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Concept
CDNA library
Summary
A cDNA library is a combination of cloned cDNA (complementary DNA) fragments inserted into a collection of host cells, which constitute some portion of the transcriptome of the organism and are stored as a "library". cDNA is produced from fully transcribed mRNA found in the nucleus and therefore contains only the expressed genes of an organism. Similarly, tissue-specific cDNA libraries can be produced. In eukaryotic cells the mature mRNA is already spliced, hence the cDNA produced lacks introns and can be readily expressed in a bacterial cell. While information in cDNA libraries is a powerful and useful tool since gene products are easily identified, the libraries lack information about enhancers, introns, and other regulatory elements found in a genomic DNA library.
cDNA is created from a mature mRNA from a eukaryotic cell with the use of reverse transcriptase. In eukaryotes, a poly-(A) tail (consisting of a long sequence of adenine nucleotides) distinguishes mRNA from tRNA and rRNA and can therefore be used as a primer site for reverse transcription. This has the problem that not all transcripts, such as those for the histone, encode a poly-A tail.
Firstly, mRNA template needs to be isolated for the creation of cDNA libraries. Since mRNA only contains exons, the integrity of the isolated mRNA should be considered so that the protein encoded can still be produced. Isolated mRNA should range from 500 bp to 8 kb. Several methods exist for purifying RNA such as trizol extraction and column purification. Column purification can be done using oligomeric dT nucleotide coated resins, and features of mRNA such as having a poly-A tail can be exploited where only mRNA sequences containing said feature will bind. The desired mRNA bound to the column is then eluted.
Once mRNA is purified, an oligo-dT primer (a short sequence of deoxy-thymidine nucleotides) is bound to the poly-A tail of the RNA. The primer is required to initiate DNA synthesis by the enzyme reverse transcriptase.
Official source
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