Plasmid

Plasmids are small, circular DNA molecules present outside the main chromosome. They are called extra-chromosomal because they exist separately from the organism’s genome. Despite not being essential for survival, they often provide helpful traits to the host. Plasmids are transferred between bacteria through various modes of horizontal transfer. This makes them important in sharing traits like antibiotic resistance.

Alt-text: Plasmid in Bacteria

Structure of Plasmids

Plasmids are double-stranded DNA loops typically spanning several kilobases. Some plasmids are linear, but most are circular. The ends of the strands are joined by covalent bonds, giving the looped shape. Their size can range from 1 kb to over 200 kb.

Main Components of a Plasmid

The important components of a plasmid are as follows:

Origin of Replication (ORI)

This is the place where plasmid replication begins. It usually has A-T rich regions that open easily for the replication process.

Polylinker or Multiple Cloning Site (MCS)

This short DNA section has many restriction sites. These sites allow the insertion of foreign DNA using enzymes called restriction enzymes.

Antibiotic Resistance Gene

These genes help bacteria survive in the presence of antibiotics. They are used in labs to identify cells that have taken up the plasmid.

Promoter Region

It helps in starting the transcription of inserted genes.

Primer Binding Site

A specific sequence on the plasmid where primers anneal to initiate DNA replication, PCR amplification or sequencing.

Types of Plasmids

Plasmids are grouped by the functions they provide:

Resistance Plasmids: Carry genes that help fight antibiotics.
Fertility Plasmids: Help in the transfer of DNA between bacteria.
Virulence Plasmids: Allow bacteria to infect or harm hosts.
Degradative Plasmids: Break down unusual substances.
Col Plasmids: Produce proteins that kill other bacteria.

Plasmids as Vectors

Plasmids are commonly used as vectors in genetic engineering. A vector is a DNA molecule that carries foreign genetic material into a host cell. Not all plasmids are vectors.

Plasmid vectors are typically engineered to be small for easy handling for research purposes. They must contain an ORI, an MCS, and a marker gene. These elements help in cloning and tracking the inserted gene. Due to their features, plasmids are ideal for copying genes in large numbers.

Key elements of a Plasmid Vector

Vector ElementFunction

Origin of Replication (ORI)Enables plasmid replication in host cells

Selectable MarkerFor selecting bacteria containing the desired plasmid

Multiple Cloning Site (MCS)Allows insertion of foreign DNA

Promoter RegionPromotes transcription of the target gene

Primer Binding SiteStart point for PCR Amplification and Sequence Verification

Popular Cloning Vectors

pBR322 and Ti plasmid are among the popular cloning vectors. Here are the insights into the same:

pBR322 Plasmid

pBR322 is a well-known E. coli plasmid vector. It has genes for resistance to ampicillin and tetracycline. The plasmid also includes an ORI and many restriction sites. These features allow scientists to insert foreign DNA and select successful clones.

Ti Plasmid

Ti plasmid is found in Agrobacterium tumefaciens, a bacterium that causes tumours in plants. It is used to create transgenic plants. The Ti plasmid inserts a DNA segment (T-DNA) into plant cells. Scientists replace the disease-causing genes with useful ones. This modified plasmid can transfer DNA, but no longer causes harm.

Ti plasmids also have vir genes, which help move the T-DNA into the plant genome. Different Ti plasmids can produce different plant compounds called opines. This tool is very important in plant genetic engineering.

Recombinant Plasmids

A recombinant plasmid has foreign DNA inserted into it. This plasmid is made in the lab for cloning or protein production. The foreign DNA is joined using DNA ligase, and the recombinant plasmid is then introduced into bacteria by a process called transformation.

Cells with recombinant plasmids are selected using antibiotic resistance or colour-based markers. For example, a disrupted gene may stop enzyme production, helping to find recombinant cells.

Advantages of Using Plasmids

The benefits of using plasmids in research and gene transfer are as follows:

Easy to isolate due to small size
Can replicate on their own
Help in gene transfer and cloning
Useful in large-scale DNA or protein production
More stable due to their circular shape

Summary

Plasmids are small, circular, extra-chromosomal DNA molecules that replicate independently. They often carry beneficial genes like antibiotic resistance and are widely used as vectors in genetic engineering. Key components of a plasmid include ORI, selectable markers, and MCS. The different types of plasmids are resistance, fertility, and virulence plasmids. Popular vectors like pBR322 and Ti plasmids help in cloning and creating transgenic organisms.

FAQs

Q1. What are the applications of plasmids?

Plasmids are used for protein production, like insulin, to introduce foreign genes like the Bt toxin gene into crops for pest resistance, delivering therapeutic genes and to study gene expression in cells.

Q2. How do plasmids contribute to antibiotic resistance spread in microbial populations?

Plasmids often carry resistance genes and can be transferred via conjugation or transformation. It allows rapid transfer of resistance traits across different bacterial species.

Q3. What is plasmid copy number?

Plasmid copy number refers to the average number of copies of a plasmid present in a single bacterial cell. High copy number facilitates better expression of genes.