Inserting a DNA Sample into a Plasmid

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Plasmids are similar to viruses, but lack a protein coat and cannot move from cell to cell in the same fashion as a virus.

Plasmid vectors are small circular molecules of double stranded DNA derived from natural plasmids that occur in bacterial cells. A piece of DNA can be inserted into a plasmid if both the circular plasmid and the source of DNA have recognition sites for the same restriction endonuclease.

The plasmid and the foreign DNA are cut by this restriction endonuclease (EcoRI in this example) producing intermediates with sticky and complementary ends. Those two intermediates recombine by base-pairing and are linked by the action of DNA ligase. A new plasmid containing the foreign DNA as an insert is obtained. A few mismatches occur, producing an undesirable recombinant.

The new plasmid can be introduced into bacterial cells that can produce many copies of the inserted DNA . This technique is called DNA cloning.