Conjugation is "a form of sexual reproduction seen in some algae, some bacteria and ciliate protozoan. A tube (pilus) joins the two cells, growing out from one or both cells. Genetic information can then be passed from one cell (male) to the other cell (female).
Plasmids that can be transferred in this way are called conjugative plasmids - not all plasmids are conjugative plasmids. The F-plasmid is responsible for conjugation in Escherichia coli - it encodes the proteins that are needed to produce the pilus that joins the two cells together. There are only one or two copies of the F-plasmid in each cell, it is aproximately 100 kb in length and has many genes that control its maintenance in the cell and its transmission between cells. Conjugation is started by physical contact between the two cells. It is controlled by a multisubunit, macromolecular protein structure, that is within the cytoplasm and inner membrane of the bacterium. DNA transfer always occurs along with plasmid replication. This is done by rolling-circle replication. This is where "replication starts with a singe-stranded cleavage at a specific sugar-phosphate bond in a double stranded circle. This cleavage produces two chemically distinct ends. DNA is synthesised by the addition of successive deoxyribonucleotides to the 3' end with simultaneous displacement of the 5' end from the circle. As replication proceeds around the circle, the 5' end 'rolls out' as a tail of increasing length". The transfer of the F plasmid requires only a few minutes to complete.