Kingdom Monera - Plasmids

Description:

Plasmids are extrachromosomal rings of DNA present in bacteria. Plasmids were discovered by Hayes and Lederberg in 1952. They are capable of replicating independent of nucleoid. Plasmids which remain associated with nucleoids are called episomes.

Plasmids can act as vectors hence are used in genetic engineering for introduction of genes. They can pass from one bacterium to another, hence are called transfer plasmids. Plasmids contains non-vital genes which may or may not be useful to bacteria. Plasmids which does not provide any useful trait to the cells are called cryptic plasmids.

Types of Plasmids −

F-plasmid

• Contain genes for conjugation or fertility.
• F plasmid is called fertility factor or F-factor.
• A bacterium having F plasmid is called male or donor bacterium.
• It develops sex pili for conjugating with female bacterium which are devoid of F-factor.

R-plasmid

• Carry genes for resistance against common antibiotics like chloramphenicol, tetracyclin, streptomycin and sulphonamide.

• Pathogenic bacteria having R-plasmid are very difficult to be treated.

Col-plasmid

• Also called colicinogenic factors.
• Produce toxins called colicins or bacteriocin.
• They can get transferred from one bacterium to another just like F-plasmid or R-plasmid.