- A plasmid is a small, circular, extrachromosomal double stranded DNA that has a capacity to replicate independently.
- Discovered by Laderberg in 1952.
- Naturally occur in bacteria
- The genes carried in plasmid benefits the survival of the organism by providing them with genetic advantages like antibiotic resistance, etc.
- Replicate independently and code for their own transfer.
- Sizes ranges from 1 Kbp to several mbp.
- Some produces virulence factor that help in defense and nutrient utilization.
- Plasmids can also provide bacteria with the ability to fix nitrogen.
Elements of Plasmids
|origin of Replication (ORI)||It is the DNA sequence which directs initiation of plasmid replication by recruiting bacterial transcriptional machinery.|
|Antibiotic Resistance Gene||These genes allows for selection of plasmid containing bacteria by providing a survival advantage to the bacterial host.|
|Multiple Cloning Sites (MCS)||Short segment of DNA which contains several restriction sites allowing for easy insertion of DNA. In expression plasmids, the MCS is often downstream from a promoter.|
|Insert||It is a foreign DNA cloned into the multiple cloning sites.|
|Promoter region||Drives transcription of the target gene.|
|Selectable marker||It is used to select for cells that has successfully taken up the plasmid for the purpose of expressing the insert.|
|Primer binding site||A short single-stranded DNA sequence used as an initiation point for PCR amplification or sequencing. Primers can be exploited for sequence verification of plasmids.|
- Nicked open circular DNA
- Relaxed circular DNA
- Linear DNA
- Supercoiled or covalently closed circular DNA
- Supercoiled denatured DNA
- Nicked open circular DNA: it has one strand cut. A double stranded circular DNA molecule that has been nicked in one of the strands to allow the release of any super helical turns present in the molecule. The open circular form migrates more slowly during gel electrophoresis than a covalently closed circular molecule of the same size due to associated differences in conformations, or shape, of the molecule. Or Plasmid with one DNA strand cut or “nicked”; this releases the supercoiling and leaves a large, floppy circle with slow mobility in agrose.
- Relaxed circular DNA: when the DNA helix has the normal number of base pairs per helical turn, it is in the relaxed state. DNA is fully intact with both strands but supercoiling is removed.
- Linear DNA: plasmid with cut in both strands at same place. Therefore, it has free ends. Usually migrate between nicked circular DNA and supercoiled forms.
- Supercoiled or covalently closed circular DNA: DNA is fully intact with both strands uncut, and with a twist built in, resulting in a compact form.
- Supercoiled denatured DNA: small quantities occur following excessive alkaline lysis; both strands are uncut but are not correctly paired, resulting in a compacted plasmid form.