunit contains ribosomal RNA and proteins, not DNA, and the RNA component is essential for catalysis and decoding, while proteins help stabilize the structure. The nucleolus is the cellular region where ribosome assembly begins, not the subunits themselves. In bacteria and eukaryotes, these two subunits differ in size (for example, bacteria have a small and a large subunit that assemble into a 70S ribosome, while eukaryotes form an 80S ribosome from their own small and large subunits).