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In molecular biology, a cDNA library refers to a complete, or nearly complete, set of all the mRNAs contained within a cell or organism. Because working with mRNA is difficult (as mRNA is unstable and is easily degraded by RNases which can be found even on the skin), researchers use an enzyme called reverse transcriptase which will produce a DNA copy of each mRNA strand. Referred to as cDNA these reverse transcribed mRNAs are collectively known as the library.

Such a library has several uses. A cDNA of an eukaryotic organism (for example, a human) can be cloned into a prokaryotic organism (for example, E. coli) and expressed (translated into the appropriate protein) there (with limitations, for example posttranslational modification). A cDNA library is also important for analysis through bioinformatics. The complete cDNA library of an organism gives the total of the proteins it can possibly express. Also, the cDNA sequence gives the genetic relationship between organisms through the similarity of their cDNA.

Contents 1 cDNA Library Construction 2 cDNA Library vs. Genomic DNA Library 3 Full-length cDNA libraries 4 Screening a cDNA library 5 External links 6 See also

cDNA libraries are prepared from total or enriched Poly(A)+ single stranded mRNA that is converted into a double-stranded DNA copy of the message using the enzyme reverse transcriptase. The cDNA fragments can be inserted into an appropriate plasmid, phage, or cosmid vector for maintenance and cloning. The population of recombinant vectors will represent the entire set of expressed genes in the cell from which the RNA was isolated.

In Eukaryotic DNA, the genes may be very long because they are full of introns. The advantage of a cDNA library is the introns are spliced out and the mRNA sequence can be used as a template to create DNA (via Reverse Transcriptase) to collect the preferred genes. Prokaryotic DNA is much shorter and mRNA is difficult to isolate so the genomic DNA library is preferred in this case.

Differently than standard cDNA libraries, in full-length cDNA libraries most of the cDNA represent complete original mRNA molecules. These are produced by implementation of normal methods for cDNA libraries preparation. The main technologies are known as "cap-trapper" and "oligo-capping".

1. using PCR 2. by hybridisation 3. Expression cloning 4. using Antibodies(western blotting)

• Functional Annotation of the Mouse database
• examples of cDNA synthesis and cloning

Genomic library