Many antibiotics inhibit prokaryotic translation without disrupting protein synthesis of the host cell. These antibiotics work via all of the following mechanisms EXCEPT

Options

A. Antibiotics bind to the ribosomal 50S subunit to prevent translation
B. Antibiotics bind to the ribosomal 40S subunit to prevent translation
C. Antibiotics bind 70S ribosomes at similar sites and with similar efficacies
D. Antibiotics bind free ribosomes in the cytoplasm

Answer

To answer the question about the mechanisms by which antibiotics inhibit prokaryotic translation, we will analyze the options provided and identify the one that does not apply.

### Step-by-Step Solution:

1. **Understanding Prokaryotic Ribosomes**:
- Prokaryotic ribosomes are classified as 70S ribosomes, which consist of two subunits: the larger 50S subunit and the smaller 30S subunit.
- This is important because antibiotics target these specific components to inhibit protein synthesis in bacteria.

2. **Analyzing the Options**:
- **Option 1**: "Antibiotics bind to ribosomal 50S subunit to prevent translation."
- This is a valid mechanism. Many antibiotics, such as chloramphenicol and erythromycin, bind to the 50S subunit and inhibit protein synthesis.

- **Option 2**: "Antibiotics bind to ribosomal 40S subunit to prevent translation."
- This option is incorrect. Prokaryotic ribosomes do not have a 40S subunit; this is a feature of eukaryotic ribosomes. Therefore, antibiotics cannot bind to a non-existent 40S subunit in prokaryotes.

- **Option 3**: "Antibiotics bind to 70S ribosomes."
- This is also a valid mechanism. Since 70S ribosomes consist of both the 50S and 30S subunits, antibiotics can act on the whole ribosome.

- **Option 4**: "Antibiotics bind to free ribosomes in the cytoplasm."
- This is a valid mechanism as well. In prokaryotes, ribosomes are typically free in the cytoplasm and can be targeted by antibiotics.

3. **Identifying the Exception**:
- From the analysis, Option 2 is the only statement that does not accurately describe a mechanism by which antibiotics inhibit prokaryotic translation, as there is no 40S subunit in prokaryotic ribosomes.

### Conclusion:
The correct answer is **Option 2**: "Antibiotics bind to ribosomal 40S subunit to prevent translation."

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