Hint: A plasmid is a small, extrachromosomal DNA molecule present within a cell. It is physically separated from the chromosomal DNA and can replicate independently. They are most commonly found in bacteria in the form of small circular, double-stranded DNA molecules.
Complete Answer: The term plasmid was introduced in 1952 by the American molecular biologist Joshua Lederberg to state "any extrachromosomal hereditary determinant." Lederberg began his medical studies at Columbia's College of Physicians and Surgeons while continuing to perform experiments. He was inspired by Oswald Avery's discovery of the importance of DNA and began to investigate his hypothesis that bacteria did not simply pass down exact copies of genetic information, contrary to prevailing opinion. Such passing down of information would make all cells in a particular lineage essentially clones. Lederberg wrote to Edward Tatum, a leading American geneticist, proposing a collaboration. In 1946 and 1947, Lederberg took a leave of absence to study under the mentorship of Tatum at Yale University. Lederberg and Tatum showed that the bacterium Escherichia coli entered a sexual phase during which it shared genetic information through bacterial conjugation. They shared the Nobel Prize of 1958 for their discovery. In order for plasmids to replicate independently within a cell, they must be in possession of a stretch of DNA which can act as an origin of replication. The self-replicating unit, in this case, the plasmid, is called a replicon. A typical bacterial replicon consists of a number of elements, such as the gene for plasmid-specific replication initiation protein (Rep), repeating units called iterons, DnaA boxes and an adjacent AT-rich region. Smaller plasmids make copies of themselves with the help of host replicative enzymes, while larger plasmids carry genes specific for the replication of those plasmids.
Note: The term, plasmid, initially indicated any bacterial genetic material that exists extrachromosomal for at least part of its replication cycle. Since the description, extrachromosomal includes bacterial viruses, the notion of plasmid was refined over time to comprise genetic elements that reproduce autonomously. Later in 1968, the definition was narrowed to genetic elements that exist exclusively or predominantly outside the chromosome and can replicate autonomously.