One of the following pairs is incorrect

Options

A. Plasmid-small piece of extrachromosomal DNA in bacteria
B. interferon-an enzyme that interferes with DNA replication
C. cosmid-a vector for carrying large DNA fragments into host cells.
D. myeloma-antibody producing tumor cells.

Answer

To determine which of the given pairs is incorrect, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Analyze the first option: Plasmid**
- A plasmid is an extra-chromosomal piece of DNA found in bacteria.
- It is not a small piece of chromosomal DNA; rather, it is a circular DNA molecule that can replicate independently of chromosomal DNA.
- **Conclusion**: The statement "plasmid small piece of chromosomal DNA in bacteria" is incorrect because plasmids are extra-chromosomal, not chromosomal.

2. **Analyze the second option: Interferon**
- Interferons are proteins that are released by host cells in response to viral infections.
- They are not enzymes that interfere with DNA replication; instead, they function as signaling molecules to help the immune response.
- **Conclusion**: The statement "interferon an enzyme that interferes with DNA replication" is incorrect.

3. **Analyze the third option: Cosmid**
- Cosmids are hybrid vectors that can carry larger DNA fragments compared to plasmids.
- They are indeed used for cloning large DNA fragments into host cells.
- **Conclusion**: The statement "cosmid a vector for carrying large DNA fragments into host cells" is correct.

4. **Analyze the fourth option: Myeloma**
- Myeloma cells are a type of tumor cell that produces antibodies.
- They are indeed antibody-producing tumor cells, often used in the production of monoclonal antibodies.
- **Conclusion**: The statement "myeloma antibody producing tumor cell" is correct.

### Final Conclusion:
The incorrect pairs among the options are:
- **Option A**: Plasmid (incorrectly described as a small piece of chromosomal DNA)
- **Option B**: Interferon (incorrectly described as an enzyme that interferes with DNA replication)

Since the question asks for one incorrect pair, the answer is **Option B: Interferon**.