Plasmids are able to replicate in the host cell independent of the control of chromosomal DNA. Plasmids like pBR322 is used as a cloning vector in E. coli organisms. The pBR322 plasmid has resistance site for two antibiotics - tetracycline and ampillicin. If we ligate a foreign DNA at the site of tetracycline resistance the recombinant plasmids will not be tetracycline resistant. We can obtain the recombinants by growing them on ampicillin-resistant medium. The restriction site for EcoRI is neither on ampicillin resistance gene nor tetracycline resistant gene. Thus it will be resistant to both these antibiotics