Plasmids are small , circular , extrachromosomal , double-stranded dna molecules . They are capable of self-replication and contain genes that confer some properties , such as antibiotic resistance , virulence factors . Plasmids are not essential for cellular survival .

质粒是一小的，环状的，染色体外的双股dna分子，能自我复制，带有基因，赋予细菌某些性状如耐药性，毒力等。质粒不是细菌生存所必须的。