Plasmid 

Tsutomu Watanabe: Pioneered research on R-plasmids and antibiotic resistance in bacteria.

Joshua Lederberg: Contributed to the understanding of bacterial conjugation, which is closely tied to plasmid transfer.

Herbert Boyer and Stanley Cohen: Revolutionized molecular biology by developing recombinant DNA technology using plasmids.

1.0Structure of Plasmid

Plasmids are small, circular, double-stranded DNA molecules separate from a cell's chromosomal DNA. These extrachromosomal DNA fragments were first discovered by lederberg (1952). These entities carry genes that offer various genetic advantages to bacteria, including antibiotic resistance. Ranging in size from about one thousand DNA base pairs to several hundred thousand, plasmids vary significantly in length. Plasmid are non-essential for bacteria but may give advantages.

Certain plasmids, such as Col (colicinogenic) factors, are responsible for coding proteins known as colicins, which possess antibiotic properties capable of eliminating other bacteria. In contrast, R factors are a distinct class of plasmids that grant bacteria resistance to antibiotics. These plasmids, both Col and R factors, have the ability to transfer themselves from one bacterial cell to another, enabling rapid dissemination through bacterial populations.

Plasmids that are either connected to the cell membrane or integrated into the bacterial chromosome are termed episomes. These structures possess the capability to exist independently as well as integrate into the host cell's genetic material, offering additional flexibility in their replication and transmission among bacterial cells. This unique feature of episomes contributes to their potential to persist and disseminate genetic information within bacterial populations.

2.0Types of Plasmid

on the basis of conjugation plasmids are classified into conjugative and  non-conjugative respectively : 

Conjugative Plasmid

A conjugative plasmid is a type of extrachromosomal DNA molecule that possesses the capability to transfer itself from one bacterial cell to another through a process known as conjugation. These plasmids carry genes responsible for the synthesis of sex pili and the machinery required for the formation of a physical bridge between donor and recipient bacteria. This bridge allows the direct transfer of the plasmid from the donor to the recipient, promoting the exchange of genetic material, including advantageous traits such as antibiotic resistance genes. Conjugative plasmids play a significant role in the horizontal transfer of genetic information among bacteria.

Non-Conjugative Plasmid

A non-conjugative plasmid is a type of extrachromosomal DNA molecule that lacks the inherent ability to transfer itself from one bacterial cell to another through conjugation. Unlike conjugative plasmids, non-conjugative plasmids do not carry the necessary genes for the synthesis of sex pili and the machinery involved in the direct cell-to-cell transfer process. As a result, these plasmids rely on other mechanisms, such as transformation or transduction, or the assistance of conjugative plasmids for their horizontal transfer between bacterial cells. Non-conjugative plasmids may still carry beneficial genes, but their dissemination within bacterial populations depends on alternative means of genetic exchange.

3.0Special Types of Plasmids

F Plasmid

A fertility plasmid, also known as an F plasmid (F for "fertility"), is a type of plasmid that carries genes responsible for the formation of sex pili and facilitates the transfer of genetic material during a process called conjugation. Conjugation is a mechanism of horizontal gene transfer that allows the direct transfer of DNA from one bacterial cell (donor) to another (recipient).

Degradative Plasmid

A degradative plasmid is a type of plasmid that carries genes encoding enzymes responsible for the degradation of specific substances, often complex organic compounds. These plasmids play a crucial role in the metabolic versatility of bacteria, allowing them to utilize and break down substances that might be present in their environment.

R Plasmid

A resistance plasmid, also known as an R plasmid, is a type of plasmid that carries genes conferring resistance to antibiotics or other toxic substances. These plasmids are particularly significant in the context of bacterial adaptation and survival in environments where these substances are present. The resistance genes on R plasmids encode various mechanisms that enable bacteria to withstand the harmful effects of specific antibiotics, making them an important factor in the development of antibiotic resistance.

Col Plasmid

A colicinogenic plasmid, often referred to as a Col plasmid, is a type of plasmid that carries genes responsible for the production of colicins. Colicins are proteins with antibacterial activity, and they can kill or inhibit the growth of closely related bacterial strains. Colicinogenic plasmids play a role in the competitive dynamics of bacterial populations, providing a selective advantage to the host bacterium.

Virulence Plasmid

A virulence plasmid is a type of extrachromosomal DNA molecule found in certain bacteria, especially pathogenic ones, that carries genes contributing to the virulence of the bacterium. Virulence refers to the ability of a microorganism to cause disease. Virulence plasmids often harbor genes that encode factors involved in the infection process, evasion of host defenses, and the establishment of the bacterium within the host organism.

4.0Plasmid Cloning Vectors

Two main types of vector system are used in cloning one is bacteriophage and other is plasmid. A plasmid vector is a small, circular DNA molecule used as a carrier to introduce foreign genes into a host organism, typically a bacterium. These vectors serve as tools in genetic engineering, allowing the insertion, replication, and expression of specific genes in the host organism. Plasmid vectors are commonly employed in molecular biology and biotechnology for various applications, such as the production of recombinant proteins or the creation of genetically modified organisms.

Here are some key features and components of plasmid vectors:

• Origin of Replication (ori): Plasmids contain an origin of replication, which is a specific DNA sequence that initiates the replication process. This ensures that the plasmid is replicated when the host cell divides.
• Selectable Marker Genes: Plasmid vectors often carry genes that confer resistance to certain antibiotics (e.g., ampicillin or kanamycin). These genes serve as selectable markers, allowing researchers to easily identify and select cells that have successfully taken up the plasmid.
• Multiple Cloning Site (MCS): Also known as a polylinker, the multiple cloning site is a region on the plasmid vector where foreign DNA can be inserted. It contains multiple unique restriction enzyme recognition sites, facilitating the cloning of DNA fragments.
• Promoter and Terminator Sequences: Plasmid vectors typically include promoter and terminator sequences. The promoter initiates the transcription of the inserted gene, while the terminator signals the end of transcription.
• Reporter Genes: In some cases, plasmid vectors may carry reporter genes, such as the green fluorescent protein (GFP), which allows researchers to visually identify cells that have successfully taken up the plasmid.
• Origin of Transfer (oriT): For plasmids involved in conjugation (horizontal gene transfer between bacteria), an origin of transfer is present, facilitating the transfer of the plasmid from one bacterium to another.

pBR322 Plasmid : pBR322 is a well-known plasmid that was widely used in molecular biology research and genetic engineering. The p stands for “plasmid” and BR in the name of two scientists “bolivar” and “rodriguez”. It was one of the first widely used cloning vectors and played a crucial role in the development of recombinant DNA technology. Here are some main characteristics of the pBR322 plasmid:

• Origin of Replication: High-copy-number ori for rapid replication.
• Selectable Markers: Ampicillin resistance (amp^r) and tetracycline resistance (tet^r) for antibiotic selection.
• Multiple Cloning Site (MCS): Polylinker region with unique restriction enzyme sites.
• Beta-Lactamase Gene (bla): Encodes beta-lactamase for ampicillin resistance.
• Tetracycline Resistance Gene (tet^r): Provides resistance to tetracycline antibiotics.
• Size: Approximately 4,361 base pairs; circular DNA.
• Restriction Sites: Multiple unique restriction enzyme recognition sites in the MCS.

pBR322 played a crucial role in the development of recombinant DNA technology, allowing researchers to clone and manipulate genes. Its use laid the foundation for the construction of subsequent plasmid vectors with improved features.

5.0Ti Plasmid

The Ti plasmid, or tumor-inducing plasmid, is a distinctive genetic element found in certain strains of the soil bacterium Agrobacterium tumefaciens. This plasmid is renowned for its unique capability to induce the formation of tumors or galls in the plant tissues it infects. The primary vehicle for this tumorigenic ability is a specific region of the Ti plasmid known as transfer DNA (T-DNA). When Agrobacterium tumefaciens infects plants, it transfers this T-DNA into the plant cells, where it becomes integrated into the host genome. 

Diagram of Ti plasmid :  

Transfer DNA (T-DNA): The Ti plasmid carries a specific region called transfer DNA (T-DNA), which is transferred into the plant cells during the infection process. This T-DNA integrates into the plant genome and is responsible for causing the formation of tumors.

Virulence Genes: The Ti plasmid carries various virulence genes that enable Agrobacterium tumefaciens to infect and transform plant cells. These genes play a critical role in the transfer of T-DNA and the subsequent induction of tumors.

Opine Synthesis: Opines are unique compounds produced by transformed plant cells in response to the presence of the Ti plasmid. Agrobacterium tumefaciens can utilize opines as a nutrient source, and this metabolic interaction is facilitated by opine synthesis.

Genetic Engineering Tool: The Ti plasmid is widely utilized as a tool in plant genetic engineering. Researchers can modify the T-DNA region to introduce specific genes into plants, leading to the expression of desired traits such as insect resistance or enhanced nutritional content.

Binary Vector System: In genetic engineering applications, the Ti plasmid is often used in a binary vector system. This involves two separate plasmids—the Ti plasmid with T-DNA and a helper plasmid containing the necessary virulence genes. The binary vector system enhances the efficiency and precision of genetic modifications.

Selectable Markers: Like other plasmids used in genetic engineering, the Ti plasmid may carry selectable markers such as antibiotic resistance genes. These markers aid in the identification and selection of transformed plant cells.