Fosmids

Fosmids are used when preparing genomic libraries for genome sequencing.  Fosmids are circular DNA of bacterial origin – technically plasmids – but where typical plasmids exist in high copy number (up to 100 copies per cell) and possess small (3 to 6 kb) inserts, fosmids are present as a single copy in a cell and may possess inserts upwards of 40 kb.  Fosmids are advantageous because they produce stable libraries for genome sequencing.  They have a tendency to provide fairly uniform coverage, so they are optimal for closing gaps in whole genome alignments.  In addition to genome sequencing, they have also been used for metagenomics and expression studies.

Furthermore, Fosmids contain the F plasmid (a fertility plasmid that directs conjugal transfer of DNA between bacteria) origin of replication and a cos site. They are similar to cosmids but have a lower copy number in E. coli, which means that they are less prone to instability problems.

Fosmid vectors are derived from random shearing – which yields more uniform coverage when comparing against other library cloning methods.