There are various ways for gene cloning. There are two main libraries which are used to isolate different DNAs, these are known as genomic library and cDNA library. The main difference between these both is, genomic library consists of DNA fragments that represent the whole genome of an organism and cDNA library, messenger RNA is taken from specific cells of an organism and them cDNA is made from that mRNA in a reaction which is catalyzed by an enzyme.
It is a total DNA, which consist of introns, exons, junk DNA and much more. In genomic library, DNA is broken up into smaller pieces, inside a cell. After this, all these small pieces are inserted in a vector to produce the library. This genomic library consists of all the DNA of the cell. Genes contain all of their introns. Genomic DNA is the translation of the entire genome. Genomic library do not code the entire region on one codon because of the size of introns. They contain more sequence information than cDNA because it is prepared from total genomic DNA. There is no splicing mechanism in a genomic library and that is why expression of genes taken from genomic library is difficult.
cDNA library is made by the following process, first of all one cell or tissue type is selected. Then messenger RNA is isolated from that tissue or cell. A DNA copy of mRNA molecule is made using reverse transcriptase enzymes. So unlike genomic library, cDNA contains only that DNA which is present in the mRNA. There are no introns and no DNA sequences. They make a large portion of eukaryotic genomes. In cDNA libraries, all clones are full length. Moreover, a cDNA clone is needed to transect cells for protein production or for cell based assays.
- Genomic library contain fragments of DNA from the entire cell or tissue while cDNA library has only expressed genes sequences.
- Genomic library represents both, coding and non-coding regions. But cDNA library represents only expressed part of the genome.
- There is no splicing mechanism in genomic library.
- Genomic library is difficult to express while cDNA library has only coding sequence and that is what makes its expression easy.
- Genomic library has introns but cDNA library has no introns.
- Vectors used in genomic library are somewhat different than those used in cDNA library.