Which of the following is used in Genetic Engineering?

Plasmids are small, circular, double-stranded DNA molecules found mainly in bacteria. They sit apart from the chromosomal DNA and replicate on their own, which is why they work as vectors in genetic engineering. A gene of interest is spliced into a plasmid, the recombinant plasmid is introduced into a host cell, and the cell then copies and expresses the foreign gene. The Ti plasmid of Agrobacterium tumefaciens is the classic tool for plant transformation, and pBR322 is a standard laboratory cloning vector.

Why the others fail:

• Plastids (chloroplasts, chromoplasts) are plant organelles with their own DNA. They are used in chloroplast transformation research but are not the standard vector.
• Mitochondria are energy-producing organelles and are not used as routine cloning vehicles.
• Ribosomes are protein-synthesis machinery made of rRNA and proteins. They carry no cloning DNA and are not a genetic engineering tool.

Core concept: vectors in recombinant DNA technology. This links to the Biotechnology section of GS Paper III and to Mains themes such as GM crops and biosafety.

Trap: Candidates pick plastid or mitochondria because both have their own DNA. The test is whether the element is a vector that carries foreign DNA into a host cell, and plasmids are the answer.