Integrative Plasmid

Episome

Under certain conditions some plasmids may integrate into the bacterial chromosome. They are called  episome or integrative plasmids. At this stage they replicate along with the bacterial chromosome.

The plasmids in this way are classified into 2 types

Relaxed plasmids

They are the ones which are normally maintained at multiple copies per cell.

Stringent plasmids

They are the ones which have a limited number of copies per cell.

In this case of plasmid replication, the plasmid DNA is integrated in the bacterial chromosome and grows along with the cell. It uses the bacterial machinery for division. A good example of this type of replication is Ti Plasmid, often used in agricultural genetic engineering experiments. It completely uses the cell genetic mechanism to grow.

Fig. 35. Replication of plasmid

Types of plasmid

• Fertility plasmid :
  
  That contains the tra genes required for conjugation else known as F plasmids. For example F plasmids of E coli.

Fig. 36. Fertility plasmid

• Resistance Plasmid or R plasmids
  
  Carry genes of resistance to one or more antibacterial agents such as ampicilin, etc

• Col plasmid :
  
  It contains genes for production of bacteriocins, proteins that kill other bacteria example col E1.

• Virulent plasmids:
  
  Which in turn convert bacteria into a pathogen. For example, the Ti plasmid of agrobacterium tumefaciens induces crown gal disease on dicot of the palnts

Table 2. Size of some plasmids as vectors