Plasmids are small, Circular, Double Stranded DNA molecules that is distinct from the chromosomal DNA of the cell. The term Plasmid was discovered by Tatum. And was coined by a Lederberg and Hays. Along with main genetic material nucleoid and There is also present an extra chromosomal genetic material called plasmid. In modern age, different species of bacteria has different types of Plasmids which are used in genetic engineering. Some of them are helpful while others are harmful. Depending upon their working and nature different types of plasmids along with characteristics are as follow.
Characteristics of Plasmid
- The plasmid consists of extra chromosomal coiled circular DNA. It is called extra chromosomal because of presence of additional DNA along with naked chromosome.
- Plasmid is smaller in size as compared to nucleoid that’s why it is also called as Mini Chromosome.
- It does not depend on the main DNA (Nucleoid) for replication. That’s why plasmid shows autonomous replication.
- Genes of plasmids are not important for its bacterial growth and viability (surviving). It means bacteria can also show its metabolic activities for survival. That’s the reason, plasmid is also known as non-essential element or dispensable element of bacteria.
Types of Plasmids
Depending on the state of plasmid. The plasmid is divided into two types.
Depending on the functionality of plasmid the plasmid is of four types.
- Fertility plasmid containing bacteria shows sex pili (Finger Like Projections) formation. The bacteria with finger like projections is called male bacteria or donor bacteria.
- The male bacteria are also called F+
- Bacteria without F. Plasmid does not show sex pili formation. Therefore, it is known as female bacteria. The female bacteria are also called as Recipient bacteria.
- The female bacteria are also called F–
Ananda Mohan Chakrabarty (Info: is an Bengali American microbiologist, scientist, and researcher, most notable for his work in directed evolution and his role in developing a genetically engineered organism using plasmid transfer). He discovered “Petroleum Ether Bacteria” also called as Oil Eating Bacteria. The scientific name is Pseudomonas putida. This bacterium contains four different types of plasmid. These plasmids produce four different types of enzymes that digest four different carbohydrates of petroleum ether.
R-Plasmids consist of cyclic double-stranded DNA molecules. They replicate independently of the chromosomes and transmit through successive cell divisions. The bacteria which consists of R-Plasmid are also called as High Frequency Recombinant Bacteria. In short (Hfr). They show resistance to following substances:
The Plasmid that produces an antibiotic (colicin) used by the host to kill other strains of bacteria is called Col-Plasmid. The Col-plasmid usually kills sensitive bacteria. Depending on the presence or absence of this bacteria this plasmid the bacteria are of two types Sensitive and Non-Sensitive Bacteria.
Ti-Plasmid or Tumor Inducing plasmid are double stranded circular DNA present in Agrobacterium tumefaciens. Common name is Soil Born bacteria. Which are use to transduce their genetic material to plants.
Function of Sex Pili
- The sex pili joins male and female bacteria by forming a tube called Conjugation Tube. The conjugation tube is also called as Cytoplasmic Bridge.
- The genetic material is transferred through this cytoplasmic bridge from ale bacteria to female bacteria. That is the reason male bacteria is called Donor Bacteria and female bacteria is called Recipient Bacteria.
Types of Plasmids with Different Examples
Following are the different types of plasmids along with their examples.
- Fertility F Plasmid/Sex Plasmid
This type of plasmid contains tra genes They are capable of conjugation and result in the expression of sex pili.
For Example: F Plasmid of E. coli.
- Degradative Plasmid
This type of plasmid is able to digest unusual substances like toluene and salicylic acid.
For Example: Tol Plasmid of Pseudomonas putida.
- R Plasmid (Resistance Plasmid)
This type of plasmid makes the bacteria resistant to antibiotics, heavy metals, UV Rays and can also make it resistant to multiple antibiotics.
For Example: Staphylococcus aureus, Enterobacteria, Shigella, Salmonella
Bacteria Resistant to Heavy Metals
Bacteria resistant to heavy metals are as follow:
For Example: Escherichia coli, Pseudomonas aeruginosa, Pseudomonas florenscens, Pseudomonas syringae
- Col Plasmid
Col stands for colicin, a toxin produced by bacteria.
For Example: Escherichia coli (Colicin), Pseudomonas aeruginosa (Pyocin), Bacillus megaterium (Megacine).
- Ti Plasmid
Ti Plasmid stands for tumor inducing plasmid.
For Example: Agrobacterium tumefaciens (Soil Born Bacteria).