The modern biotechnology especially recombinant DNA technology is indebted to this extra chromosomal DNA in bacteria called plasmids. In 1972, Paul Berg along with Boyer and Cohen, incorporated a DNA fragment into a plasmid creating the first recombinant DNA molecule. Later this was transformed into a bacterium creating the first genetically engineered organism. Even now, plasmids play a major role as vectors in rDNA technology.
Definition: Plasmids are extra chromosomal, double stranded, circular DNA molecules present in bacteria.
Stringent vs Relaxed plasmids
Plasmids are either stringent or relaxed. If the plasmid occur free in the cytoplasm, then it is called as relaxed as it can replicate independent of bacterial genome replication.
|Relaxed Plasmid: capable of independent replication|
If the plasmid is integrated to the bacterial chromosome, then called as episome (on the chromosome). Such plasmid can replicate only when bacterial genome replicates and called as stringent. Relaxed plasmids are used as vectors for gene transfer in rDNA technology. Plasmids encode genes for specific proteins.
|Stringent Plasmid: Replicate only with bacterial genome replication|
Different kinds of bacterial plasmids:
- F-plasmids (fertility factor or sex factors): Bacterial cells with F plasmids are called F+ cells and acts as donor cells or +strain during bacterial conjugation. F factors are involved in bacterial conjugation.
- R-plasmids (resistance plasmids): These plasmids encode genes that confer bacterium resistance against antibiotics. R plasmids are easily transferred from one strain to another offering antibiotic resistance to neighbouring cells easily.
- Col-plasmids: these plasmids carry genes that produce bacterial toxins called colicins.
- Metabolic plasmids carry genes involved in the production of metabolic enzymes.
Major Types of Bacterial Plasmids:
- Endo genote: the original genome of the recipient cell
- Exo genote: The DNA fragment, transferred from a donor cell to the recipient cell
- Merozygotes: are partial diploids
- Transformation: uptake of DNA fragments from the surroundings through cell wall.
- Conjugation: unidirectional transfer of genetic material from a donor bacterium to recipient bacterium through cell to cell contact via sex pili.
- Transduction: is the virus mediated gene transfer.
- Prophage: phage DNA integrated with the bacterial chromosome
- Hfr(high frequency recombination): In Hfr, the F plasmid is integrated on the chromosome forming episome or recombinant DNA (formed between chromosomal DNA and F plasmid), Such bacterial strains having rDNA will undergo conjugation and recombination at high rate or frequency.
- Plasmid: extra chromosomal, double stranded, circular DNA molecules present in bacteria
- Induction: refers to the dissociation of prophage from the bacterial chromosome and often lytic cycle is followed.