Plasmid - Structure, Types, Characteristics, Functions and Uses

Plasmid: A plasmid is a small, circular, double-stranded DNA molecule distinct from a cell's chromosomal DNA. Plasmids naturally occur in bacterial cells and are also found in some eukaryotes. These genetic structures often carry genes that provide bacteria with specific advantages, such as antibiotic resistance. Plasmids vary in size, ranging from approximately one thousand to hundreds of thousands of DNA base pairs. During cell division, plasmids are replicated so that each daughter cell receives a copy.

What is Plasmid?

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Plasmid Structure

Mammary Glands

Types Of Plasmid

1. Resistance Plasmids (R plasmids): These plasmids carry genes that provide resistance to antibiotics, disinfectants, and other harmful substances. The spread of resistance plasmids is a major concern in public health.
2. Virulence Plasmids: These plasmids contain genes that increase the pathogenicity of bacteria. They may encode toxins or proteins that help bacteria adhere to host cells.
3. Degradative Plasmids: These plasmids carry genes that encode enzymes allowing bacteria to degrade unusual compounds, such as pollutants or pesticides.
4. Col Plasmids: Col plasmids carry genes that encode bacteriocins, proteins that kill other bacteria, giving the host bacteria a competitive advantage.
5. Fertility F Plasmids (F plasmids): These plasmids carry genes involved in bacterial conjugation, allowing bacteria to transfer genetic material to each other.

Okazaki Fragments

Plasmid Characteristics

1. Extrachromosomal Nature: Plasmids are small, circular DNA molecules that exist outside the bacterium's chromosomal DNA. They are not part of the bacterium's essential genetic material.
2. Independent Replication: Plasmids can replicate independently of the chromosomal DNA. They possess their own origin of replication (ori) which initiates the replication process.
3. Small Size: Plasmids are considerably smaller than chromosomal DNA, typically ranging from 1,000 to 200,000 base pairs in length. In contrast, chromosomes can be millions or even billions of base pairs long.
4. Carriage of Genes: Plasmids often carry genes that provide advantageous traits to the host bacterium. These genes can code for functions such as antibiotic resistance, toxin production, or the ability to metabolize unusual compounds.
5. Transferability: Plasmids can be transferred from one bacterium to another through conjugation, enabling bacteria to share genes and potentially acquire new characteristics.
6. Variable Copy Number: The number of copies of a plasmid within a bacterial cell can vary. Some plasmids exist in low copy numbers (one or two copies per cell), while others can exist in high copy numbers (hundreds or even thousands of copies per cell).

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Plasmid Function

1. Antibiotic Resistance: Plasmids frequently contain genes that provide resistance to antibiotics. These genes may encode enzymes that break down antibiotics or modify bacterial cell walls to prevent antibiotic entry.
2. Environmental Adaptation: Plasmids can help bacteria survive in harsh conditions, such as extreme temperatures, high salt concentrations, or exposure to toxic substances. Some plasmids carry genes encoding enzymes that detoxify heavy metals.
3. Reproductive Functions: Certain plasmids contain genes involved in bacterial reproduction, including those that regulate conjugation, the process of genetic material transfer between bacteria.
4. Virulence: Virulence plasmids encode genes that contribute to bacterial pathogenicity, enabling them to cause disease. These plasmids may contain genes encoding toxins or proteins that aid in the adherence of bacteria to host cells.

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Plasmid Examples

1. pUC19: This is a commonly used cloning vector in molecular biology. It carries genes for ampicillin resistance and the enzyme beta-galactosidase, which allows for the easy selection and identification of transformed bacteria.
2. pBR322: Another widely used cloning vector, pBR322 carries genes for resistance to ampicillin and tetracycline. It also contains a multiple cloning site (MCS) that allows for the insertion of DNA fragments for cloning.
3. pGLO: This plasmid is used in biotechnology to study gene expression. It carries the gene for green fluorescent protein (GFP), which can be easily visualised under UV light when expressed in bacteria.
4. Ti plasmid: Found in the soil bacterium *Agrobacterium tumefaciens*, the Ti plasmid is responsible for causing crown gall disease in plants. It carries genes that allow the bacterium to transfer a portion of the plasmid (T-DNA) into the plant's genome, leading to the formation of tumours.
5. R plasmids: These plasmids carry genes that confer resistance to antibiotics. They can be transferred between bacteria, leading to the spread of antibiotic resistance genes.
6. ColE1: A high-copy-number plasmid commonly used for the expression of recombinant proteins in bacteria. It replicates quickly and efficiently, allowing for the production of large amounts of protein.

Sexual Reproduction In Flowering Plants

Plasmid Vector

1. Origin of replication (ori): Plasmid vectors possess an origin of replication, which is a specific DNA sequence enabling the plasmid to replicate independently from the host cell's chromosome. This ensures the replication of the foreign DNA carried by the plasmid when the host cell divides.
2. Selectable markers: Plasmid vectors often contain genes that provide resistance to antibiotics, such as ampicillin or kanamycin. These markers facilitate the selection of cells that have incorporated the plasmid, as only those cells survive in the presence of the antibiotic.
3. Multiple cloning site (MCS): The MCS is a segment of the plasmid vector containing various unique recognition sites for restriction enzymes. These sites allow for the straightforward insertion of foreign DNA into the plasmid using restriction enzymes and DNA ligase.
4. Promoter and reporter genes: Plasmid vectors may include a promoter region, which is a DNA sequence initiating the transcription of the inserted gene. Additionally, they may carry reporter genes, such as the green fluorescent protein (GFP) gene, enabling the visual detection of gene expression in the host cell.
5. Size and copy number: Plasmid vectors come in different sizes and copy numbers. Some are high-copy-number vectors, meaning they replicate extensively within the host cell, while others are low-copy-number vectors, replicating at a lower rate. The choice of vector depends on the specific application and experimental requirements.

Plasmid Uses

1. Gene Cloning
2. Gene Expression Studies
3. Creation of Genetically Modified Organisms (GMOs)
4. Production of Recombinant Proteins
5. Studies on Drug Resistance
6. Gene Therapy