Unit 3 :Plasmid genetics S.Y.Micro Sem 4 genetics
Unit 3 :Plasmid genetics
1. Types of Plasmid
2. Propertie of Plasmid
3. Plasmid Replication
4. Plasmid incompatibility
5. Plasmid curing
6. Plasmid amplification concepts
Plasmid Definition
A plasmid is a small, circular piece of DNA that is different than the chromosomal DNA.
Plasmids are defined as extra-chromosomal genetic elements, occurring chiefly in bacteria and rarely in eukaryotic organisms.
In bacteria, plasmids are circular double-stranded DNA molecules which contain genes controlling a wide variety of functions.
In yeast (Saccharomyces cerevisiae) an RNA plasmid has been found.
This simplified figure depicts a bacterium’s chromosomal DNA in red and plasmids in blue.
Functions of Plasmids
Plasmids have many different functions. They may contain genes that enhance the survival of an organism, either by killing other organisms or by defending the host cell by producing toxins.
Some plasmids facilitate the process of replication in bacteria.
Properties of Plasmids:
Plasmids are self-replicating elements, yet they are largely dependent on the host cell for their reproduction, because they use the host cell replication machinery.
The first plasmid to be discovered was the sex-factor or F plasmid (F stands for fertility) of E. coli K12.
This plasmid confers the ability to an E. coli cell (F+) to conjugate with another lacking this plasmid (P cell).
The F-plasmid can exist in two alternative states, viz. it can either remain free in the cell or it can be integrated into the E. coli chromosome. Plasmids with such property are known as episomes.
General Types of Plasmids
1.Conjugative and Non-Conjugative
There are many ways to classify plasmids from general to specific.
One way is by grouping them as either conjugative or non-conjugative.
Bacteria reproduce by sexual conjugation, which is the transfer of genetic material from one bacterial cell to another, either through direct contact or a bridge between the two cells.
Some plasmids contain genes called transfer genes that facilitate the beginning of conjugation.
Non-conjugative plasmids cannot start the conjugation process, and they can only be transferred through sexual conjugation with the help of conjugative plasmids.
2.Incompatibility
Another plasmid classification is by incompatibility group.
In a bacterium, different plasmids can only co-occur if they are compatible with each other.
An incompatible plasmid will be expelled from the bacterial cell.
Plasmids are incompatible if they have the same reproduction strategy in the cell.
Specific Types of Plasmids
There are five main types of plasmids: fertility F-plasmids, resistance plasmids, virulence plasmids, degradative plasmids, and Col plasmids.
1.Fertility F-plasmids
Fertility plasmids, also known as F-plasmids, contain transfer genes that allow genes to be transferred from one bacteria to another through conjugation.
Bacteria that have the F-plasmid are known as F positive (F+), and bacteria without it are F negative (F–).
2.Resistance Plasmids
Resistance or R plasmids contain genes that help a bacterial cell defend against environmental factors such as poisons or antibiotics.
Some resistance plasmids can transfer themselves through conjugation. When this happens, a strain of bacteria can become resistant to antibiotics.
3.Virulence Plasmids
When a virulence plasmid is inside a bacterium, it turns that bacterium into a pathogen, which is an agent of disease.
The bacterium Escherichia coli (E. coli) has several virulence plasmids.
E. coli is found naturally in the human gut and in other animals, but certain strains of E. coli can cause severe diarrhea and vomiting.
Salmonella enterica is another bacterium that contains virulence plasmids.
4.Degradative Plasmids
Degradative plasmids help the host bacterium to digest compounds that are not commonly found in nature, such as camphor, xylene, toluene, and salicylic acid.
These plasmids contain genes for special enzymes that break down specific compounds.
5.Col Plasmids
Col plasmids contain genes that make bacteriocins (also known as colicins), which are proteins that kill other bacteria and thus defend the host bacterium.
Bacteriocins are found in many types of bacteria including E. coli, which gets them from the plasmid ColE1.
Plasmid Replication:
There are two methods for the replication of plasmids. Among the two mechanisms, replication can occur by any one of the mechanisms:
1. theta mode for both unidirectional and bi-directional pathways
2. rolling circle mode.
1. Mechanism of Theta Plasmid Replication:
§ DNA unwinds at the ori site from where the replication begins.
§ It then creates the structure where the whole replicational machinery assembles.
§ Since the structure resembles the Greek letter theta (θ), its name has been derived from it.
§ The process gets initiated by the RNA primer.
§ Then deoxyribonucleotides are added which extends the process.
§ The replication process may proceed in one (unidirectional) or both directions (bi-directional).
§ In the first case (unidirectional), a single replication fork moves around the circle until it returns to its point of origin. and then the two daughter DNAs separate.
§ In the other case (bidirectional replicational) two replication forks begin at ori then it travels to the opposite until they meet at some point on the other side of the molecule.
§ This is the most common mode of DNA replication.
§ The theta mechanism is the most common form especially in Gram-negative bacteria like the proteobacteria.
§ Commonly used plasmids, including ColE1, RK2, and F, as well as the bacteriophage P1, use this type of replication
Plasmid incompatibility:
Generally, two closely related plasmids cannot coexist in a bacterial cell. In the population of progeny cells derived from a cell containing two such plasmids, the proportion of cells having only one of the two plasmids increases with every cell division. This is known as plasmid incompatibility.
On the other hand, two different unrelated plasmids, e.g. F plasmid and ColEl can exist together without any difficulty, because these plasmids belong to two different incompatibility groups. Whereas, two F-plasmids cannot coexist in the same cell.
One mechanism by which a plasmid already resident in a cell prevents the entry of a second similar plasmid into the same cell is by surface exclusion.
Plasmid curing:
Bacteria possess extra chromosomal DNA,called plasmids. Often it carries functional genes for the resistance of bacteria (example: Aromotic compound degrading genes).
Plasmid curing is a process of completely removing plasmids of bacteria by means of chemical agents such as Acriflavin or acridine orange
Plasmid Amplification:
Another important point of plasmid replication is that chromosomal DNA synthesis and plasmid DNA synthesis are independent of each other, though, in both, DNA synthesis is followed by replication.
Thus it is possible to stop chromosomal DNA synthesis and replication without affecting
plasmid DNA synthesis and replication.
Such situation can be practically created by adding chloramphenicol to a bacterial culture.
This antibiotic specifically inhibits prokaryotic protein synthesis.
When it is added to a growing bacterial culture, chromosomal DNA synthesis is inhibited, but plasmid DNA synthesis and replication.
The net result is that each bacterial cell contains large number of plasmid copies. This is known as plasmid amplification.
When a specific gene which has been transferred (cloned) to a plasmid requires to be isolated, plasmid amplication becomes a useful tool, because of high plasmid DNA concentration in the total cellular DNA.
Reference:
1. https://biologydictionary.net/plasmid/
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