BiotechnologyHigh NEET PriorityClass 12
Plasmid
Definition
A plasmid is a small, circular, double-stranded DNA molecule found in bacteria and some eukaryotes. It replicates independently of chromosomal DNA. Plasmids are commonly used in genetic engineering as vectors to introduce foreign DNA into host cells and produce recombinant proteins.
Key Points for NEET
- 1Small circular DNA, separate from chromosomal DNA
- 2Self-replicating (has origin of replication)
- 3Often carries antibiotic resistance genes
- 4Used as vectors in recombinant DNA technology
- 5Examples: pBR322, pUC series
Example
pBR322 plasmid used to clone insulin gene for producing human insulin
Asked in NEET
NEET 2024NEET 2023NEET 2022
Common Mistakes to Avoid
- ✗Confusing plasmids with chromosomal DNA — plasmids are extrachromosomal, circular, and self-replicating
- ✗Thinking all bacteria have plasmids — plasmids are optional, not essential for bacterial survival
- ✗Forgetting that plasmid vectors need selectable markers (antibiotic resistance genes) for screening
Quick Revision Notes
- ⚡pBR322: first artificial plasmid vector; has ampR and tetR genes + multiple restriction sites
- ⚡Good vector needs: origin of replication, selectable marker, restriction sites, small size
- ⚡Insertional inactivation: foreign DNA inserted into antibiotic resistance gene → gene disrupted → screening
- ⚡Ti plasmid (Agrobacterium): natural plant genetic engineering vector; T-DNA integrates into plant genome
Related Terms
VectorRecombinant DNACloningBacteriaGene Transfer
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