The term plasmid was coined by Joshua Lederberg. In microbiology, a plasmid is an extrachromosomal genetic element found in many bacterial species. Plasmids are circular DNA molecules that replicate independently of the chromosome of the bacteria. They aren’t required for the bacterium’s survival, but they may provide a selection advantage. Colicinogenic factors are a type of plasmid that controls the creation of proteins called colicins, which have antibiotic properties and can kill bacteria. R factors are a type of plasmid that provides antibiotic resistance to bacteria. Some Col factors and R factors can move from one cell to another, allowing them to spread quickly throughout a bacterial population. An episome is a plasmid that is linked to the cell membrane or integrated into the bacterial chromosome. Plasmids are incredibly useful tools in molecular biology and genetics, particularly in the field of genetic engineering. They are essential in gene cloning, recombinant protein manufacturing (for example, human insulin), and gene therapy research.

Plasmid Structure

Plasmids are non-essential extrachromosomal elements. They are beneficial, but they are not found in every organism of the species. Plasmids are not part of the genome, and the same plasmid can exist in various species and be transmitted from one to the other. Plasmids have their own replication origin (ORI), and they reproduce with the cell, so that each daughter cell has a copy of the plasmid as well. Aside from the replication origin, it frequently contains genes for antibiotic resistance, toxin synthesis, and other valuable genes that may be essential for cell survival.

Characteristics of Plasmid

The following are the characteristics of a plasmid:

• At least one gene is usually always carried by plasmids.
• Plasmids can also make bacteria capable of nitrogen fixation.
• Plasmids can be found in different numbers in a single cell, ranging from one-to-many hundreds.
• Larger plasmids have fewer copies than smaller plasmids.
• There are some plasmids that have no effect on the phenotypic of the host. Cryptic plasmids are the name for these forms of plasmids.

Types of Plasmids

There are 5 kinds of plasmids namely:

• Resistance Plasmids
• Virulence Plasmids
• Degradative Plasmids
• Col Plasmids
• Fertility F Plasmids

Plasmid Vector

In DNA recombinant technology, plasmids and bacteriophages are widely utilized as cloning vectors.

• Plasmids are a useful tool in genetic engineering and biotechnology because they are simple to modify and proliferate.
• Plasmids are created artificially in the lab for genetic engineering reasons.
• A replication origin, cloning site, and selection marker are all present in the lab-grown plasmids that are utilized as a vector.
• Restriction enzymes (molecular scissors) are used to cut DNA at certain locations, resulting in sticky ends.

By recombining DNA segments as required and inserting them into the bacteria cell to get the desired protein, Herbert Boyer and Stanley Norman Cohen jointly discovered recombinant DNA technology.

• DNA Ligase is then used to introduce the desired genes.
• The recombinant DNA molecule is subsequently transformed and injected into the host bacteria cell.
• Using host DNA polymerase, the recombinant plasmid multiplies.
• The plasmid pSC101 from Salmonella typhimurium was the first to be utilized as a cloning vector. They demonstrated that a frog gene can be expressed in a bacterial cell.
• The plasmid from E. coli is commonly used as a cloning vector.

Ti Plasmid

In the bacteria Agrobacterium tumefaciens, the Tumour inducing or Ti plasmid is found.

It’s now commonly employed as a cloning vector to transfer desired genes to the host plant, resulting in transgenic plants. The following are the major properties of Ti plasmid:

1. Ti plasmids are classified according to the genes that code for various opiates, such as leucinopine, nopaline, and octopine.
2. Many dicotyledonous plants are pathogenic to this species. Crown gall disease is caused by it in plants.
3. By introducing a DNA fragment known as T DNA into normal cells, Agrobacterium tumefaciens can turn them into tumor cells and begin generating chemicals that the bacterium needs.
4. It loses its pathogenic potential after introducing the desired gene onto Ti plasmid, but it can still insert the desired gene into the plant cell.
5. It comprises vir or virulence genes, which transmit the T-DNA region to plant cells and incorporate it into the genome of the plant.
6. The Ti plasmid can be tweaked to insert the desired genes if necessary.
7. The bacterium Agrobacterium tumefaciens has been dubbed “nature’s genetic engineer.”

CONCLUSION

Plasmids come in a variety of sizes, ranging from a few thousand DNA base pairs to hundreds of thousands. When a bacterium divides, all of the plasmids in the cell are duplicated, giving each daughter cell a copy of each plasmid. Bacteria can also exchange plasmids through a process known as conjugation.Scientists have used plasmids as a tool for cloning, transferring, and modifying genes. Vectors are plasmids that are used in research for these purposes. By adding DNA fragments or genes into a plasmid vector, researchers can make a recombinant plasmid. This plasmid can be introduced into bacteria via the transformation process. Bacteria can therefore be employed as factories to copy DNA fragments in vast quantities since they divide quickly.