Which statement describes the difference between bacterial and eukaryotic ribosomes and its relevance to antibiotic selectivity?

• A. Both have 70S ribosomes; antibiotics target DNA replication instead.
• B. Bacteria have 80S ribosomes; eukaryotes have 70S; this difference prevents any antibiotic targeting.
• C. Bacteria have 70S ribosomes (50S and 30S subunits) vs eukaryotes' 80S; this difference allows drugs like tetracyclines and macrolides to target bacterial ribosomes.
• D. Both have 80S ribosomes; only Gram-positive bacteria are affected by ribosome inhibitors.

Answer: (C)

The main idea is that antibiotics can selectively inhibit bacterial protein synthesis because bacterial ribosomes differ in structure from human ribosomes. Bacteria use 70S ribosomes, made of a 50S large subunit and a 30S small subunit. Eukaryotic cells use 80S ribosomes, composed of a 60S large subunit and a 40S small subunit. This difference means many antibiotics bind specifically to sites on the bacterial 70S ribosome (such as the A site for tetracyclines or the 50S subunit for macrolides) and disrupt translation, while the eukaryotic 80S ribosome is not affected in the same way. That selective binding underlies why these drugs can target bacteria without broadly harming human cells. As additional context, some antibiotics can affect mitochondrial ribosomes because they resemble bacterial ribosomes, which can contribute to side effects.