Which statement about ribosome sizes is true?

Which statement about ribosome sizes is true?

• 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 composition reflect where in the cell the ribosome operates and its evolutionary origin. Prokaryotes have a 70S ribosome, made of a 30S small subunit and a 50S large subunit. Eukaryotic cells in the cytosol use an 80S ribosome, consisting of a 40S small subunit and a 60S large subunit. Organelles such as mitochondria, which are descended from bacteria, retain bacterial-like ribosomes around 70S. So, the statement that prokaryotic ribosomes are 70S (30S+50S) and that mitochondria have 70S ribosomes matches this pattern and is true. The idea that all ribosomes are 70S is incorrect because the cytosolic eukaryotic ribosome is 80S. Saying prokaryotic ribosomes are 60S misstates the subunit structure (they are 30S+50S, totaling 70S). And mitochondrial ribosomes being 90S isn’t aligned with their usual description as 70S in this context.

Ribosome size and subunit composition reflect where in the cell the ribosome operates and its evolutionary origin. Prokaryotes have a 70S ribosome, made of a 30S small subunit and a 50S large subunit. Eukaryotic cells in the cytosol use an 80S ribosome, consisting of a 40S small subunit and a 60S large subunit. Organelles such as mitochondria, which are descended from bacteria, retain bacterial-like ribosomes around 70S.

So, the statement that prokaryotic ribosomes are 70S (30S+50S) and that mitochondria have 70S ribosomes matches this pattern and is true. The idea that all ribosomes are 70S is incorrect because the cytosolic eukaryotic ribosome is 80S. Saying prokaryotic ribosomes are 60S misstates the subunit structure (they are 30S+50S, totaling 70S). And mitochondrial ribosomes being 90S isn’t aligned with their usual description as 70S in this context.