How do prokaryotic ribosomes compare to eukaryotic ribosomes in size and composition?

• A. All ribosomes are 70S
• B. Prokaryotic ribosomes are 70S (30S+50S); mitochondria have 70S
• C. Prokaryotic ribosomes are 60S; eukaryotic ribosomes are 60S
• D. Mitochondrial ribosomes are 90S

Answer: (B)

Ribosome size and subunit makeup differ between prokaryotes and eukaryotes, reflecting their distinct cellular organization and evolutionary history. Prokaryotic ribosomes are 70S in total, built from a 30S small subunit and a 50S large subunit. In contrast, eukaryotic cytoplasmic ribosomes are larger, about 80S, composed of a 40S small subunit and a 60S large subunit. Mitochondria (and chloroplasts in plants) contain ribosomes that resemble the bacterial type and are typically described as 70S, with 30S- and 50S-like subunits. So the statement that prokaryotes have 70S ribosomes and that mitochondria also have 70S ribosomes best captures the size and composition pattern. The other options misstate subunit sizes or mix up the distinct ribosome types (for example, cytoplasmic eukaryotic ribosomes are not 60S, and 90S refers to a pre-ribosomal particle, not a mature ribosome).