Plasmids

 Plasmids

Summary

• Plasmids are small, free DNA molecules, mostly occurred in prokaryotes.
• They are essential for microbial adaptation, gene transfer, and antibiotic resistance.
• Plasmids are essential to biotechnology for cloning, gene therapy, and vaccine manufacturing.
• Addressing issues related to global health and the environment requires an understanding of plasmid biology.

Keywords

Plasmid, DNA, Bacteria, Prokaryotes, Eukaryotes, Circular DNA, Double-stranded DNA, Origin of replication (ori), Copy number, Horizontal gene transfer, Antibiotic resistance.

Definition

Small, circular, double-stranded DNA molecules called plasmids occur in cells apart from the chromosomes. Although they are most frequently found in bacteria, they may also be found in some eukaryotic creatures and archaea.

Plasmids, in contrast to the bacterial chromosome, can confer selection benefits like antibiotic resistance but are not necessary for the host's survival in typical circumstances.

Historical Background

Plasmids were discovered in the 1940s, but the term "plasmid" was first introduced by Joshua Lederberg in 1952. Plasmids were first investigated in the field of bacterial genetics, but they attracted a lot of interest as molecular biology and biotechnology advanced, especially with the invention of recombinant DNA technology in the 1970s.

Structural Characteristics

Shape

Mostly circular (though some linear plasmids exist).

Size

Varies from 1 Kb (kilobase pairs) to over 200 Kb.

Replication

Possess an origin of replication (ori), allowing them to replicate independently.

Copy Number

Can be low (1–5 copies per cell) or high (10–100s of copies per cell).

Coding Capacity

May carry a few to several genes, typically related to survival advantages.

Types

Plasmids are categorized according on the roles that the genes they contain play:

Plasmid Type	Explanation
Fertility (F) plasmid	Contain tra genes that are essential for facilitating bacterial conjugation or genetic transfer. E.g., the F plasmid in E. coli.
Resistance (R) plasmid	Offer defense against poisons, heavy metals, or antibiotics. Contribute to harmful bacteria's resistance to many drugs
Virulence Plasmid	Possess genes that increase the host's pathogenicity. Present in Yersinia, Shigella, and Salmonella species
Degradative Plasmid	Encode enzymes that break down uncommon materials, such as insecticides and toluene. Permit bacteria to endure under hostile conditions.
Col Plasmid	Make bacteriocins, which are proteins that kill other bacteria, such as colicins.
Cryptic Plasmid	Plasmids that don't encode recognized characteristics and have an unknown purpose

Significance

• Plasmids encourage genetic diversity and adaptation, which increases genetic flexibility.
• Through conjugation, transformation, or transduction, horizontal gene transfer plays a significant role in the dissemination of characteristics like antibiotic resistance.
• Commonly employed as vectors in genetic engineering for the creation of vaccines, gene expression, and cloning
• Degradative plasmids aid in the removal of environmental contaminants through bioremediation.
• Managing antibiotic resistance and creating antimicrobial therapies depend on an understanding of plasmids.

Applications in Research and Biotechnology

Cloning Vectors

pBR322, pUC19, and other engineered plasmids are utilized to introduce foreign DNA into host organisms.

Protein Expression

Plasmids are used to express proteins like growth hormones, insulin, or enzymes.

Gene Therapy

Therapeutic genes are delivered into human cells using plasmid-based vectors.

DNA Vaccines

Plasmids that contain pathogen genes to elicit immunological responses.

Mechanisms of Plasmid Transfer

• Conjugation: Direct transfer of plasmid between two bacteria through pilus.
• Transformation: Taken up free plasmid from the environment.
• Transduction: Plasmid transfer occurs through the use of a bacteriophage acting as a vector.

Regulation and Replication

Regulatory components found in plasmids include:

• Replication origin (ori): Starts the replication of DNA
• Regulatory genes: Control the number of plasmid copies maintained within the cell.
• Partitioning systems: Make sure that cells are properly segregated during division.

Challenges with Plasmids

• If plasmid stability is not adequately maintained, it may be lost during cell division.
• The dissemination of resistance genes poses a major challenge to public health.
• Plasmids with a lot of copies might cause host cells stress.