libraries

Meaning: A DNA library is a collection of DNA fragments, each inserted into a vector such as a plasmid or bacteriophage and carried in host bacteria, so a sequence of interest can be found and copied. A genomic library holds restriction fragments of the whole genome, so it includes introns, promoters and other noncoding DNA. A cDNA library is made from mRNA with reverse transcriptase, so it holds only sequences that were transcribed and has no introns. Clones are found by hybridization with a labeled complementary probe.

On the MCAT: Passages ask which library suits a goal. To express a eukaryotic gene in bacteria, which cannot splice introns, use cDNA. To study promoters, regulatory regions or intron structure, use genomic. cDNA libraries differ between tissues or conditions because the mRNA present differs; genomic libraries from different tissues of one organism are essentially the same. You may also be asked to order the steps: cut or copy the DNA, ligate it into vectors with DNA ligase, transform bacteria, screen with a probe.

Common confusion: A cDNA library is not a copy of the genome. It reflects only the genes transcribed in that cell at that time. The c stands for complementary, not circular.