The DNA of most bacteria is contained in a single circular molecule, called the bacterial chromosome. The chromosome, along with several proteins and RNA molecules, forms an irregularly shaped structure called the nucleoid. This sits in the cytoplasm of the bacterial cell. In addition to the chromosome, bacteria often contain plasmids — small circular DNA molecules. Bacteria can pick up new plasmids from other bacterial cells during conjugation or from the environment.
They can also readily lose them — for instance, when a bacterium divides in two, one of the daughter cells might miss out on getting a plasmid. For this reason, plasmids can copy themselves independently of the bacterial chromosome, so there can be many copies of a plasmid — even hundreds — within one bacterial cell. What are the basic properties of the genomes of bacteria?
Most bacteria have a genome that consists of a single DNA molecule i. In addition, bacteria may have one or more smaller circular DNA molecules, called plasmids, that contain usually non-essential genes. In general, a single bacterium will be replicating its DNA whenever possible i. Once the genome is completely replicated, the two circular DNAs separate and the cell divides. Bacteria can also transfer plasmids to one another through a process called conjugation.
Scientists have taken advantage of plasmids to use them as tools to clone, transfer, and manipulate genes. Plasmids that are used experimentally for these purposes are called vectors.
Researchers can insert DNA fragments or genes into a plasmid vector, creating a so-called recombinant plasmid. The fact that plasmids are smaller and in greater number than the host chromosome make plasmids easier to isolate in pure form, which is why researchers commonly use them for studying DNA in the laboratory.
Plasmids are thus a fundamental tool of recombinant DNA technology. Eric D.
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