Why are plasmids often used in laboratory gene manipulation?

• A. They replicate independently of chromosomal DNA.
• B. They are the only DNA in the cell.
• C. They encode the cell's energy metabolism.
• D. They carry viral genomes.

Answer: (A)

Plasmids are small circular DNA molecules that replicate independently of chromosomal DNA. This independent replication lets scientists insert a gene of interest into the plasmid and have it copied many times inside the cell without integrating into the host genome. That means you can produce numerous copies of the gene, study or express it, and recover it easily, all while using selectable markers to maintain cells that carry the plasmid. The cell’s essential genes and energy metabolism stay on the chromosome, not on the plasmid, so plasmids provide a flexible, non-disruptive tool for gene manipulation.