Do All Bacteria Have Plasmids

What is a Plasmid?

A plasmid is a tiny, extrachromosomal DNA molecule within a cell that is physically segregated from chromosomal DNA and can replicate alone. They are generally found as small circular, double-stranded DNA molecules in bacteria. Plasmids are occasionally present in archaea and eukaryotic organisms. In nature, plasmids often carry genes that boost the survival of the organism and bestow selective advantage such as antibiotic resistance. Plasmids are usually very tiny and include only additional genes that may be useful in certain circumstances. Artificial plasmids are widely utilized as vectors in molecular cloning, serving to ride the replication of recombinant DNA progressions within host organisms.


Properties and Characteristics of Plasmids

  • In order for plasmids to replicate unassisted within a cell, they must acquire a stretch of DNA that can work as an origin of replication. The self-replicating unit, the plasmid, is named a replicon.

  • Plasmids often carry at least one gene. Many of the genes carried by a plasmid are effective for the host cells. For instance, encouraging the host cell to survive in an environment that would contrarily be fatal or restrictive for growth.

  • Plasmids can provide bacteria with the capacity to fix nitrogen. Some plasmids have no noticeable effect on the phenotype of the host cell or its usefulness to the host cells cannot be assumed, and these plasmids are labeled as cryptic plasmids.

  • Naturally occurring plasmids differ greatly in their physical properties. Their size can vary from very small mini-plasmids of less than 1-kilobase pairs to very huge megaplasmids of several megabase pairs.


Do All Bacteria Have Plasmids?

Yes, Plasmids naturally exist in all bacterial cells. Each bacterial cell has its own plasmid, that is transmitted during a process of conjugation. These plasmids have a great benefit to the bacterial cell as they deliver DNA containing identifiable antibiotic resistance gene sequences which would lead to the production of antibiotic resistance determinants.


In some cases, bacteria need to pick up a new plasmid from the environment, as they readily lose them during the antibiotic resistance and during the process of conjugation, as one of the daughter cells may not receive the plasmid.


Here are a Few Important Functions of Plasmids in a Bacterial Cell:

  • It helps in their survival by producing toxins.

  • Facilitate the process of replication in bacteria.

  • Few plasmids contain genes that help in food digestion.

  • The R plasmids help a bacterial cell by defending against environmental factors such as antibiotics, poison, etc.


Role of Plasmids in Bacterial Cell

  • Plasmids Support Bacteria to Survive Stress: Plasmids contain just a few genes, but they make a big difference to their bacterium. The genes are usually not important for the bacterium’s day-to-day survival. Instead, they help the bacterium to withstand occasional stressful situations. For instance, many plasmids contain genes that make the host bacterium immune to an antibiotic. Other plasmids comprise genes that enable the host to digest unusual materials or to kill other kinds of bacteria.

  • Plasmids Make Themselves Indispensable: Keeping a plasmid is hard work for a bacterial cell because replicating DNA uses up energy. However, by preserving its bacterial host from stress-related death, a plasmid maximizes its opportunities of being kept around. Under stressful conditions, bacteria with the plasmid will live longer and have more chances to pass on the plasmid to daughter cells or other bacterias. Bacteria without the plasmid are less inclined to survive and reproduce.

  • Plasmids in Biotech: Transmitting DNA: Plasmids have always been the key to the growth of molecular biotechnology. They act as delivery vehicles or vectors so as to introduce foreign DNA into bacteria.


Did You Know?

  • A plasmid is a DNA molecule that is separate from the chromosomal DNA and that can replicate (copy itself) independently.

  • The term plasmid was mainly introduced by the American molecular biologist Joshua Lederberg in the year 1952.

  • Plasmid length varies from 1 to over 1,000 kilobase pairs.

  • Plasmids can be found in all three main domains - Archaea, Bacteria, and Eukarya.

  • Unlike viruses, plasmids are "naked" DNA and do not encode genes essential to cover the genetic material for transfer to a new host.

  • Plasmids are transferable genetic components, or "replicons", capable of independent self-copying in a suitable host.

  • Plasmids are double-stranded and, in many cases, circular.

  • Plasmids mostly occur naturally in bacteria but are sometimes found in eukaryotic organisms.

  • The number of identical plasmids within a single cell can vary from one to thousands.

  • Plasmids are often correlated with conjugation, a means of horizontal gene transfer.

  • Plasmids also can provide bacteria with a capacity to fix elemental nitrogen or to degrade complicated organic compounds.

FAQ (Frequently Asked Questions)

1. Why Plasmids are Excellent DNA Delivery Vectors?

Years after their first use, plasmids are nonetheless crucial laboratory tools in biotechnology:

  • Scientists can force bacteria to keep them. Almost all plasmids that are utilized to deliver DNA incorporate genes for antibiotic resistance. Once bacteria have been treated with a plasmid, scientists grow them in the presence of antibiotics.

  • They are copied independently regardless of whether the bacterial host is replicating its own DNA.

  • They are circular. DNA that is circular is well fitted to integrate extra DNA sequences. That is because it can be cut open without falling apart, then snap back together once fresh DNA has been incorporated.

2. What do you Mean by Plasmid Families and Incompatibility?

Two different plasmids that belong to a similar family cannot co-exist in an identical cell. This is known as incompatibility. Plasmids were initially categorized by incompatibility and so plasmid families are often understood as incompatibility groups and are appointed by letters of the alphabet. Plasmids of the identical incompatibility group have very similar DNA sequences in their replication and partition genes, although the different genes they transmit may be very different. It is quite possible to have two or more plasmids in the same cell as long as they belong to different families. So, a P-type plasmid will heartily share the same cell with a plasmid of the F-family.