What are Plasmids and Vectors?  

The DNA molecule that exists in a circular shape and is smaller in size which is capable of its replication is called Plasmids. In other words, it is called extra-chromosomal plasmid DNA. Vectors are the molecule which is capable of carrying genetic material which can be transferred into another cell and further carry out replication and expression. Plasmids can act as vectors.  


Plasmids are the smaller and circular DNA molecules or DNA fragments that do not depend on the chromosomal DNA of the cell for their replication. As it is capable of its replication, it is called extrachromosomal DNA. It is naturally surviving in prokaryotes including bacterial cells and few cells of eukaryotes. Plasmids exhibit certain benefits in the bacterial cells by providing antibiotic drug resistance for their survival. These molecules are varied in length starting from 1000 base pairs of DNA sequences to 100s of 1000s of base pairs. Bacteria when undergoing division make sure that its daughter cell contains a copy of all the plasmid available within them. Conjugation also facilitates the plasmid transfer between bacterial cells. This acts as a tool for molecular cloning, gene transfer, and manipulating genes. 

"Extra-chromosomal DNA or plasmid inside bacteria"

Structure of Plasmids  

Most of the plasmids are circular in shape while some exist in linear form as well. The circular shape is achieved with the help of the 2 double-stranded ends which are connected by covalent bonds. The size of the molecule is small which is about a few kilobases to several 100 kilobases. The three important components of plasmid are the origin of replication, multiple cloning sites, and antibiotic resistance genes.  

·  Origin of replication: This is the location in a plasmid that initiates the replication process. It also contains several elements for regulation that contribute to its functions. This particular location in the plasmid contains many A-T base pairs as they are comparatively easier to separate during replication.  

·  Multiple cloning sites: It is otherwise referred to as polylinker. This is most important in experimental studies as it is the DNA's short sequence that provides certain cleavage sites for the restriction enzyme such as EcoRI to perform its role. It helps for both the restriction process and ligates process by facilitating insertion in an easy manner.  

· Antibiotic resistance gene: This provides resistance to certain antibiotics which act as a challenge for treating several diseases.  

· Promoter region: This plays a significant role in producing the desired protein by promoting the transcription process of the gene that has to be targeted.  

· Primer Binding site: The site act as a starting point for the amplification by PCR and verification of sequences.  

·  Selectable marker: This is used for bacterial selection that contains the targeted plasmid. For example, certain antibiotic resistance genes for tetracycline and ampicillin can be used for this purpose.  

"Structure of plasmids"

Types of Plasmid

Plasmids are available in various types such as fertility plasmids, col plasmids, degradative or degenerative plasmids, resistance plasmids, and virulence plasmids.  

Fertility plasmids

Significant in reproduction as they contain genes that are responsible for the sex pilus production and the production of enzymes that are essential for the process of conjugation.  

Resistance plasmids

 It is also called antimicrobial resistance plasmids. This is useful in providing resistance to antibiotics. This is categorized into two categories namely the narrow and the broad host range.  

="Resistance plasmids"

Col plasmid

 Colicines are the toxic proteins produced by bacteria with col plasmids. Shigella, Salmonella, and E. coli are the few bacteria that use these proteins to kill the opponent bacteria. Col plasmids are available in varieties and some examples of col plasmids are Col B, Col E2, and E3.  

Degenerative plasmids

 These plasmids play a vital role in the host to digest or degrade the compounds that xenobiotic. Thus, it is used by researchers for removing various contaminants that pollute the environment. This process of removing polluting substances from the environment with the help of biodegradative microbes is referred to as bioaugmentation.  

Virulence plasmids

 It provides the organism with a series of characteristics which include functions that are virulence in nature. This facilitates their survival with several benefits. These are generally bigger in size and but are available in less quantity. In simple words, it can be stated that these plasmids are responsible for the conversion of bacteria into a pathogen.  

Other plasmids

Apart from these types of plasmids, there are several other plasmids which include Crptic plasmids, recombinant plasmids, metabolic plasmids, suicide plasmids, and conjugative plasmids. Among these plasmids, the Crptic plasmid's function is not identified yet. Recombinant plasmids are used specifically in studies due to their ability to get modified in the laboratories. The metabolic plasmid improves the host's metabolism and the conjugative plasmid helps in the promotion of self-transfer. Suicide plasmids are the ones in which replication is filed when they are shifted from one cell to the other.  

Plasmid Vectors  

Plasmid Vector act as a molecule to carry genetic information from one cell to another or for carrying foreign DNA sequence into another host cell. Its capability of replication on its own and its ease of modification convert them into an efficient tool in life science industries. Almost every molecular biology laboratories work on the plasmids for its construction and designing purpose. The interested gene has to be inserted into a vector which is achieved using various DNA cloning techniques. Ligation independent, gateway, restriction enzyme ligation cloning and Gibson assemble are the techniques facilitating the gene insertion.  

Plasmid vectors can exist in two forms namely cloning vectors and expression vectors. Cloning vectors are significant in the modification, replication, and storage of a certain sequence of genes in a temporary way. Expression vectors are important for the potential transcription process of the vector carried genes in which it is designed in such a way that it includes the region of enhancers and promoters.  

Note: Not all the plasmids are vectors, but all the vectors are plasmids.  

Apart from the role of plasmid vectors, plasmid also exists as reporter plasmids, viral vector plasmids, genome engineering plasmids, and gene knockdown plasmids. Thus, plasmids are playing a vital role not only in producing recombinant DNA, protein synthesis, and protein expression but also in the modeling of diseases, the discovery of drugs, and genome editing as well.     

Context and Applications

This topic is significant in the professional exams for both Bachelors and Master courses. Some of the courses are listed below. 

B.S. Genetics.    

B.S. Biochemistry and Molecular biology    

B.S. Ecology and Evolutionary biology    

M.Sc. Biological science    

Masters in Biotechnology   

Masters in Genetics      

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