During conjugation one cell, the donor or male cell, makes contact with another, the recipient or female, and DNA is transferred directly from the donor into the recipient. The ability to act as a donor in conjugation is determined by the presence of a transmissible plasmid in the cell ; the transmissible plasmid contains the genetic information for conjugation of the host cell with a suitable recipient and for transfer of the plasmid DNA into it. Such a transmissible plasmid is known as a transfer factor or sex factor. Only cells that contain a sex factor are male and can act as donors; those lacking a sex factor are female and act as recipients.
Transfer of DNA between cells by conjugation requires direct contact between the donor and the recipient cell. Transfer factors genes that code for the production of a protein appendage on the surface of the donor cell—a specialized fimbria 1 to 2 μm long known as a sex fimbria or pilus. The tip of the pilus attaches to the surface of a recipient cell and holds the two cells together. The DNA of the transfer factor then passes into the recipient cell. The presence of a pilus is essential for the transfer of DNA, but it is unclear whether the DNA passes along it or is transferred across some other connecting bridge.
Many transmissible plasmids contain genetic markers such as genes for antibiotic resistance or colicine production and then the DNA transferred is not merely the basic transfer factor but also these associated genes. It is thought that one strand of the circular DNA of the transfer factor is nicked open at a specific point and the free end is passed through into the recipient cell.
The DNA is replicated during transfer so that each cell receives a copy. The analogy with sex is somewhat strained here as the recipient or female acquires a transfer factor and is converted into a male, able to conjugate with further females and convert them in turn! In this way a transfer factor may rapidly spread through a whole population of susceptible cells; this process is sometimes described as infectious spread of a plasmid, or `infectious heredity'.