PREPARING AND CLONING A DNA LIBRARY:
In molecular biology, a library is a collection of DNA fragments (specifically genes) from a particular species that is stored and propagated in a population of micro-organisms through the process of molecular cloning.
There are two main types as follows-
1. Genomic Library
2. cDNA Library
1. Genomic Library
Collection of all clones of DNA fragments that represent the complete genome of an organism.
For construction of genomic library of an organism, its entire genome (DNA) is isolated and cut into fragments using suitable restriction endonuclease.
These fragments are then inserted into cloning vectors.
The recombinant vectors are then transferred into suitable organism such as bacteria or yeast.
One fragment in each host cell and then these transformed organisms are cultured to produce their clones and are stored.
When genomic DNA is fragmented, it is not known which fragment has the desired gene.
Therefore all the fragments are cloned and copies of each are stored separately.
Screening for the desired gene can be done through complementation or using probes.
2. cDNA Library
cDNA is complementary DNA.
It is produced using mRNA by the process called reverse transcription.
This process is called 'Teminism', named after its discoverer Temin and Baltimore.
Mostly for eukaryotic organisms cDNA library is constructed.
In eukaryotes there are coding and non-coding sequences in the genes.
The mRNA carrying only the transcribed coding sequences is translated into protein.
When mRNA of different cells, tissues and organs at different time and phases in the life cycle of an organism are isolated.
Using reverse transcriptase enzyme converted into cDNA.
A library having cDNA for each and every type of structural/functional protein can be constructed by inserting it into a suitable vector.
It is cloned in a proper host such as E.coli.
Production of human proteins such as inteferons, insulin, and blood clotting factor VIII can be done using bacteial cultures having cDNA.
