The best option for carrying large fragment of passenger DNA into host cell can be:

Options

A. cosmid
B. lambda phage
C. plasmid
D. BAC

Answer

To determine the best option for carrying large fragments of passenger DNA into a host cell, let's analyze the provided options step by step:

### Step-by-Step Solution:

1. **Understanding the Question**: We need to identify which vector is most suitable for transporting large fragments of DNA into a host cell.

2. **Evaluating Option 1: COSMID**
- **Analysis**: Cosmid vectors can carry larger DNA fragments than standard plasmids but are still limited in size. They are hybrid vectors that combine features of plasmids and bacteriophages.
- **Conclusion**: Not the best option for very large DNA fragments.

3. **Evaluating Option 2: Lambda Phage**
- **Analysis**: Lambda phage is a type of bacteriophage that can carry DNA into bacterial cells. However, it is primarily used for smaller DNA fragments and is not optimal for large DNA inserts.
- **Conclusion**: This option is also not suitable for large DNA fragments.

4. **Evaluating Option 3: Plasmid**
- **Analysis**: Plasmids are circular DNA molecules that can replicate independently within a bacterial cell. They are great for cloning smaller DNA fragments but have size limitations for larger inserts.
- **Conclusion**: This option is not ideal for carrying large fragments of DNA.

5. **Evaluating Option 4: BAC (Bacterial Artificial Chromosome)**
- **Analysis**: BACs are specifically designed to clone large fragments of DNA, typically ranging from 100 to 300 kb. They are derived from the F-plasmid and can accommodate much larger DNA inserts than plasmids or cosmids.
- **Conclusion**: This is the best option for carrying large fragments of passenger DNA into a host cell.

### Final Answer:
The best option for carrying large fragments of passenger DNA into a host cell is **Option 4: BAC (Bacterial Artificial Chromosome)**.

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