DNA libraries

A genome is huge and its genes are scattered. So scientists cut the DNA into fragments, clone each fragment in a vector, and keep the whole collection. This is a DNA library: like a library of books, any fragment can be found and taken out later.

Genomic library. The organism's whole genomic DNA is cut with a restriction enzyme. The fragments are joined into vectors, put into bacteria, and the bacteria grown. Together, all the clones represent the entire genome.

Uses: sequencing a whole genome; finding genes, including those that are not expressed; studying evolution; and comparing healthy and diseased tissue.

cDNA library. Genes are expressed in a tissue-specific way: insulin is made only by the beta cells of the pancreas, albumin only by the liver. So the mRNA in a tissue reflects the genes active there. To make a cDNA library:

• collect the mRNA from one tissue;
• copy it into the first strand of DNA with reverse transcriptase;
• remove the mRNA with RNase;
• make the second strand with DNA polymerase, giving double-stranded cDNA;
• clone the cDNA in vectors.

So a cDNA library holds only the genes that tissue was expressing, and libraries from different organs, or from the same organ in health and disease, are different.

Uses: finding out which genes are active in a tissue; isolating a particular gene (without its introns, ready to express in bacteria); and making cDNA probes to search a genomic library.