Plasmids are crucially important within the world of molecular biology. With the new greater emphasis on genetics and molecular biology on the MCAT, I believe that plasmids will often be appearing within passages going forward.
A plasmid is a DNA molecule, often thousands of base pairs long, which is capable of autonomous replication. Although plasmids sometimes appear in eukaryotic organisms, they exist almost exclusively in the world of bacteria.
Although linear plasmids are known, most are circular like the bacterial chromosome. All plastids have a replication origin, a unique DNA sequence at which DNA replication is initiated, so they are capable of independent replication.
Plasmids are crucial tools of the molecular biologist. They are often used as vectors to carry genetic material from one organism to another. Plasmid vectors usually contain one or several restriction sites, making cleavage and insertion possible, and they usually contain a gene for a trait that can be selected against to segregate recombinant cells from those which did not receive the plasmid, usually antibiotic resistance (R-factor).
Plasmids used as vectors can be taken up through transformation (a natural process by which bacteria import DNA fragments from their environment across their cell mentranes) or the plasmid may have tra-genes, F-factor, enabling them to form the conjugation bridge, pilus, for plasmid transfer between bacterial cells. Plasmids which do not engage in conjugation are often piggy-backed on an F-factor plasmid in recombinant DNA experiments. One of the things the textbooks don't convey well is that in actual molecular biology practice, 'ligation and transformation' is the much more commonly utilized pathway for introducing plasmid DNA to bacterial cells than conjugation using an F-factor plasmid as the vector.