One reason is because researchers can manipulate bacteria to retain plasmids by introducing genes for antibiotic resistance into the plasmids. After treatment, bacteria are grown in the presence of antibiotics. Only bacteria carrying the plasmid with antibiotic resistance genes will survive, while others are killed by the antibiotic. Another reason is plasmids are circular DNA molecules, which is a structural feature that makes them highly adaptable for adding additional DNA sequences. This circular structure allows the DNA to be opened, new DNA sequences inserted, and then closed back up without disrupting its integrity. A third reason is because plasmids have the ability to replicate independently within bacterial hosts, even when the host cell is not actively replicating its own DNA. Each time a plasmid replicates, any DNA it carries (including introduced DNA sequences) is also copied.