<P> Pili are antigenic . They are also fragile and constantly replaced, sometimes with pili of different composition, resulting in altered antigenicity . Specific host responses to old pili structure are not effective on the new structure . Recombination genes of pili code for variable (V) and constant (C) regions of the pili (similar to immunoglobulin diversity). </P> <P> Conjugative pili allow for the transfer of DNA between bacteria, in the process of bacterial conjugation . They are sometimes called "sex pili", in analogy to sexual reproduction, because they allow for the exchange of genes via the formation of "mating pairs". Perhaps the most well - studied is the F pilus of Escherichia coli, encoded by the F plasmid or fertility factor . </P> <P> A pilus is typically 6 to 7 nm in diameter . During conjugation, a pilus emerging from the donor bacterium ensnares the recipient bacterium, draws it in close, and eventually triggers the formation of a mating bridge, which establishes direct contact and the formation of a controlled pore that allows transfer of DNA from the donor to the recipient . Typically, the DNA transferred consists of the genes required to make and transfer pili (often encoded on a plasmid), and so is a kind of selfish DNA; however, other pieces of DNA are often co-transferred and this can result in dissemination of genetic traits throughout a bacterial population, such as antibiotic resistance . Not all bacteria can make conjugative pili, but conjugation can occur between bacteria of different species . </P> <P> Some pili, called type IV pili, generate motile forces . The external ends of the pili adhere to a solid substrate, either the surface to which the bacterium is attached or to other bacteria . Then, when the pili contract, they pull the bacterium forward like a grappling hook . Movement produced by type IV pili is typically jerky, so it is called twitching motility, as opposed to other forms of bacterial motility such as that produced by flagella . However, some bacteria, for example Myxococcus xanthus, exhibit gliding motility . Bacterial type IV pili are similar in structure to the component flagellins of archaeal flagella . </P>

What is the function of pili in a prokaryotic cell