Plasmids

Plasmids are found in bacterial cells and are circular pieces of DNA that are separate from the bacterial genome. They have the ability to replicate independently of the host cell, however, their replication is dependent on particular enzymes being present in the host cell. Whilst most plasmids are circular, not all of them are.

In circular plasmids replication occurs bidirectionally, (In two directions- clockwise and anticlockwise). The replication begins at the origin site (oriC) it then radiates outwards in both directions with the replication occurring at sites called ‘replication forks’. Both replication forks then reach the terminal region (terC) at the same time. Here the two DNA strands detach and are now separate plasmids.

Plasmids are often used as vectors to create transgenic bacteria, by inserting genes which are not naturally present in the bacterial genome. Genes are inserted into the vector by cutting it at specific restriction sites with enzymes called restriction endonuclease.

Plasmids are an ideal vector as there is not only room for the desired gene, but also a distinguishing gene. This is usually an antibiotic resistance gene, which makes it easier to observe which bacteria have been transformed successfully. Plasmids can be transferred between bacteria independently of human involvement in the process of conjugation, this horizontal gene transfer contributes to bacterial colonies becoming resistant to a specific selection pressures.