Polysome refers to :

Options

A. ribosome in the cytoplasm
B. cluster of ribosomes
C. single unit of ribosome associated with ER
D. ribosomes present in the nucleolus

Explanation

### Step-by-Step Solution:

1. **Understanding the Term "Polysome":**
- A polysome, also known as a polyribosome, is a complex formed by multiple ribosomes that are simultaneously translating a single strand of messenger RNA (mRNA).

2. **Identifying the Options:**
- The question provides four options:
1. Ribosome in the cytoplasm
2. Cluster of ribosomes
3. Single unit of ribosome associated with ER
4. Ribosomes present in the nucleolus

3. **Analyzing Each Option:**
- **Option 1: Ribosome in the cytoplasm**
- This option is incorrect because while ribosomes are present in the cytoplasm, they are not specifically referred to as polysomes unless they are in a cluster translating the same mRNA.

- **Option 2: Cluster of ribosomes**
- This option is correct. Polysomes are indeed clusters of ribosomes linked together by a single mRNA molecule, allowing for the simultaneous synthesis of multiple copies of a polypeptide.

- **Option 3: Single unit of ribosome associated with ER**
- This option is incorrect because a single ribosome does not constitute a polysome. Polysomes involve multiple ribosomes working together.

- **Option 4: Ribosomes present in the nucleolus**
- This option is also incorrect. Ribosomes are assembled in the nucleolus but are not referred to as polysomes. Polysomes are found in the cytoplasm or associated with the rough endoplasmic reticulum during protein synthesis.

4. **Conclusion:**
- After analyzing all the options, the correct answer is that polysomes refer to a "cluster of ribosomes."

### Final Answer:
Polysome refers to a cluster of ribosomes.

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