Scientists created DNA libraries, which represent collections of cloned DNA. Depending on how a library is constructed, it may contain genes and noncoding regions of DNA.
Generally speaking, there are two main types of libraries, genomic DNA libraries and complementary DNA (cDNA) libraries. Genomic Libraries Ideally, a genomic library consists of many overlapping fragments of the genome, with at least one copy of every DNA sequence in an organism’s chromosomes, which in summary span the entire genome.
In making a genomic library, chromosomal DNA is extracted from cells or tissues and cut randomly with restriction enzymes, and the resulting fragments are inserted into vectors. Complementary DNA (cDNA) Libraries Complementary DNA (cDNA) libraries offer certain advantages over genomic libraries and continue to be a useful methodology for specific approaches to gene cloning and other applications.
This is primarily because a cDNA library contains DNA copies—called cDNA—which are made from mRNA molecules isolated from cultured cells or a tissue sample. A cDNA library therefore represents only the genes being expressed in cells at the time the library was made—unlike a genomic library, which contains all of the DNA, coding and noncoding, in a genome.
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