dear sir n mam i just want to know why plasmid is circular..???
As we know plasmids are double stranded closed DNA molecules, they lack complex protiens like histones that are present in the human Chromosomes, and thus the plasmid DNA are freely suspended into the bacterial cytoplasm.
The cytoplasm being a fluid, the closed structure will try to reduce the surface energy which happens by covering a larger area(circular)[surface tension concept]. Hope you got your answer :-)
In add the plasmid is circular for a reduction thermodinamics energy