Final answer:
Some drugs harmful to both bacteria and humans inhibit protein synthesis by targeting the 70S ribosomes (option 4) found in both bacterial cells and human mitochondria. Tetracycline and chloramphenicol are examples of antibiotics that can inadvertently affect human mitochondrial protein synthesis.
Step-by-step explanation:
The reason why some drugs that inhibit protein synthesis in bacteria also harm humans is because eukaryotic mitochondria have 70S ribosomes similar to those in bacteria. Although human cells are eukaryotic and generally have larger 80S ribosomes that are distinct from the smaller 70S ribosomes found in bacteria, the mitochondria within human cells still contain these 70S ribosomes. This similarity can result in certain antibiotics that target bacterial ribosomes inadvertently affecting mitochondrial ribosomes in human cells, leading to toxicity.
Antibiotics like tetracycline and chloramphenicol interrupt protein synthesis in bacteria by binding specifically to the bacterial ribosome. Tetracycline blocks the A site, inhibiting tRNA from bringing amino acids for protein chain elongation, while chloramphenicol blocks peptidyl transfer, which is a crucial step in the formation of a new peptide bond during protein synthesis. When these antibiotics affect mitochondrial ribosomes, they can disrupt protein synthesis within human cells.
It's important to understand that while most antibiotics are designed to target prokaryotic ribosomes and are thus effective against bacteria, the presence of 70S ribosomes in human mitochondria does make them susceptible to interference by these drugs, which can cause the harmful side effects in some cases.