Practice question

Plasmids are extrachromosomal genetic elements independent of main chromosome, typically small circular double-stranded DNA molecules capable of autonomous replication using their own origin of replication. They are nonessential for viability under standard conditions but confer selective advantages like antibiotic resistance, toxin production, or metabolic pathways. Copy number ranges from single to hundreds per cell. They transfer horizontally via conjugation if bearing tra genes. Laboratory cloning vectors derive from natural plasmids like ColE1 and F, engineered for replication and selection markers.