(b) : A plasmid is a DNA molecule separate from the chromosomal DNA and capable of autonomous replication. In many cases, it is typically circular and double-stranded. It usually occurs in bacteria, and is sometimes found in eukaryotic organisms. The size of plasmids varies from 1 to over 400 kilobase pairs (kbp) There may be one copy, for large plasmids, to hundreds of copies of the same plasmid in a single cell. The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in 1952 .