Genomic Libraries, cDNA Libraries and their applications

Genomic Libraries are population of fragments of DNA cloned into a bacterial host. Genomic DNA is restriction digested to small fragments and cloned into suitable vector and maintained in bacterial host.

cDNA library is prepared by isolating mRNA and converted into cDNA by reverse transcription. cDNA is then cloned into plasmid and transformed into bacterial host.
The main advantage of cDNA library is that the use of mRNA eliminates the introns, making it is easy for gene expression studies.

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What is application of plasmid vactor?