What are plasmids? functions, definition, and types

Plasmids in bacteria and other cells have a significant function in bioengineering. The meaning of plasmids, their functions, and their types will be explained in this article.

Table of Contents

Definition of plasmids in biology

Plasmids are small extrachromosomal DNA molecules found within cells that can replicate independently because it is not attached to chromosomal DNA.

What are plasmids

Plasmids are extrachromosomal structures in bacteria cells that have the ability to self-replicate.

They do not merge with the host cell’s genetic material because they remain autonomous. Plasmids can be genetically altered and used in recombinant DNA technology. This is due to their ability to carry 20 genes at once.

Plasmids are most commonly found in bacteria, but they are also found in archaea and multicellular organisms. Plasmids typically contain at least one gene, and many of the genes carried by plasmids are beneficial to their host organisms. Despite the fact that they have separate genes from their hosts, they are not considered independent life.

Plasmids functions

  • The primary function of plasmids is to transport antibiotic-resistant genes throughout the human or animal body. Many human and animal diseases can be treated in this manner.
  • Another function of plasmids is to carry genes involved in metabolic activities that aid in the digestion of pollutants from the environment.
  • They can also produce antibacterial proteins, which can be used in the production of pharmaceuticals.
  • Plasmids can also carry genes that increase the pathogenicity of bacteria, which causes diseases such as anthrax and tetanus.
  • Plasmids serve a variety of purposes. They may contain genes that improve an organism’s survival. They do this by killing other organisms or defending the host cell by producing toxins.
  • Some plasmids in bacteria aid in the replication process.
  • Because plasmids are so small, they typically only contain a few genes that perform a specific function (as opposed to a large amount of noncoding DNA).

Plasmid structure

A diagram of plasmids.
A diagram showing the structure of plasmids

In terms of structure, plasmids are composed of circular double chains of DNA. The circular structure of plasmids is made possible by covalent bonds connecting the two ends of the double strands.

The molecules are also small in size, especially when compared to the DNA of organisms, ranging from a few kilobases to several hundred kilobases.

Although many plasmids have a covalently closed circular structure, others have a linear structure and do not form a circular shape.

Plasmids structures are made up of three major components, which are as follows.

  1. Origin of replication (replicon)
  2. Polylinker (multiple cloning sites)
  3. Antibiotic resistance gene

Origin of replication (replicon)

The origin of replication is the point in the strand where replication begins. This location is mostly made up of A-T base pairs, which are easier to separate during replication in plasmids.

In comparison to an organism’s DNA, which has many replication origins, plasmids have only a few replication origins due to their small size. They contain a number of regulatory elements at the replication origin that aid in the process (e.g. Rep proteins).

Polylinker (multiple cloning sites)

The polylinker (MCS) is one of the most important parts of the plasmid molecule. A polylinker is essentially a short sequence of DNA with a few sites for cleavage by restriction enzymes.

As a result, MCS facilitates the insertion of DNA via ligation or restriction enzyme digestion. Different polylinkers can cut the strand at the site of cleavage. Thus, one of the restriction enzymes can cut the plasmid at specific points along with the site, allowing for DNA insertion.

Antibiotic resistance gene

One of the most important components of plasmids is the antibiotic resistance gene. These genes play an important role in drug resistance (to one or more antibiotics), making some diseases more difficult to treat.

Plasmids are well-known for their ability to transfer from one bacterial species to another via a process known as conjugation (contact between cells that is followed by the transfer of DNA content). They are capable of conferring antibiotic resistance properties to other bacteria species in the process.

While plasmid replication gives bacteria an advantage (resistance to certain antibiotics), it also has an impact on bacterial cell division due to the additional replication burden. As a result of reduced cell division, bacteria with plasmids are outnumbered by bacteria without plasmids.

Classification of plasmids

  • Conjugative and non-conjugative
  • Incompatibility

Conjugative and non-conjugative

Plasmids can be classified in a variety of ways, ranging from general to specific. One method is to categorize them as conjugative or non-conjugative. Bacteria reproduce sexually, which is the transfer of genetic material from one bacterial cell to another via direct contact or a bridge between the two cells. Some plasmids contain transfer genes, which aid in the initiation of conjugation. Non-conjugative plasmids cannot initiate the conjugation process and can only be transferred sexually with the assistance of conjugative plasmids.


Another way to categorize plasmids is by incompatibility group. Different plasmids can only co-occur in a bacterium if they are compatible with one another. A plasmid that is incompatible with the bacterial cell will be expelled. Plasmids are incompatible if they use the same reproduction strategy in the cell; this allows the plasmids to occupy a specific territory within the cell without interfering with other plasmids.

Types of plasmids

  1. Resistance plasmids
  2. Degradative plasmids
  3. Fertility plasmids
  4. Col plasmids
  5. Virulence Plasmids

Among the other types of plasmids are plasmids that have been altered in the laboratory and introduced into bacteria for research purposes are known as recombinant plasmids, metabolic plasmids that increase the host’s metabolism, conjugative plasmids that increase self-transfer, and suicide plasmids are plasmids that fail to replicate when transferred from one cell to another.

Resistance plasmids (R plasmids)

Resistance plasmids, also known as antimicrobial resistance plasmids, are plasmids that carry genes that play an important role in antibiotic resistance. They also play an important role in bacterial conjugation, producing conjugation pili that transfer the R plasmid from one bacterium to another.

Resistance plasmids are classified into two categories namely, the narrow-host-range group (this group is frequently replicated within a single species) and the group with a wide host range that is easily transferred between bacteria species (this type of resistance plasmid has been found to carry a variety of antibiotic resistance genes).

The transfer of antibiotic resistance genes to drug-sensitive bacteria can result in the bacteria developing resistance to a variety of drugs.

Degradative plasmids

Degradative plasmids, as opposed to other plasmid types, allow the host organism to degrade/break down xenobiotic compounds. Xenobiotic compounds, also known as recalcitrant substances, are a class of compounds that are released into the environment as a result of human activity and are thus not naturally occurring or common in nature.

Degradative plasmid hosts are found in the groups IncP-1, IncP-7, and IncP-9, and include species such as Ochrobactrum anthropi, Rhizobium sp, Burkholderia hospita, Escherichia coli, and Pseudomonas fluorescens, among others.

While degradative plasmids aid in the degradation of xenobiotic compounds, their behavior varies depending on a number of factors such as replication capacity and stability. Plasmids found in the IncP-1 group, for example, have been shown to have a broad host range as well as a high transfer frequency.

Differences in the behavior of different degradative plasmids have thus been shown to result in differences in their interactions with their respective hosts.

Fertility plasmids

Fertility plasmids (F plasmids), like many other plasmids, have a circular structure and a size of about 100 kb. The following are some of the main components of the F plasmid:

  • Transposable components (IS2, 1S3, and Tn1000)
  • Sites of replication (RepFIA, RepFIB, and RepFIC)
  • The source of conjugative transfer (oirT)
  • Origins of replication

The F plasmid is important in reproduction because it contains genes that code for the production of sex pilus as well as enzymes needed for conjugation. The F plasmid also includes genes involved in their own transfer. As a result, they enhance their own transfer from one cell to another during conjugation.

The cells that process the F plasmids are referred to as donors, while those that lack this factor are referred to as recipients. Transfer factors, on the other hand, are plasmids that enhance the host cell’s ability to behave like a donor.

During conjugation, a donor cell (bacteria) with sex pili (1-3 sex pili) binds to a specific protein on the recipient’s outer membrane, triggering the mating process.

The pili retract after the initial binding, allowing the two cells to bind together. Following this, the donor’s DNA is transferred to the recipient, and the F plasmid is transferred as well. As a result, the recipient gains the F factor and the ability to produce the sex pilus involved in conjugation.

Col plasmids

Col plasmids enable bacteria to produce toxic proteins known as colicines. Toxins are used by bacteria such as E. coli plasmids, Shigella, and Salmonella to kill other bacteria and thus thrive in their respective environments.

There are various types of Col plasmids that produce various types of colicines/colicins. Col B, Col E2, and Col E3 are a few examples of Col plasmids. Their distinctions are also distinguished by differences in their modes of action.

For example, Col B causes cell membrane damage in other bacteria (in the absence of the plasmid), and Col E3 has been shown to cause nucleic acid degradation in target cells.

Some Col plasmids, like fertility plasmids, have been shown to carry elements that improve their transmission from one cell to another. Hence, the Col plasmids can be transferred from one cell (donor) to another via conjugation or mating, particularly in cells with the F factor (fertility plasmids) (recipient).

As a result, the recipient develops the ability to produce toxins that kill or inhibit the growth of target bacteria that lack the plasmid.

Virulence Plasmids

Bacteria that are pathogenic in nature, as opposed to other harmless bacteria, carry genes for virulence factors that allow them to invade and infect their respective hosts.

The virulence factors in some of these bacteria are the result of the organisms’ own genetic material. Others, on the other hand, are affected by genetic elements from extra-chromosomal DNA. Although other sources of such elements exist, such as transposons, plasmids are among the most common mobile genetic elements.

In terms of pathogenicity, virulence plasmids are important because they can assist bacteria in effectively adapting to their respective environments. This is due to the virulence plasmid’s ability to enable the organism to express a variety of virulence-associated functions, allowing the organism to thrive in its environment.

Virulence plasmids, like other types of plasmids, can be passed from bacterium to bacterium. Aside from the virulence gene, plasmids have been shown to contain other important elements that aid in transmission and maintenance.

As a direct consequence, they are larger in size but few in number. This ensures that they do not add to the organism’s burden during cell division.

Cell division and cell maintenance typically necessitate the use of energy. The cells are spared the significant metabolic burden that would be required for the maintenance and genome duplication of numerous plasmids by having a low number of virulence plasmids.

Vectors plasmids

A vector is any molecule that contains genetic material that, when transferred to another cell, can be replicated and expressed. Based on this definition, it is easy to understand why the terms “vector” and “plasmids” are sometimes used interchangeably. This does not imply that all plasmids are vectors.

One of the most distinguishing features of plasmid vectors is their small size. Aside from their size, they are distinguished by a replication origin, a selective marker, and multiple cloning sites.

Inside the cell, ideal plasmid vectors have a high copy number. As a result, it ensures a large number of the target gene for cloning. This also ensures that the gene of interest grows during genomic division. Furthermore, the plasmid can contain a marker gene as a visual marker to aid in determining whether cloning was successful.

In the relationship between plasmids and vector, plasmids have been shown to be some of the best vectors for cloning due to their multiple cloning sites. Because of this property, restriction enzymes can cleave various regions of the plasmid for cloning.

Plasmid isolation

Plasmid isolation is required to obtain purified plasmid DNA for procedures such as cloning, PCR, and transfection. The procedure entails employing a variety of techniques to obtain plasmid DNA from host cells for use in molecular biology.

The following steps are involved in plasmid isolation.

  • Cell growth (growth of bacterial cells) entails cultivating bacteria containing plasmids in a specific shaken culture. Antibiotics may be used to prevent the growth of other undesirable bacteria in this situation.
  • Centrifugation – After bacterial growth, the cells are pelleted using centrifugation. After the supernatant has been removed, plasmid isolation can begin.
  • The classical method, also known as alkaline lysis, is one of the most common techniques for isolation.

Application of plasmids

Humans have discovered numerous applications for plasmids and have developed software to record the DNA sequences of plasmids for use in a variety of techniques as listed below.

  • Plasmids are used in genetic engineering to amplify or duplicate, specific genes.
  • A plasmid is a type of vector in molecular cloning. A vector is a DNA sequence capable of transporting foreign genetic material from one cell to another, where the genes can be expressed and replicated. Plasmids can be used to clone short segments of DNA.
  • Plasmids can also be used to replicate proteins in large quantities, such as the protein that codes for insulin.
  • Furthermore, plasmids are a means of transferring genes into human cells as part of gene therapy. This aspect is still being studied.

FAQ on plasmids

Is it easier to transform bacteria with plasmid DNA?

Yes, it is easier to transform bacteria with plasmid DNA since plasmids are able to replicate inside their host.

Do eukaryotes have plasmids?

Yes, plasmids in eukaryotes do exist.

What happens when plasmids are used to produce a desired protein?

When plasmids are used to produce a desired protein, the desired gene is inserted into the plasmid, and the plasmid is returned to the bacterium by transformation.

How are plasmids different than chromosomes?

The primary distinction between plasmid and chromosome is that plasmid is a circular double-stranded extra-chromosomal DNA structure of bacteria, whereas chromosome is a well-organized thread-like structure containing genomic DNA tightly coiled with proteins.

How does rolling-circle replication of plasmids proceeds?

Rolling-circle replication of plasmids proceeds in a unidirectional manner from their dso.

Which of the types of plasmids allow a bacterial cell to kill its competitors?

Bacteriocin factors are the type of plasmids that allow a bacterial cell to kill its competitors.

What happens after genetically modified plasmids are inserted into bacteria?

The bacteria that have genetically modified plasmids transcribe the plasmid-encoded gene into mRNA. The mRNA is therefore translated into a protein that is used by the bacteria.

What is the function of plasmid in gene-cloning projects?

In gene-cloning projects, plasmids function to be a site that allows DNA fragments to be inserted.

How are plasmids transferred?

Plasmids are mostly transferred from one bacterium to another (even from different species) via conjugation. Frequently, genetic engineers use plasmids, which are part of the mobilome to transfer genetic material from one host to another.

In what type of cells can plasmids be found?

Plasmids are naturally found in bacterial cells and in some eukaryotes. Although, plasmids have been found in all of the following types of cells except animal cells.

What are Escherichia coli plasmids?

E.coli plasmids are E.coli that have been found to have a wide range of plasmid types, including plasmids associated with virulence. There are several types of E. coli virulence plasmids, including those required for enterotoxigenic E. coli virulence.

What are cloning plasmids?

Cloning plasmids or cloning vectors are plasmids that are primarily used for DNA propagation. They replicate to high copy numbers in E. coli and have a multiple cloning site (also known as a polylinker) with restriction sites for inserting a DNA fragment.

How are recombinant plasmids used to move genes between species?

Recombinant plasmids use conjugation to move genes between species.

How are enzymes used during gene transfer involving plasmids?

Enzymes like endonuclease restriction enzymes are used to cut the DNA at specific base sequences in a selected plasmid.

What are resistant plasmids?

They are any of the conjugative or nonconjugative plasmids that carry antibiotic or antibacterial drug resistance genes in bacteria.

Why do you think plasmids and bacterial cells are such important tools in genetic engineering?

Plasmids and bacterial cells are important tools in genetic engineering because they are used to introduce foreign genetic material into various cell types. After inserting the foreign DNA fragment into the plasmid, the recombinant DNA molecule is transformed into the recipient cell. The antibiotic resistance of the plasmid used selects the transformed cells.

What are expression plasmids?

An expression plasmid, also known as an expression construct, is a plasmid or virus that is designed to promote gene expression in cells. The vector is used to introduce a specific gene into a target cell, and it can hijack the cell’s protein synthesis mechanism to produce the protein encoded by the gene.

How are plasmids important in biotechnology?

Plasmids are important in biotechnology because they are they serve as vectors, or delivery vehicles, for introducing foreign DNA into bacteria. They are being used to insert genes into bacteria in order to encourage the production of therapeutic proteins such as human insulin.

Where are plasmids found in eukaryotic cells?

Plasmid DNA is found inside the nucleus of eukaryotes, but it is extrachromosomal in nature.

How are lentiviral packaging plasmids done?

Lentiviral packaging plasmids or particles are packaged in producer cell lines such as HEK293T cells.

What are plasmids?

Plasmids are small circular DNA molecules that are used in the laboratory manipulation of genes.

Can phage λ carry larger DNA fragments than plasmids?

Yes, phage λ can carry larger DNA fragments than plasmids.

Where are bacterial plasmids located?

They are located in the cytoplasm of the bacterial cell and the genes contained in bacterial plasmids can be involved in providing genetic advantages, such as antibiotic resistance.

What are genscript plasmids?

GenScript’s are industrial grade plasmid DNA that can assist your experiments in achieving highly efficient cell transfection, thereby improving experimental outcomes in areas such as protein expression, antibody production, and other research projects.

Genomic libraries can be constructed using either bacterial plasmids or what other vector?

Genomic libraries can be constructed using either bacterial plasmids or Escherichia coli.

Are plasmids self replicating?

Yes, plasmids are extrachromosomal DNA molecules that self-replicate in Gram-negative and Gram-positive bacteria, as well as some yeast and other fungi.

Do prokaryotes have plasmids?

Yes, plasmids in prokaryotes do exist.

What is the difference between transposable elements and plasmids?

Transposable elements differ from plasmids in that only plasmids transfer genetic material between genomes whereas transposon transfer genetic material between chromosomes within the same genome.

Plasmids that govern their own transfer are known as?

Plasmids that govern their own transfer are known as conjugative plasmids.

What are shRNA plasmids?

The plasmids called shRNA plasmids are plasmids that knockdown gene expression.

How are genes inserted into plasmids?

The desired piece of DNA or gene is extracted from its original DNA source using a restriction enzyme and then ligated into the plasmid.

What do plasmids do?

Plasmids confer genetic advantages on bacteria, such as antibiotic resistance.

Do gram negative bacteria have plasmids?

Yes, gram negative bacteria have plasmids.

How are plasmids transmitted?

Plasmids are transmitted mostly through conjugation.

Which two tools are vital to genetic engineering of plasmids?

Restriction enzymes and vectors are the 2 vital tools in the genetic engineering of plasmids.

Do plasmids often encode for proteins?

Yes, plasmids often encode for proteins.

Can you outline a basic technique for gene transfer involving plasmids?

The gene transfer process can be broken down into four key steps: isolation of the gene and vector (via PCR), digestion of the gene and vector (via restriction endonuclease), ligation of the gene and vector (via DNA ligase), and the selection and expression of the transgenic construct.

What do plasmids carry?

Plasmids often carry the information for antibiotic resistance.

Does chloroplast have plasmids?

Yes, some chloroplasts have plasmids.

Cloning human genes into the plasmids of bacteria has enabled scientists to?

Cloning human genes into the plasmids of bacteria has enabled scientists to create multiple copies of genes.

What organisms have plasmids?

Plasmids are most commonly found in bacteria, but they are also found in archaea and multicellular organisms.

How are chromosomal islands similar to plasmids?

Chromosomal islands are similar to plasmids in that they are both used in gene transfer.

Do archaea have plasmids?

Yes, archaeans, like bacteria, lack internal membranes and their DNA exists as a single loop known as a plasmid.

Where are plasmids located?

Plasmids are located in the cytoplasm and nucleotides of bacteria and other cells.

Do plasmids typically carry _ genes?

Yes, plasmids do carry a small number of genes.

A video from addgene plasmids 101 briefly explaining plasmids

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