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What is a plasmid?

Plasmids are the small circular or linear double-stranded DNA molecules (ranging from a few to several hundred kilobases) found in bacterial cells but also present naturally in archeae and eukaryotes. The plasmids are not packaged inside a chromosome, and have no distinct 5’ or 3’ beginning or end. They can autonomously replicate independently of the chromosome inside the cell, and provide one or more benefits to the host such as resistance to antibiotics, degradative functions, and virulence. Plasmids also have the ability to transfer from one cell to another during the process of bacterial conjugation (contact between cells followed by transfer of DNA content), sometime spreading genes that are advantageous for survival. Horizontal gene transfer plays a critical role in bacterial evolution. As horizontal spread of antibiotic resistance genes (ARGs) among bacteria is driven by plamids, plasmids need to be well monitored and analyzed using methods like complete plasmid DNA sequencing after plasmid extraction.

Figure 1. Plasmids are present in prokaryotes.

The structure of a plasmid

Although a number of plasmids have a covalently closed circular structure, some plasmids are linear. The plasmids are usually artificial and have at least an origin of replication (OR), antibiotic resistance gene (ARG) and multiple cloning site (MCS).

Figure 2. The major elements of plasmids.

Table 1. Common elements of plasmids.

Element

Description
Origin of Replication (OR) A specific location largely composed of A-T base pairs allows initiation of replication within a plasmid.
Selection Marker Antibiotic resistance gene (ARG) plays an important role in drug resistance allowing for selection of bacteria with plasmids. In addition to ARGs, some plasmids also have other selection markers in nature or by artificial introduction.
Multiple Cloning Site (MCS) / Polylinker Contain several sites for cleavage by restriction enzymes allowing for easy insertion of DNA.
Promoter Region A region is involved in recruiting transcriptional machinery.
Primer Binding Site

A short DNA sequence on a single strand is typically used for PCR amplification or DNA sequencing.

What are plasmids used for?

Plasmids are commonly adopted in the life sciences as vectors for introduction of foreign DNA into another cell. The ability of plasmids to self-replicate within a cell and the ease of modifying plasmids make them attractive tools for the biologists or bioengineer. Nearly all molecular biology labs need to design, modify and construct a plasmid. A gene of interest is typically inserted into a vector through different cloning methods such as restriction enzyme ligation, ligation independent cloning, Gateway cloning and Gibson assembly. How the cloning method is chosen depends on the plasmid. Following the cloning process, the reconstructed vector containing a gene of interest are transformed into bacterial cells and selectively grown on plates with antibiotics.

Plasmids can be used as clonal vectors primarily for replication, modification, and temporary storage of a specific desire gene sequence, or as expression vectors for efficient transcription of the gene carried on the vector that is engineered to include promoter regions and enhancers. Other types of plasmids include gene knock-down plasmids, reporter plasmids, genome engineering plasmids and viral plasmids. Plasmids are widely used for experiments including fluorescent imaging, DNA recombination, protein expression, mass protein production, disease modeling, drug discovery, as well as genome editing.

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