Plasmid: small, usually ring-shaped molecule of deoxyribonucleic acid (DNA), Plasmids are present in almost all bacteria and may also be found in some yeasts and other fungi, protozoa, and even some plants and animals. They are separate from chromosomes.
Plasmids generally carry fewer genes than do chromosomes, and the genes that they carry are useful, but not essential, to the survival of the cell. Most bacteria have only one chromosome under normal circumstances, but may contain 1 to 100 or more copies of a given plasmid. Plasmid Replication Site Example: pAmp
Recombinant DNA, like most other DNA technologies, relies on restriction enzymes, specialized enzymes that act like molecular scissors to cut DNA strands at certain points in a base sequence, which may be four to eight bases long. This creates “sticky ends.” Another enzyme, called DNA ligase, glues an isolated gene to a vector—a fragment of DNA that is able to transfer from one organism to another (plasmids). In this way, scientists cut and paste pieces of DNA from different sources together to create molecules that will then be known as recombinant DNA.
The ability to combine human and bacterial DNA has given rise to a number of medical advances. The first commercial use of a recombinant plasmids came in 1982, when scientists created a genetically engineered bacterium able to produce human insulin. Insulin is a hormone that helps regulate blood sugar and is needed by many people with diabetes.