Explain the Recombinantion of DNA ?

Recombinant DNA is made by combining DNA from more than one source - often from very different species. The technique is now the basis for many biotechnology advances. Usually, a human or other animal gene is inserted into the bacterium E. coli, the most common bacterium in the human intestine. Because this bacterium divides in as little as 20 minutes, biologists are able to clone (produce many copies of) the gene in a short time, and harvest the proteins they produce. The technique for introducing DNA into E. coli is as follows:

1. The DNA is cut from chromosomes, usually human or animal, by use of a specific restriction enzyme, each of which cuts DNA at a specific spot between nucleotides.

2. The DNA is inserted into a plasmid, a small, circular piece of DNA found in E. coli which is not part of the large main molecule. First, the plasmid DNA must be cut using another restriction enzyme. These enzymes leave short pieces of unpaired DNA strands at the ends of the cut segments. These "sticky ends" bond by base-pairing to other single-stranded DNA, binding the gene segment into the plasmid.

3. The plasmid containing the foreign gene is inserted into another E. coli bacterium by mixing the plasmids with E. coli cells that have been made relatively permeable by means of osmotic shock, a drastic change in concentration of the medium surrounding the cells. Some of the plasmids enter the bacterium, where they replicate along with the bacterial DNA.

4. Recombinant cells are segregated and cloned; that is, billions of copies are grown in large vats, and the product is extracted from the culture medium and purified.