FOREIGN DNA PROPERTIES
In order to insert foreign DNA into a plasmid, scientists first must determine why they are recombining the DNA. If they simply want to look at the DNA sequence or identify regions of DNA, they can use genomic DNA from a cell. If they want to express a protein, they need to use complementary, or cDNA, which is made by reverse transcribing mRNA. The major difference between cDNA and genomic DNA is that cDNA lacks introns and can be cut and pasted directly into a plasmid. The lack of introns in cDNA also enables scientists to put less DNA into the plasmid, which increases the chances that DNA will be correctly inserted. If protein expression is the final goal, scientists often will cut DNA with two different enzymes, leaving two different sticky ends so that the foreign DNA can fit in only one direction.