A ribosome is composed of

Options

A. A single unit
B. Two subunits
C. Three subunits
D. Four subunits

Answer

To answer the question "A ribosome is composed of," we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding Ribosomes**:
- Ribosomes are known as the "protein factories" of the cell because they are the sites where protein synthesis occurs.

2. **Composition of Ribosomes**:
- Ribosomes are not single entities; they are made up of two distinct parts known as subunits.

3. **Types of Subunits**:
- Each ribosome consists of a large subunit and a small subunit.
- The large subunit is responsible for joining amino acids together to form a polypeptide chain.
- The small subunit is involved in reading the messenger RNA (mRNA) during protein synthesis.

4. **Subunit Sizes in Different Organisms**:
- In prokaryotic cells, the ribosomal subunits are designated as 50S (large) and 30S (small).
- In eukaryotic cells, the ribosomal subunits are designated as 60S (large) and 40S (small).

5. **Conclusion**:
- Based on the information gathered, we conclude that ribosomes are composed of two subunits. Therefore, the correct answer to the question is the second option: "two subunits."

### Final Answer:
A ribosome is composed of **two subunits**.

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