Explain the importance of (a) ori, (b) amp^R and (c) rop in the E. coli vector .

(a) Importance of ori. This  sequence helps in initiation of replication. A foreign piece of DNA has to be linked to this sequence in order to replicate within the host cells. It also controls the copy number of linked DNAl.

(b) Importance of amp^R. It is antibiotic resistance gene for ampicillin. It serves as a cloning sites where foreign DNA can be inserted. It may also helps in selection of transformants. 

(c) Importance of rop. It codes for the proteins involved in the replication of plasmid.