DNA (GENE) LIBRARY

A gene library is the collection of different DNA sequences from an organism where each sequence has been cloned into a vector for ease of purification, storage and analysis. 

A gene library should contain a certain number of recombinants with a high probability of containing any particular sequence. This value can be calculated if you know the genome size.

A genomic library, also clone bank or gene bank, is a set of DNA from a single organism, ideally though not necessarily containing its whole genomic DNA sequence. The DNA from the source organism of interest is divided into multiple fragments and packaged within cloning vectors such that each carries a portion of it. The vector DNA can then be inserted into host organisms - commonly a population of bacteria - for amplification and retrieval. 

Type of Gene Library

There are two categories of libraries on the basis of source of DNA used:

    (i)            Genomic library (where genomic DNA is used), and

  (ii)            cDNA library (where cDNA or complementary DNA is produced using mRNA).

Genomic Library

It is a set of clones that represent the complete genome of an organism. All fragments of DNA inserted into vectors for further propagation into suitable host represent the whole genome of an organism.

For building of a genomic library the entire genomic DNA is Isolated from host cells/tissues, purified and broken
randomly into fragments of correct size for cloning into a suitable vector. There are two fundamental ways of fragmenting genomic DNA randomly: physical shearing (e.g. pipetting, mixing or sonication) and partial restriction enzyme digestion (by using limiting amount of restriction enzyme the DNA is not digested at every recognition sequence). Using these methods genomic DNA is broken randomly into smaller fragments.

Following Fig. shows a total of 6 random DNA fragments obtained after physical shearing. Vector isolated from bacterium is also digested with the restriction enzyme which digests genomic DNA. The fragments of genomic DNA are inserted into the vector. Each vector consists of diff of DNA. The Recombinant DNA molecules are transferred into bacterial cells bacteriophage particles is assembled.

The genomic library for organisms with smaller genome size (e.g. E. coli) can be construe, in plasmid vector. Only 5,000 clones (of the average size of 5,000 bp) results in 99% chance cloning the entire genome of 4.6 x 10 bp. In addition, libraries from organisms with superior genomes are constructed using phase, cosmid, BAC or YAC vectors.

 cDNA Library

The mRNAs are highly processed representatives of genes which express under definite conditions. The mRNAs cannot be directly cloned because they are unstable hence mRNAs

are converted into cDNAs. The library prepared from complementary or copy DNA (cDNA) is called cDNA library. The cDNA library stand for the DNA of only eukaryotic organisms, not the prokaryotic ones. Because genomic DNA of eukaryotes contains introns (A gene segment that is transcribed from DNA which later on is excised from the primary transcript during processing to a functional  RNA molecule and Exon is the coding region of the gene that are present in processed mRNA), regulatory regions, repetitiv The cDNA library can be constructed by using mRNA because mRNAs are the highly processed, intron-free representatives of DNA having only coding  sequence. Keeping in view the aim of the experiment, DNA molecule is either enzymatically fragmented or maybe procured from cDNA or genomic libraries. Getting a cDNA fragment of its Known function or characterizing its sequence is very difficult. A cDNA fragment is prepared directly by using mRNA as template. It follows many biochemical methods.

Therefore, establishment of genomic library of eukaryotes is not meaningful.