Plasmid
A plasmid is a small, circular, double-stranded DNA molecule found in bacteria and some eukaryotes that replicates independently of chromosomal DNA. Plasmids typically carry accessory genes — such as antibiotic resistance genes — that confer a selective advantage to the host cell. In biotechnology, plasmids are widely used as vectors to clone, transfer, and express foreign genes in host organisms.
Common Types of Plasmids and Their Features
| Plasmid Type | Size (kb) | Copy Number | Key Feature | Common Use |
|---|---|---|---|---|
| Resistance (R) plasmid | 80–100 | 1–3 (low) | Antibiotic resistance genes | Selective marker in cloning |
| pUC19 | 2.7 | 500–700 (high) | lacZ reporter gene | Blue-white colony screening |
| Ti plasmid | 200 | 1–2 | T-DNA region | Plant transformation |
| 2-micron plasmid | 6.3 | 40–60 | Yeast origin of replication | Yeast expression vectors |
| Cosmid | 40–50 | 5–10 | cos sites for phage packaging | Genomic library construction |
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Related Terms
Recombinant DNA
Recombinant DNA (rDNA) is artificially engineered DNA formed by joining sequences from two or more different organisms using molecular biology techniques such as restriction endonucleases and DNA ligase. The resulting hybrid molecule can be introduced into a host cell where it replicates and, if properly constructed, directs the synthesis of a desired protein. rDNA technology underlies the production of medicines such as human insulin, erythropoietin, and growth hormone.
Gene Cloning
Gene cloning is the process of making multiple identical copies of a specific gene or DNA fragment by inserting it into a vector (such as a plasmid or bacteriophage) and replicating it inside a host organism, typically Escherichia coli. The technique involves cutting both the target DNA and the vector with the same restriction enzyme to generate compatible sticky ends, ligating them with DNA ligase, transforming the recombinant construct into host cells, and selecting colonies that contain the insert. Gene cloning is foundational to modern biotechnology and medicine, enabling the production of recombinant proteins (such as insulin), gene therapy constructs, diagnostic probes, and transgenic organisms.
From Greek "plasma" (something formed or moulded) + "-id" (diminutive suffix). The term was coined by Joshua Lederberg in 1952 to describe extrachromosomal genetic elements that could replicate autonomously.