What is a DNA library?

In humans, there are approximately 25,000-35,000 genes among the 3 billion DNA base pairs in the human genome. Genes are nothing but sequences of DNA that code for the synthesis of RNA molecules used to make proteins. In order to study a gene, a researcher needs to isolate it from all the other genes in an organism’s DNA. One method involves the construction of a DNA library. It is a collection of DNA fragments that come from one organism and are stored in another organism, often a bacterium such as E.coli.

Before we open the doors and venture inside a DNA library, it is essential to get back to basics with terms such as DNA, mRNA and cDNA.

DNA or deoxyribonucleic acid molecules, which contain ATGC base pairs, make up the chromosome. Moreover, DNA molecules consist of two specific regions: exons and introns -- the former code for proteins while the latter i.e., introns are non-coding regions of DNA that are not involved in protein production. The DNA of eukaryotic organisms contains both exons and introns. However, prokaryotes lack introns.

When DNA is transcribed, it forms mRNA (messenger RNA), which when translated codes for a particular protein.

Now, everyone is familiar with the term library. You go to a college library to search specific books and they are so arranged that it takes no time to find the one you want. In genetics, a DNA library is constructed to help scientists find specific genes.

A genomic DNA library will contain the entire genome of an organism:

Digest DNA with restriction enzymes: In this process, large human double-stranded DNA molecules are cleaved with restriction enzymes. As a result, millions of genomic DNA fragments are formed. The same procedure is followed for creating libraries of other organisms.

Insert into plasmids: Each DNA fragment is inserted into a plasmid (autonomously replicating circular extra-chromosomal DNA) and is sealed with the enzyme DNA ligase. Once a human DNA fragment is attached to a plasmid DNA, it is called a recombinant DNA molecule.

Make bacteria take up recombinant DNA: Recombinant DNA molecules are then introduced into a bacterial cell, where the foreign DNA molecules replicate and make thousands of similar copies of the same DNA fragment. This is also called cloning where similar DNA molecules are formed. Finally a genomic DNA library is constructed.

A complimentary DNA library contains just the protein-coding DNA content; it lacks regulatory elements and promoter regions, in other words the non-coding sequences. The starting point is messenger RNA which is transcribed by the enzyme reverse transcriptase into cDNA.

NCBI. "Construction of a human genomic DNA library," retrieved on July 10, 2009 from DNA library.

NCBI, "Genomic DNA library," retrieved on July 10, 2009 from genomic DNA library.

NCBI, "Making a cDNA library," retrieved on July 10, 2009 from cDNA library.

Sample contents.