Plasmids are suitable vectors for gene cloning because :

Options

A. these are small circular DNA molecules, which can integrate with host chromosomal DNA
B. these are small circular DNA molecules with their own replication origin site
C. these can shuttle between prokaryotic and eukaryotic cells
D. these often carry antibiotic resistance genes

Answer

### Step-by-Step Solution:

1. **Understanding Plasmids**:
- Plasmids are small, circular, extra-chromosomal DNA molecules found in prokaryotic cells, particularly bacteria. They are distinct from the chromosomal DNA of the bacterium and can replicate independently.

2. **Characteristics of Plasmids**:
- Plasmids have the ability to replicate independently of the bacterial chromosomal DNA.
- They often carry important genes, such as those conferring antibiotic resistance.

3. **Defining Vectors**:
- A vector is a DNA molecule used to carry foreign DNA (the insert) into another cell, where it can be replicated and expressed.

4. **Key Features of a Suitable Vector**:
- A suitable vector must have an origin of replication (ORI) to ensure that it can replicate independently within the host cell.
- It should also be able to carry foreign DNA effectively.

5. **Evaluating the Options**:
- **Option A**: "These are small circular DNA molecules which can integrate with host chromosomal DNA."
- This is incorrect because plasmids do not integrate into the host DNA; they replicate independently.
- **Option B**: "These are small circular DNA molecules with their own replication origin sites."
- This is correct because plasmids have their own ORI, allowing them to replicate independently.
- **Option C**: "These can shuttle between prokaryotic and eukaryotic cells."
- This is incorrect; while some plasmids can be used in both types of cells, it is not a defining characteristic of all plasmids.
- **Option D**: "These often carry antibiotic resistance genes."
- This is true, but carrying antibiotic resistance genes alone does not make them suitable vectors for gene cloning.

6. **Conclusion**:
- The most appropriate reason why plasmids are suitable vectors for gene cloning is that they are small circular DNA molecules with their own replication origin sites (Option B).

### Final Answer:
Plasmids are suitable vectors for gene cloning because they are small circular DNA molecules with their own replication origin sites.

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