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let's begin with the term what's the vector? Vector is any DNA molecule that has the capacity to replicate inside the host to which our desired gene is inserted for cloning or simply it is a desired DNA carrier or the vehicle that carries our desired DNA. let's take an example to understand better suppose this is gene x, so I need to transfer this gene x into this host bacterium, I need a vehicle for that, that vehicle is a vector, the most common vector is plasmid what we do is we'll be cutting the plasmid using restriction enzyme then attaching our gene x or desired gene and join it with ligase enzyme to form a recombinant DNA molecule and this recombinant DNA molecule is transferred into the host or bacterium so your vector acts as the desired gene carrier.

Now the second point what is the minimum requirement of a vector first and the foremost thing is origin of replication. It is a sequence that is needed or essential for replicating inside the host organism the second aspect is selectable markers. Selectable markers helps in selection of transformed colonies or host organisms with our gene of interest. and the third point is the presence of restriction sites or restriction enzyme sites that should be within selectable markers here as you can see the Bam H1 site is within the selectable marker that helps in selection of recombinant colonies by insertional inactivation. So these are the three minimum of a vector. Now let us move into different types of vectors starting with the most common one that is the plasmid vector as you see this is the plasmid, plasmids are extra chromosomal double stranded circular DNA molecule that is present in bacteria that is widely used as a gene cloning vector. We have already discussed this --------- this is a pBR 322 a genetically modified or a genetically engineered plasmid with ori, two selectable markers ampicillin resistance region or tetracycline resistance region and there are many restriction sites, restriction sites within the selectable markers also. So plasmid is, a modified plasmid is widely used as a gene cloning vector with an insert size of 6 to 15 kb widely used for protein expression, as plasmid is known to bacteria, once inside the bacteria it can replicate very easily and it is capable of expressing inside bacteria forming the protein product. The advantages is it is very easy to handle, it can replicate inside the host bacterium and is very stable also. But the major disadvantage is the insert size the 6 to 15 kb means plasmid can accommodate only small fragments of DNA.