cDNA library
/ see-DEE-en-AY /
A genomic library stores every page of the genome, including the vast stretches that are never read in a given cell. But often you only care about the genes a cell is actually using. A cDNA library captures exactly that: a collection of DNA copies made from the messenger RNAs that a cell was expressing — a snapshot of its active genes.
To build it you start not from DNA but from the cell's mRNA. The enzyme reverse transcriptase reads each mRNA and synthesizes a complementary DNA strand against it; this is the 'c' in cDNA — complementary DNA. The RNA is then removed and the second DNA strand built, giving double-stranded cDNA that you clone into vectors just like genomic fragments. The key difference: because mature mRNA in eukaryotes has already had its introns spliced out, the cDNA contains only the coding exons — no introns, no intergenic junk. It is a clean, intron-free copy of the gene's working sequence. And because abundant mRNAs are copied more often, the library reflects which genes are highly expressed in that tissue.
cDNA libraries are indispensable for two reasons. First, only an intron-free cDNA can be expressed in bacteria, which cannot splice — so to make a human protein in E. coli you must clone the cDNA, not the genomic gene. Second, comparing cDNA libraries from different tissues or conditions reveals which genes are switched on where — the foundation of studying gene expression. The catch: a cDNA library only contains genes that were being transcribed in the source cells; a gene that was silent leaves no mRNA and so never appears.
To clone the insulin gene for drug production, you do not use the genomic gene (with its introns that bacteria cannot remove). Instead you make a cDNA library from pancreatic beta cells, which are loaded with insulin mRNA, and pull out the intron-free insulin cDNA — ready for E. coli to transcribe and translate into protein.
cDNA = intron-free copy of expressed mRNA, made by reverse transcriptase.
A cDNA library is biased toward what was being expressed: it omits genes that were off in the source tissue, and abundant transcripts are over-represented while rare ones may be missed entirely.