Plasmids refer to small and circular DNA molecules which can replicate independently. But, they are not dependent on an organism’s chromosomal DNA for replication. Due to this feature of plasmids, they are often termed as extra-chromosomal DNA.

Even though plasmid was initially discovered as a member of Enterobacteria acae, several observations showed that they occur naturally in multiple microorganisms across the world.

Let us move further with plasmid structure, components and other related information which are quite important for your NEET 2020.

Structure of Plasmids

According to the plasmid structure, it consists of double chains of DNA which are circular. Both the ends of the double strands are attached by covalent bonds which gives it a round structure. However, there are some plasmids which show a linear structure and is not circular. The size of these molecules ranges from few kilobases to hundreds of kilobases.

Plasmid Components

In general, plasmids hold three fundamental components, such as:

  • Replication Origin (Replicon)

This component mentions the particular location within the strand where replication starts. In case of plasmids, this area mostly comprises A-T base pairs, which splits up easily during replication.

  • Polylinker

The polylinker (multiple cloning sites) is an integral part of a plasmid. The reason for this is that students can master a lot of things about cloning. Essentially, MCS is a short sequence of DNA which includes several restriction enzyme sites for separation.

For this reason, polylinkers allow effortless DNA insertion by restriction enzyme digestion or ligation. At the point of separation, multiple polylinkers can slit the chain. So, any one restriction enzyme has the ability to cut the plasmid to enable insertion of DNA.

  • Antibiotic Resistance Gene

This is another major plasmid component that has an important part in drug resistance. As a result, treatment of few diseases becomes difficult. By conjugation, plasmids can shift from one kind of bacteria to another. Within this process, they have the ability to give out antibiotic resistance features to other bacteria types.

Besides the main ones, there are two other plasmid components:

  • Promoter region

  • Primer binding site

Kinds of Plasmids

The major types of plasmids include:

  1. Resistance

  2. Degradative

  3. Fertility

  4. Col

  5. Virulence

Plasmid Vector

A molecule piece that contains genetic material which has the feature of replication and can be expressed after transferring to another cell is called a vector. Due to this definition, the terms “plasmid” and “vector” are sometimes swapped. However, you must keep in mind that every plasmid is not a vector.

A plasmid vector has a significant feature, i.e., its size is small. Besides the size, they can be identified by replication origin, MCS and a selectable marker.

Complete plasmid vectors contain a high number of copies in the cell. So, it makes sure high target gene numbers for the purpose of cloning. Furthermore, it also makes sure the increase in the gene of interest throughout genomic division. Additionally, the plasmid can hold a marker gene which helps to ascertain the successful cloning process.

Due to their MCS, plasmids are introduced to best vectors, and restriction enzymes can divide different plasmid regions for cloning.

In the past years, usage of these vectors has made it possible for the introduction of recombinant DNA to host cells for observation purposes. For example, because of this kind of cloning, researchers are able to:

  • Observe gene expressions.

  • Arrange genome of a variety of species.

  • Study different cellular mechanisms.

One widely used E. coli cloning vector is pBR322. The next section will give you an idea about pBR322 plasmid.

Take a look!


pBR322 vector holds gene for ampicillin and tetracycline resistance and has a length of 4361 base pairs. It includes rop gene and origin of replication of pMB1. The restriction sites for 40+ restriction enzymes of this vector are unique, where eleven of these are located in TetR gene. Two sites, namely – Clal and Hindlll, are present in TetR gene’s promoter. Also, there are six chief restriction sites within the AmpR gene.

The circular order is equipped with numbers where zero forms the middle of EcoRI site, and the number increases across TetR gene. Moreover, ampicillin resistance gene is equal to penicillin beta-lactamase. There are two promoters for beta-lactamase such as P1 and P3, where the former is created artificially, and the latter is a natural one. P2 and P1 are on opposite strands but in the same area, and P2 also begins transcription towards TetR gene direction.

The following picture represents pBR322 diagram

(image will be uploaded soon)

Do It Yourself

1. Which of the following is the tiniest plasmid and is a complete cloning vector?

a) RP4 b) ColE1 c) F d) PUC8

2. Which gene among these aids in determining transformed cells?

a) Vector b) Selectable marker c) Plasmid d) Structural gene

Hard Work is the Key to Success

With regards to the intense competition around, you must score considerably in exams to stay ahead of others. Furthermore, if you are a medical aspirant and will appear for NEET, prepare your Biology lessons thoroughly. In the case of the plasmid topic, starting from the plasmid definition to all its kinds and components, every portion is essential.

Apart from working hard, candidates must also follow a disciplined lifestyle by following a proper study schedule, eating healthy food and taking adequate rest. You can also try meditation or yoga, which will assist you to stay calm and fresh.

All the best!

FAQ (Frequently Asked Questions)

1. Which Organisms Contain Plasmids?

Ans. Plasmids are present in bacteria, archaea and multiple eukaryotes (plants and yeast).

2. What is the Meaning of 322 in pBR322?

Ans. In pBR322 plasmid, 322 is the number that either distinguishes it from other kinds of plasmids, or it denotes the synthesis order.

3. Which are the Kinds of Plasmids Other Than the Five Major Ones?

Ans. The other plasmids apart from the main ones are recombinant, cryptic, metabolic, conjugative and suicide plasmids.

4. What are the Primary Parts of F Plasmid?

Ans. The main parts of F plasmid are a transposable element, replication origin regions, replication sites and conjugative transfer origin.