Origin of replication (ori): This is a sequence from where the replication starts and thus any piece of DNA, when linked to this sequence, is also replicated within the host cells. Thus, an alien DNA that is linked with the origin of replication can replicate and multiply itself in the host organism. This is known as cloning or making multiple identical copies of any template DNA. Ori sites are also responsible for controlling the copy number of the alien or linked DNA. So, if many target DNA copies are required, they should be cloned in a vector whose origin supports a high copy number.