C-DNA library:
A cDNA library is a combination of cloned cDNA fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of an organism. cDNA is produced from fully transcribed mRNA found in the nucleus and therefore contains only the express genes of an organism. Similarly, tissue-specific tDNA libraries can be produced. In eukaryotic cells, the matured mRNA is already sliced hence the cDNA produced lacks introns and can be readily expressed in a bacterial cell while the information in cDNA libraries is a powerful and useful tool since gene products are easily identified but the library lack information about enhancers, introns and other regulatory elements found in the genomic DNA library.
cDNA is created from a matured mRNA from a eukaryotic cell with the use of an enzyme known as reverse transcriptase. In eukaryotes, a poly a tail distinguishes mRNA from tRNA and rRNA and therefore can be used as a primer site in reverse. Once mRNA is tagged as a complementary primer which binds to the poly a tail providing a free 3` OT that can be extended by reverse transcriptase to create the complementary DNA strand. The mRNA is removed by using an RNA as an enzyme leaving a single strand cDNA is converted into a double-stranded DNA with the help of DNA polymerase. Restriction endonuclease, DNA ligases are then used to clone the sequence into bacterial plasmids. This can now be stored in a cDNA library.
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