Source: Wikimedia Commons — pBR322 plasmid map showing key features
Feature Labels and Functions
| Feature | Position (approx.) | Function | NEET Significance |
|---|---|---|---|
| ori (origin of replication) | ~2535 bp | Initiates autonomous plasmid replication in E. coli | Without ori, plasmid cannot replicate |
| ampR gene | ~3293-4153 bp | Encodes β-lactamase (degrades ampicillin) | Selectable marker — all transformants grow on amp |
| tetR gene | ~86-1276 bp | Encodes tetracycline efflux pump | Site of insertional inactivation |
| BamHI site | Within tetR | 6-bp recognition site (GGATCC) | Cloning site in tetR for insertional inactivation |
| SalI site | Within tetR | 6-bp recognition site | Cloning site in tetR |
| EcoRI site | Near tetR/ori junction | 6-bp recognition site (GAATTC) | Frequently tested restriction site |
| PvuI, PstI sites | Within ampR | Cloning sites in ampR | Can be used for insertional inactivation of ampR |
Key Relationships:
- Insert in tetR → tetR disrupted → Recombinant grows on ampicillin only
- Insert in ampR → ampR disrupted → Recombinant grows on tetracycline only (rare use)
- Insert in ori → plasmid cannot replicate → no colonies
- Both markers intact → Non-recombinant → grows on both antibiotics