Why are plasmids circular, while DNA is not? Furthermore, why do plasmids occur naturally in bacteria?
In fact the chromosomes of many bacteria ARE circular. This is a necessity, because linear DNA needs a way to copy its ends. Why plasmids exist is an open question. They appear to exist to promote their own existance. They usually carry genes that give any host a selective advantage over their neighbors (like antibiotic resistance). They probably arose by using origins of replication borrowed from some bacterial chromosomes. Hope this is an answer you can use.
