Plasmids are extra-chromosomal molecules of DNA, mostly in the form of double-stranded circles. Plasmids are mostly found in prokaryotes, but may be found in some archaea and eukaryotes. Plasmid usually encodes non-essential genes that are not required during normal growth conditions. The plasmid may encode gene that governs the cell's physiology aspect in order to survive in harsh environment such as antibiotic resistance.
Figure 1. Structure of bacteria.