What is true of plasmids?

Options

A. They are found in viruses
B. They are main parts of chromosomes
C. They are widely used in gene transfer
D. They contain genes for vital activities

Answer

**Step-by-Step Solution:**

1. **Understanding Plasmids**: Plasmids are small, circular, double-stranded DNA molecules that are distinct from a cell's chromosomal DNA. They are primarily found in prokaryotic organisms, such as bacteria, and are not part of the chromosomal DNA.

2. **Analyzing the Statements**:
- **Statement 1**: "They are found in viruses."
- This statement is false. Plasmids are not found in viruses; viruses consist of a protein coat and either DNA or RNA, but not plasmids.

- **Statement 2**: "They are the main part of the chromosome."
- This statement is also false. In bacterial cells, the main part of the chromosome is the nucleoid (or genophore), while plasmids are separate entities that exist in the cytoplasm.

- **Statement 3**: "They are widely used in gene transfer."
- This statement is true. Plasmids can act as vectors in genetic engineering, allowing for the transfer of genes between organisms. They have a high copy number and can carry additional genes, making them useful in biotechnology.

- **Statement 4**: "They contain the vital genes for the vital activities required for the cell."
- This statement is false. Plasmids typically carry non-essential genes that provide additional traits, such as antibiotic resistance or fertility factors, but they are not vital for the basic life processes of the cell.

3. **Conclusion**: Based on the analysis, the only true statement about plasmids is that they are widely used in gene transfer (Statement 3).

**Final Answer**: The true statement about plasmids is that they are widely used in gene transfer.

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