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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/
[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/
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