Plasmids

Plasmids are small, circular, double-stranded DNA molecules found in bacteria that replicate independently of chromosomal DNA and often carry antibiotic-resistance genes.

• Plasmids are small, circular, and duplex DNA molecules.
• Their natural function is to provide antibiotic resistance to the host cell.
• They replicate independently from the bacterial DNA and can exist as single or multiple copies within the bacterium.
• They are smaller than the host chromosome, which makes them easily separable from it.
• The complete DNA sequence of many plasmids is already known, which provides the precise location of restriction enzyme cleavage sites for inserting foreign DNA.

• Replication origin: The site responsible for plasmid synthesis.
• Promoter region: The site for transcription initiation.
• Recombination site: The specific location where the insert gene is placed.
• Recognition sites: Areas for endonuclease binding that surround the recombination site.
• Antibiotic resistance gene: Used to select transgenic cells.
• Repressor sequence: Found in some plasmids only.

• Cutting the DNA: Both the foreign DNA (gene to be inserted) and the plasmid DNA are cut into specific fragments using restriction enzymes.
• Joining the DNA: Annealing (joining) occurs between the foreign DNA and the plasmid, and they are sealed together using the enzyme DNA ligase.
• Recombinant DNA: The product of this recombination is known as recombinant DNA, containing the cloned (foreign) gene.
• Transformation: The plasmid carrying the cloned DNA insert is put back into a suitable recipient bacterium, such as E. coli.
• Multiplication and Isolation: The recipient bacterium is grown in large quantities, allowing the recombinant plasmid to be isolated.
• Release: The desired foreign genes can be readily released from the recombinant plasmid by cutting it again with the specific restriction enzyme. This technology allows for large quantities of foreign genes to be isolated.

• Plasmids are small, circular, double-stranded DNA molecules that replicate independently of the host bacterial chromosome and naturally confer antibiotic resistance.
• Due to their manageable size and precisely mapped DNA sequences, plasmids serve as highly effective and easily isolatable vectors for genetic cloning.
• A typical plasmid vector includes critical functional regions such as an origin of replication, recognition sites for endonucleases, and antibiotic resistance genes for selection.
• Recombinant DNA is created by cutting both the target foreign DNA and the plasmid with specific restriction enzymes and joining them together using DNA ligase.
• This newly formed recombinant plasmid is then inserted into a host bacterium, which multiplies rapidly to produce large quantities of the desired foreign gene.