The best description is that plasmids are small, circular, double-stranded DNA molecules that carry nonessential genes which can improve a prokaryote’s survival (for example, antibiotic resistance genes).

Structure of plasmids

Plasmids are usually covalently closed circular molecules of double-stranded DNA that exist separately from the main bacterial chromosome. They are relatively small and replicate independently inside the cell, so a single prokaryote can carry multiple copies or different plasmids at once.

Importance to prokaryotes

Genes on plasmids often encode traits such as antibiotic resistance, toxin production, or metabolic abilities that give the cell an advantage under certain environmental conditions. Because plasmids can be transferred between bacteria, these beneficial traits can spread rapidly through a population, influencing adaptation and evolution.