1-3.1 Introduction:

DNA molecule used for carrying an exogenous DNA into a host organism and facilitates stable integration and replication inside the host system is termed as Vector. Molecular cloning involves series of sequential steps which includes restriction digestion of DNA fragments both target DNA and vector, ligation of the target DNA with the vector and introduction into a host organism for multiplication. Then the fragments resulted after digestion with restriction enzymes are ligated to other DNA molecules that serve as vectors.

In general, vectors should have following characteristics:

• Capable of replicating inside the host.
• Have compatible restriction site for insertion of DNA molecule (insert).
• Capable of autonomous replication inside the host (ori site).
• Smaller in size and able to incorporate larger insert size.
• Have a selectable marker for screening of recombinant organism.

1-3.2 Plasmids:

• Plasmids are naturally occurring extra chromosomal double-stranded circular DNA molecules which can autonomously replicate inside bacterial cells. Plasmids range in size from about 1.0 kb to over 250 kb.
  
  
• Plasmids encode only few proteins required for their own replication (replication proteins) and these proteins encoding genes are located very close to the ori . All the other proteins required for replication, e.g. DNA polymerases, DNA ligase, helicase, etc., are provided by the host cell.Thus, only a small region surrounding the ori site is required for replication. Other parts of the plasmid can be deleted and foreign sequences can be added to the plasmid without compromising replication.