Tetracycline inhibits mitochondrial protein synthesis due to ribosomal similarity

Steps:

• Tetracycline binds to the 30S subunit of bacterial ribosomes, blocking protein synthesis.
• Human cells use 80S cytoplasmic ribosomes, unaffected by tetracycline at therapeutic doses.
• Mitochondria contain 55S ribosomes with bacterial-like 30S subunits from endosymbiotic origins.
• Thus, tetracycline targets mitochondrial ribosomes, impairing energy production.

Why D is correct:

• Statement 3 reflects the endosymbiotic theory, where mitochondrial ribosomes evolved from bacterial ancestors, enabling tetracycline binding.

Why the others are wrong:

• A includes 1 (tetracycline's bacterial action) and 2 (mitochondrial location), but neither explains the specific ribosomal targeting.
• B includes 2 (mitochondrial location, irrelevant to mechanism) and 3.
• C includes only 2, which describes anatomy without addressing ribosomal similarity.

Final answer: D