Core Concepts

In this article, you will learn to understand what plasmids are, the different types, and their applications in the medical and scientific world.

Topics Covered in Other Articles

  1. Viruses
  2. Central Dogma of Biology
  3. Genetic Code
  4. Transcription Factors

What are Plasmids?

Plasmids are circular DNA pieces, they replicate independently from the host’s chromosomal DNA. Although they can be found in eukaryotes and archaea, they are primarily found in bacteria and unicellular molecules. These molecules, naturally, provide various benefits to the host cell, such as antibiotic resistance and degradative functions. Any natural plasmid has an origin of replication, controls the host range, and has a gene for survival.

Scientists can artificially design plasmids to introduce foreign DNA into any other cell. These lab-made plasmids also have an origin of replication, and in addition, they have a cloning site and selection marker. Bioengineers are the scientists that work on the modifications of any plasmid. Gene cloning and manipulation are carried out with the use of laboratory-made plasmids. We name these plasmid vectors. They are certainly a crucial component of recombinant DNA technology.

These double-stranded DNA molecules generally have a wide range of sizes, from approximately a thousand base pairs to hundreds of thousands of base pairs. Every plasmid in a bacterial cell is duplicated during bacterial division and then passed on to each daughter cell. Moreover, bacteria may exchange plasmids with one another through a process known as conjugation.

Plasmid Conformation

  • Origin of Replication (ORI): a DNA sequence that allows the initiation of replication.
  •  Antibiotic Resistance Gene: helps with the selection of bacteria that contain plasmids.
  •  Multiple Cloning Sites (MCS): a DNA segment that contains restriction sites. These sites allow easier DNA insertion.
  •  Insert: could be a gene, a promoter, or a DNA segment cloned, used for further studies.
  •  Promoter region: specifies the amount of recombinant DNA and the cells on which the gene will be expressed for vectors. On normal plasmids, it drives the transcription to reach target genes.
  •  Selectable Marker: The gene for antibiotic resistance enables bacterial selection.
  •  Primer Binding Site: single-stranded DNA fragment, used as the initiation point for PCR analysis.
plasmids conformation

Plasmids Types

There are lots of ways to classify plasmids, nonetheless a very common one, is to classify them by function. There are five classes.

  • Fertility (F). They contain tra genes, and are able to conjunction, straightaway, resulting in the expression of sex pili.
  •  Resistance (R). Particularly, contain genes that benefit the cell with antibiotic resistance. Also known as the R-factors.
  •  Col. Contains genes that code proteins that kill other bacteria.
  •  Degradative. Enables the digestion of unusual substances like toluene.
  •  Virulence. Turns bacterium into pathogens.

They can also be classified based on their functional groups.

  • RNA plasmids. Plasmids can also be found as single-stranded DNA and, more frequently, as double-stranded RNA molecules, despite the fact that they are typically double-stranded molecules. These molecules are non-infectious, however, it is almost impossible to distinguish an RNA plasmid apart from infectious RNAs.
  •  Chromids. Only found in 10% of bacterial species. Chromids are elements that exist between plasmids and chromosomes. They have a plasmid-like replication system, carry core genes, and encode their DNA similarly to chromosomes. Occasionally named megaplasmids or minichromosomes.

Plasmids Applications

As previously mentioned, vectors are plasmids created in a lab by genetic engineering. These vectors have great applications for biotechnology and genetic studies. We will learn about the most popular uses of vectors.

Gene Therapy

Vectors are used for gene transfer in order to express proteins that are absent from some cells. The insertion of pre-selected genes into the human genome is specifically necessary for several types of gene therapy. In this mechanism plasmids aid in the delivery of the precise therapeutic gene to the injured cell in this process. There are no mutations or immunological reactions.


Plasmid vectors are the most common way of bacterial cloning. These vectors concurrently feature a multiple cloning site that employs restriction sites and allows for the insertion of DNA. All plasmids are then delivered to the bacterium by a procedure known as transformation once the gene has been incorporated. These bacteria can survive thanks to the selectable marker that plasmids provide, that grants the bacteria the ability of antibiotic resistance. Cells are exposed to a selective medium after the transformation process, where antibiotics operate as a filter, remaining just the bacteria that have plasmid DNA.

plasmids cloning cycle

Protein synthesis

Proteins are synthesized in large quantities using vectors. This is a cheap way to create large masses of specific proteins that code for certain genes, just like insulin. It is possible to cause the bacteria to create a lot of proteins from the inserted gene, similar to how it produces proteins to transmit its antibiotic resistance.