Introduction: In the steps of cloning discussed in earlier lecture, a vehicle DNA is required to carry foreign DNA to generate a recombinant construct so that it should allow easy amplification of chimeric DNA in host. Vehicle DNA used for this purpose in genetic engineering is known as ‘vector’. In today’s lecture we will discuss the properties of different vector used for host strains.  
 
Criteria of a good vector : The vector DNA has a two main responsibility: (1) ability to carry foreign DNA, (2) ability to replicate in the host. Hence to fulfil these responsibility, a number of properties are desirable. Few crucial properties are as follows-

1. Low molecular weight-The low molecular weight or size confers a number of advantages. (1) small size vector is robust towards shear stress and easier to handle. In addition, after ligating foreign DNA into the vector, the size of the resulting recombinant DNA will be small and it will be easier to deliver the recombinant DNA into the host cell.   

2. Post entry into the host should give phenyotypic changes-Another important feature is that vector DNA should give additional phenotypic changes in the host cell so that recognition of transformed cells will be easier.   

3. Muliple cloning site with unique restriction site- A short stretch of DNA on vector DNA containing restriction sites for insertion of foreign DNA is desirable.

4. High copy number-A high copy number is desirable as it gives high amount of DNA after growing host cells.

Different vectors: As vector needs to replicate in different host strain, vector needs special additional structural features to make it suitable for a particular host strain. Why one vector doesn’t rplicate in different host strains? Replication of vector DNA is controlled by the orgin of replication and it need to be recognized by the host factor especially DNA polymerase to perform replication. Consequently, there are different types of vector DNA to suits the cloning of a foreign DNA in a particular host strain.