Ribososomes are CORRECTLY characterized by which of the following statements ?

Options

A. They are composed entirely of r-RNA
B. They are composed of two equal subunits.
C. They differ in response to antibiotics depending on whether they are eukaryotic or prokaryotic in origin .
D. They are composed of three equal subunits .

Answer

To solve the question regarding the correct characterization of ribosomes, we will evaluate each of the provided statements one by one.

### Step-by-Step Solution:

1. **Evaluate the First Statement**:
- The statement claims that ribosomes are composed entirely of ribosomal RNA.
- **Analysis**: This is incorrect because ribosomes are made up of both ribosomal RNA (rRNA) and proteins. Therefore, this statement does not correctly characterize ribosomes.

**Hint**: Remember that ribosomes consist of both RNA and proteins, not just RNA.

2. **Evaluate the Second Statement**:
- The statement suggests that ribosomes are composed of two equal subunits.
- **Analysis**: This is also incorrect. Ribosomes consist of two subunits that are not equal in size. In prokaryotes, the subunits are 50S and 30S, while in eukaryotes, they are 60S and 40S. Thus, this statement is not accurate.

**Hint**: Pay attention to the sizes of ribosomal subunits; they are unequal in both prokaryotes and eukaryotes.

3. **Evaluate the Third Statement**:
- The statement indicates that ribosomes differ in response to antibiotics depending on whether they are eukaryotic or prokaryotic in origin.
- **Analysis**: This statement is correct. Ribosomes from prokaryotic cells (such as bacteria) are different from those in eukaryotic cells, and this difference affects how they respond to various antibiotics.

**Hint**: Consider how antibiotics target specific ribosomal structures to inhibit protein synthesis, which varies between prokaryotic and eukaryotic ribosomes.

4. **Evaluate the Fourth Statement**:
- The statement claims that ribosomes are composed of three equal subunits.
- **Analysis**: This is incorrect. Ribosomes do not consist of three subunits; they are made up of two subunits that vary in size and composition.

**Hint**: Remember the basic structure of ribosomes; they have only two subunits, not three.

### Conclusion:
After evaluating all the statements, the only correct characterization of ribosomes is found in the third statement, which highlights their differential response to antibiotics based on their origin (eukaryotic or prokaryotic).

**Final Answer**: The correct characterization of ribosomes is given by the third statement.