In prokaryotes, which statement correctly describes ribosomal subunits and the assembled ribosome?

In prokaryotes, which statement correctly describes ribosomal subunits and the assembled ribosome?

• A. The small subunit is 40S, the large is 60S, forming an 80S ribosome
• B. The small subunit is 20S, the large is 40S, forming a 60S ribosome
• C. The small subunit is 30S, the large is 50S, forming a 70S ribosome
• D. The small subunit is 50S, the large is 30S, forming a 70S ribosome

Answer: (C)

In prokaryotes, the ribosome is a 70S particle made from a 30S small subunit and a 50S large subunit. The Svedberg unit (S) reflects how fast a particle sediments in a centrifuge, not its exact size, and the prokaryotic 70S ribosome is distinct from the eukaryotic 80S ribosome, which is built from a 40S small subunit and a 60S large subunit. So the description with a 30S small subunit and a 50S large subunit forming a 70S ribosome matches the prokaryotic system. The other options mix in subunit sizes that belong to either the wrong organism (eukaryotes) or incorrect subunit composition for prokaryotes.

In prokaryotes, the ribosome is a 70S particle made from a 30S small subunit and a 50S large subunit. The Svedberg unit (S) reflects how fast a particle sediments in a centrifuge, not its exact size, and the prokaryotic 70S ribosome is distinct from the eukaryotic 80S ribosome, which is built from a 40S small subunit and a 60S large subunit. So the description with a 30S small subunit and a 50S large subunit forming a 70S ribosome matches the prokaryotic system. The other options mix in subunit sizes that belong to either the wrong organism (eukaryotes) or incorrect subunit composition for prokaryotes.