A set of techniques adopted for gene cloning for a particular purpose is said to be a gene cloning strategy. There are several gene cloning strategies in recombinant technology. DNA fragments are generated by cutting the DNA with a specific restriction endonuclease. These fragments are ligated into a vector molecules and the collection of recombinant molecules is transferred into host cells, one molecule in each cell. The total number of all DNA molecules makes up the library. Two main types of libraries can be used to isolate specific DNAs: genomic and cDNA libraries.
A genomic library contains DNA fragments that represent the entire genome of an organism, whereas in case of cDNA library mRNA from an organism or from an organism or from specific cells of an organism are extracted and then complementary DNA (cDNAs) are prepared from the mRNA in a multistep reaction catalysed by the enzyme reverse transcriptase. The resulting double stranded DNA fragments are then inserted into a suitable vector and cloned, creating a population of clones called cDNA library. Since, a cDNA library is derived from mRNA, the library contains the coding region of expressed genes only, with no introns or regulatory regions. Such a library lacks the non coding DNA that makes up a large portion of many eukaryotic genomes.
|It include all possible fragments of DNA from a given cell or organism.||cDNA library carries only expressed gene sequences.|
|It is larger||It is smaller|
|It represents the entire genome of an organism having both coding and non coding regions.||It represents only the expressed part of the genome and contain only coding sequences called ESTs|
|Expression of genes taken from genomic library is difficult in prokaryotic system like bacteria due to absence of splicing mechanism.||cDNA has only coding sequences therefore can be directly expressed in prokaryotic system.|
|Vectors used genomic library include plasmid, cosmid, lambda phage, BAC and YAC in order to accommodate large fragments||Vectors used cDNA library include plasmid, phagemids, lambda phage etc to accommodate small fragments as cDNA has no introns.|
|Uses of Genomic Library||Uses of cDNA Library|