DNA libraries are extensive collections of relatively short DNA sequences that represent genome, single chromosome, or gene expression profile in a cell type. 

The DNA molecules that constitute genomes and even a single chromosome are very long. Large scale scientific analysis that could be done with them in that state is limited. Therefore, scientists shear them into small pieces to make DNA libraries. These libraries of small pieces are easier and quicker to analyze.

Let’s say you are a scientist and have a task of sequencing the entire genome. Then what you do first is to prepare a DNA library. For this, you break your genome into much smaller pieces by using restriction enzymes. If you were living in the 90s and working on a human genome project, you should also have found appropriate DNA vectors to clone the DNA pieces you obtained. Then you would put those vectors into a bacteria or yeast and amplify them. Lastly, you would isolate your vectors and sequence each of them. But, luckily you live today and we have next-generation sequencing. All you need to do is some small chemical treatments to your collection of DNAs, and then the fellows doing sequencing will accept that as a “library”.

Nevertheless, let’s not be arrogant and celebrate the old. In the old days, when the genomic sequences of many organisms were not available, scientists were able to produce insulin and human growth hormone (HGH) in bacteria. This achievement, which changed the lives of so many people, was possible with cDNA libraries (also with some remarkable scientists).

cDNA library is a representation of an mRNA expression profile from the cell type that is under investigation. To obtain this library, researchers isolate mRNAs from target cells. Then they convert these mRNAs into complementary DNAs (cDNA) and clone cDNAs to plasmid vectors. These vectors are multiplied in bacterial colonies. Each of these colonies bears a plasmid containing a cDNA that is complementary to specific mRNA. By using special techniques, researchers identify the colony that has their target gene (e.g. HGH gene). Then, they grow that colony and isolate the protein product (e.g. HGH protein). As a result, large scale commercial production becomes possible. 

Key Points

• Various types of DNA libraries are genomic, chromosomal, and cDNA libraries. 

• DNA libraries store genomic, chromosomal, and gene expression profile information in the form of short DNA sequences. 

Key Terms

plasmid: a circular DNA in bacteria and protozoa that is separate from the chromosomes.

vector: carrier DNA molecule that is used to store, amplify, express a certain gene, or any DNA sequence of interest.

DNA sequencing: generation of the exact order of nucleotides for a DNA molecule. 

mRNA: an RNA molecule that is complementary to one strand of DNA and a product of transcription process.

gene expression: synthesis of mRNA and proteins in the cell.  

DNA libraries: large collections of relatively short DNA sequences that represent a genome

restriction enzymes:enzymes that can target and cut a specific region of DNA; they are naturally found in bacteria

genome: the genetic material of an organism

cDNA: DNA synthesized from a single-stranded RNA by the enzyme reverse transcriptase