Fosmid

Fosmids are similar to cosmids but are based on the bacterial F-plasmid. The cloning vector is limited, as a host (usually E. coli) can only contain one fosmid molecule. Fosmids can hold DNA inserts of up to 40 kb in size; often the source of the insert is random genomic DNA. A fosmid library is prepared by extracting the genomic DNA from the target organism and cloning it into the fosmid vector. The ligation mix is then packaged into phage particles and the DNA is transfected into the bacterial host. Bacterial clones propagate the fosmid library. The low copy number offers higher stability than vectors with relatively higher copy numbers, including cosmids. Fosmids may be useful for constructing stable libraries from complex genomes. Fosmids have high structural stability and have been found to maintain human DNA effectively even after 100 generations of bacterial growth. Fosmid clones were used to help assess the accuracy of the Public Human Genome Sequence.

==Discovery== The fertility plasmid or F-plasmid was discovered by Esther Lederberg and encodes information for the biosynthesis of sex pilus to aid in bacterial conjugation. Conjugation involves using the sex pilus to form a bridge between two bacteria cells; this bridge allows the F+ cell to transfer a single-stranded copy of the plasmid so that both cells contain a copy of the plasmid. On the way into the recipient cell, the corresponding DNA strand is synthesized by the recipient. The donor cell maintains a functional copy of the plasmid. It later was discovered that the F factor was the first episome and can exist as an independent plasmid making it a very stable vector for cloning. Conjugation aids in the formation of bacterial clone libraries by ensuring all cells contain the desired fosmid.