|Foreign DNA / Passenger DNA
Foreign/passenger DNA is a fragment of DNA molecule which is enzymatically isolated and cloned. The gene is identified on a genome and pulled out from it either before or after cloning. The cloned foreign DNA fragment expresses normally as in parental cell. Thus, the foreign DNA fragments can be procured from a variety of sources depending on the aims and scope of cloning experiments.
Identification and characterization of DNA sequences are rather more difficult on its genome than using mRNA, if it is in pure form. If the gene product translated by mRNA is not well characterized it can be most difficult procedure for cloning. In an average cell or tissue, 1-2% of total cytoplasmic RNA population is mRNA which carry transcripts for coding various proteins. When mRNA is present in low amount it is rather difficult to isolate cDNA clones. The procedures of isolation and purification of mRNA and getting pure cDNA are given in Genes : Nature, Concept and Synthesis.
Besides cDNA, clonable DNA fragments are also isolated from the donor organisms by using restriction endonucleaees or can be procured from a gene bank (see Figure below) constructed for the same purpose.