How is recombinant DNA made?
Recombinant DNA just means taking a piece of DNA from some place and putting it together with a piece of DNA from some other place. I'll explain
GFP is a gene that produced a protein called Green Fluorescent Protein (think Jelly Fish, glowing fish). This gene is used as a "reporter gene" in molecular biology all the time.
Lets say you discovered a promoter (piece of DNA that turns a gene on), and you want to know where this promoter functions (in which cells). You make a copy of that promoter gene by PCR and clone it into a plasmid.
You make a copy of the GFP cDNA and you clone it into a plasmid. THen you cut out GFP and place it after the Promoter you discovered.
You then cut the plasmid and now you have PromoterX-GFP, so that when promoter X gets turned on, the tissue where it is expressed will turn green.
You take this and inject it into a fertilized mouse egg, let that mouse grow up, and then you've got a mouse that has cells that glow wherever that promoter is turned on. If it were a liver-specific promoter, you'd cut the mouse open and the liver would be glowing.
That piece of DNA - the one with promoterX-GFP is a recombinant piece of DNA.