Definition
Plasmids are small, circular, double-stranded DNA molecules that exist independently of the bacterial chromosome.
They replicate autonomously and can be transferred between bacteria.
Types of Plasmids
1) Fertility (F) Plasmids
Carry genes for conjugation (tra genes), enabling DNA transfer between bacteria.
2) Resistance (R) Plasmids
Carry antibiotic resistance genes, promoting the spread of drug-resistant bacteria.
3) Col Plasmids
Encode bacteriocins, protein toxins that kill competing bacteria.
4) Virulence Plasmids
Carry genes that enable bacteria to cause infections (e.g., Ti plasmid in Agrobacterium tumefaciens).
5) Metabolic Plasmids
Carry genes for specific metabolic functions, such as nitrogen fixation and organic compound degradation.
6) Cryptic Plasmids
Have no known function but contribute to genetic diversity.
Functions and Significance
Mediate horizontal gene transfer and genetic diversity.
Enable antibiotic resistance, posing a challenge in medicine.
Used in biotechnology for cloning and genetic engineering (e.g., pUC19, pBR322 plasmids).
