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An episome is an independent segment of DNA capable of attaching to a bacteria or cell as well as integrating itself into the genome. Episomes offer advantages to their hosts and are easily transferable to pass those advantages on to other members of the same species. They are a topic of interest for researchers because they play a role in bacterial virulence, and they can be useful as vectors for the insertion of genetic material into bacteria for the purpose of research.
Each episome carries a short string of potentially useful DNA that might confer an advantage such as drug resistance or the ability to thrive in harsh environments. When an episome attaches itself to a bacterium, it can replicate along with the rest of the organism, providing the same genetic heritage to the next generation of organisms. It also can add itself to the bacterial genome and become a permanent part of it.
Bacteria might swap episomes and other strands of independent DNA, like plasmids, with each other. This allows them to share advantages without having to wait for the next generation, and it can provide a mechanism for quickly spreading resistance and durability through a bacterial population. Bacteria without that segment of DNA might die off in response to antibiotics or harsh conditions, but the survivors will thrive and might multiply to fill the gap left by their fallen fellows.
One example of an episome is a virus. Viruses are not living organisms in the traditional sense but rather contain genetic material that they can use to hijack cells and force them to reproduce copies of the virus. In the process, the virus actually destroys the cell it takes over, but at that point, it has already accomplished the goal of replicating itself. Collateral damage caused by viruses and viral replication can range from a brief head cold to a serious illness.
Researchers look at episomes and similar strands of independent DNA to understand how bacteria evolve and change over time as well as how they pass on various advantages. This can be important for the study of an epidemic or outbreak, in which tracing the genetic heritage might help researchers understand how to combat the disease. Episomes also can be used to an advantage in research by inserting genetic material into cells or whole organisms. The researchers can engineer the DNA to retain the segments that allow the episome to attach and multiply, and they can clip out unnecessary sections and insert new ones.