What is a DNA library?

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A DNA library is a collection of cloned DNA fragments stored in host cells (usually bacteria), which constitute an organism’s entire genome. There are also cDNA (complementary DNA) libraries that are made through the reverse transcription of messenger RNA isolated from an organism. Such cDNA libraries give insight of which genes are actively transcribed in an organism; one drawback is that in cDNA libraries not all genes are represented.

DNA libraries are created by cutting the DNA using mechanical methods or with a restriction endonuclease, and then cloning the fragments into vectors to produce recombinant DNA. Vectors are ‘delivery vehicles’ that can carry inserted DNA into host cells; there are several types, which differ in their insert capacity (the amount of DNA that can be inserted). Some of these vectors are: plasmids, bacterial artificial chromosome (BAC), cosmids, yeast artificial chromosome (YAC), and bacteriophage λ [1]. Plasmids are the most commonly used in recombinant DNA technology, but the vector to be used in a particular experiment depends mostly on the length of the sequence or sequences to be cloned. For instance, plasmids would not be appropriate to clone large sequences of DNA, as they can only store DNA inserts no larger than 20 kb [2]. Therefore, to clone very large sequences BAC or YAC would be more suitable choices, because they can carry foreign DNA of more than 100 kb [1]. Viruses, such as bacteriophage λ, can also be used as vectors to clone relatively large fragments of DNA that cannot be cloned by plasmids. Phages insert the recombinant DNA by infecting bacteria, where the DNA sequences can then be cloned.

One of the challenges that scientists have to face when creating a library is that to make sure most of the organism’s genome is contained within the library, “the sum of the inserted DNA in the clones of the library should be three or more times the amount of DNA  in the genome” [1]. This makes the process of creating a DNA library very time consuming and expensive. Another challenge scientists could have is that, in some cases, certain fragments could be too large to be cloned. Therefore, they have to use more than one restriction endonuclease to create the library.

DNA libraries are extremely useful for biotechnology, as they allow genes of interest to be readily available to clone for further studying. These genes have numerous applications, such as:

• to create transgenic animals to study specific diseases and/or treatments;
• for genetic therapy;
• to confer a specific trait to an organism, such as inserting a gene to create a pest resistance plant. 

References:

[1] Glick, B. R., Pasternak, J. J., & Patten, C. L. (2010). Molecular Biotechnology, Principles and Applications of Recombinant DNA. Washington, DC: ASM Press.

[2] Griffiths, A.J.F., Gelbart, W.M, Miller, J.H, et al. (1999). Modern Genetic Analysis. New York: W. H. Freeman. Retrieved from: http://www.ncbi.nlm.nih.gov/books/NBK21450/