Ribosomes in Prokaryotes and Mitochondria Are 70S

Steps:

• Recall that ribosomes are classified by sedimentation coefficient (Svedberg units): prokaryotes use 70S, eukaryotes use 80S.
• Note prokaryotic ribosomes consist of 30S small and 50S large subunits, totaling 70S.
• Recognize mitochondria originated from prokaryotic endosymbionts, retaining similar 70S ribosomes.
• Confirm eukaryotic cytoplasmic ribosomes are 80S (40S + 60S), distinguishing them from mitochondrial ones.

Why C is correct:

• Prokaryotic and mitochondrial ribosomes are defined as 70S based on their subunit composition and evolutionary origin.

Why the others are wrong:

• A: 50S is only the large subunit of prokaryotic 70S ribosomes, not the full size.
• B: 60S is the large subunit of eukaryotic 80S ribosomes, not found in prokaryotes or mitochondria.
• D: 80S is the full size of eukaryotic cytoplasmic ribosomes, larger than prokaryotic or mitochondrial ones.

Final answer: C