Polyribosomes are aggregation of

Options

A. ribosomes and rRNA
B. peroxisomes
C. several ribosomes held together by a string of mRNA
D. rRNA.

Explanation

### Step-by-Step Solution:

1. **Understanding Polyribosomes**: Polyribosomes, also known as polysomes, are structures formed during the process of translation in protein synthesis.

2. **Formation of Polyribosomes**: During translation, messenger RNA (mRNA) is translated into a polypeptide chain. Multiple ribosomes can attach to a single mRNA strand simultaneously.

3. **Structure of Polyribosomes**: Each ribosome consists of two subunits: a larger subunit and a smaller subunit. When these ribosomes attach to the mRNA, they create an appearance similar to "beads on a string."

4. **Aggregation of Ribosomes**: The aggregation of these ribosomes is held together by the mRNA strand. This means that multiple ribosomes can be found translating the same mRNA molecule at the same time.

5. **Conclusion**: Therefore, polyribosomes are defined as the aggregation of several ribosomes that are held together by a single strand of mRNA.

6. **Final Answer**: Polyribosomes are aggregation of several ribosomes held together by a string of mRNA.