Deoxyribonucleic acid (DNA) contains all the genetic hereditary information of an organism. The information is most often referred to as genetic information. To study genes that are present in DNA and what regulates their activity, DNA libraries can be constructed. DNA library contains all the hereditary information of a particular organism stored in another organism, mainly bacteria and fungi. There are three different DNA libraries that differ according to what they include. These are genomic DNA libraries, metagenomic DNA libraries and contemporary DNA (cDNA) libraries.

Genomic DNA library: This is a collection of cloned DNA fragments that are a complete genome of an organism, containing coding and non coding regions referred to as exons and introns respectively. Overlapping fragments are needed to make this library, thus partial digest of the DNA material using restriction digest.. To make this library complete DNA of an organism is isolated. It is then digested with restriction enzymes, a 4bp cutter is preferred and Sau3A is mainly used, breaking it into fragments. The DNA is cloned into a vector which might be plasmids contained in organisms, in λ bacteriophages, cosmids or artificial yeast or artificial bacterial chromosomes. This library is ideal for the study of up or downstream regulatory sequences such as promoters and terminators.

Metagenomic DNA library: This is not that different to genomic DNA library, except only the sample is taken from an environment, thus it contains all the DNA of organisms that were in that particular environment at the time of collection. They are constructed mainly to study microbial ecology and for novel gene discovery. The construction is the same as for Genomic DNA library.

cDNA library: This is a collection of coding sequences of the DNA or transcribed parts of the DNA. Thus cDNA is synthesised from reverse transcribing mRNA, hence the name cDNA. Thus it will contain genes that were present in a tissue or organism under particular conditions when the sample was collected. To construct this library, complete mRNA is isolated and a complementary cDNA strain is synthesised using reverse transcriptase. The single stranded cDNA is then converted to a double strand by activity of DNA polymerase and then inserted into a vector such as bacterial plasmids and cloned to amplify it in microorganisms such as Escherichia coli. Therefore this library comes into hand when studying or creating profiles for gene activity in cells or tissues types at different developmental levels, environmental conditions and other parameters that have an effect on what genes are expressed or rather transcribed at the given time.

All these libraries can be used to screen for DNA sequence, expressed proteins and products of interest. Even though other cloning vectors are mentioned above where the DNA fragments can be inserted before being cloned into a host, the size of the DNA determines, in a way the carrier into which it can be inserted or ligated. For example, it is ideal to use DNA cosmids to store larger fragments of DNA.

When searching for particular gene whose sequence is known, a probe can be made that is actually a whole or just fragment sequence of the gene, and then this probe can be used to screen for its complementary strand in a library. If such a strand is there in the screened library it will hybridise to the probe thus confirming its presence in the particular organism or sample whose DNA was used to construct the library.

DNA libraries are important as they allow researchers to sequence DNA, to study the genes that are expresses in different conditions and events. Thus libraries can be constructed for specific studies and thus gain insight onto genes that are expressed say when a plant is exposed to drought stress, if libraries are made from this plant, genes that were expressed, the promoters and terminators involved will be deduced and thus further research on these.

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