Biotechnology and Bioinformatics (1+1)

2.1.5. Preparation of a DNA Library

A DNA library is a storehouse of gene tic information maintained in bacteria instead of books. These bacteria are clones created by recombinant DNA, and the foreign DNA they hold is the library’s store of information. DNA libraries are helpful to scientists who require a plentiful supply of particular DNA segments to do their work. These repositories of genetic information are stored in small tubes, which can easily be shipped to other researchers for study.

Each library has a unifying theme. For example, a library may contain the entire chromosomal DNA, or genome , of a given organism, or it may consist of genes that are active within certain types of cells, such as heart cells. To create a library of the human genome, DNA from all the human chromosomes would be cut into many pieces. These pieces would be randomly inserted into vector s, such as plasmid s, which would then be placed into a population of bacteria.Taken together, the entire population of bacteria would contain all the DNA of the human chromosomes.

DNA library is a collection of clone d DNA fragments. There are two types of DNA library:

• The genomic library contains DNA fragments representing the entire genome of an organism. The genomic library is normally made by  phage vectors, instead of plasmid vectors.
• The cDNA library contains only complementary DNA molecules synthesized from mRNA molecules in a cell.

The advantage of cDNA library is that it contains only the coding region of a genome. To prepare a cDNA library, the first step is to isolate the total mRNA from the cell type of interest. Because eukaryotic mRNAs consist of a poly-A tail , they can easily be separated. Then the enzyme reverse transcriptase is used to synthesize a DNA strand complementary to each mRNA molecule. After the single-stranded DNA molecules are converted into double-stranded DNA molecules by DNA polymerase , they are inserted into vectors and cloned.