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Recombinant DNA

When DNA from least two different sources is joined to form a single DNA molecule, the product is called "recombinant DNA". In this animation, we show how recombinant DNA techniques can be used to get a bacterial cell to make human insulin. Before we can understand this process, however, two new terms, plasmid and restriction enzyme, must be introduced.

A plasmid is a small circular DNA molecule that is found in the cytoplasm of many bacterial cells. As you can see in this animation, it is not part of the bacterial chromosome.

Restriction enzymes (or restriction endonucleases) are proteins that cut DNA at a specific base sequence. For example, the restriction enzyme Eco RI cuts DNA every time it encounters the sequence GAATTC.

The process of making a molecule of recombinant DNA is relatively easy to grasp. First, all of the DNA samples (in this case the plasmid and the human insulin gene) are digested with the same restriction enzyme. As you can see, when a restriction enzyme like Eco RI cuts DNA at its recognition sequence (also known as a restriction site), it generates short overhangs of unpaired bases. These overhangs are called 'sticky ends' and are complimentary to each other. Thus, if you cut two DNA molecules with the same restriction enzyme, their complimentary sticky ends will base pair and the molecules will join. Finally, just like you saw in DNA replication, DNA ligase is used to seal the nicks between the newly joined strands.

This recombinant plasmid can now be inserted into a bacterial cell. Since bacteria can be tricked into 'overexpressing' (that is, making lots of) whatever gene is inserted into them, this bacterium will make lots of human insulin, which can be used to treat people with diabetes.