<P> The archaeon Haloquadratum has flat square - shaped cells . </P> <P> Bacteria and archaea reproduce through asexual reproduction, usually by binary fission . Genetic exchange and recombination still occur, but this is a form of horizontal gene transfer and is not a replicative process, simply involving the transference of DNA between two cells, as in bacterial conjugation . </P> <P> DNA transfer between prokaryotic cells occurs in bacteria and archaea, although it has been mainly studied in bacteria . In bacteria, gene transfer occurs by three processes . These are (1) bacterial virus (bacteriophage) - mediated transduction, (2) plasmid - mediated conjugation, and (3) natural transformation . Transduction of bacterial genes by bacteriophage appears to reflect an occasional error during intracellular assembly of virus particles, rather than an adaptation of the host bacteria . The transfer of bacterial DNA is under the control of the bacteriophage's genes rather than bacterial genes . Conjugation in the well - studied E. coli system is controlled by plasmid genes, and is an adaptation for distributing copies of a plasmid from one bacterial host to another . Infrequently during this process, a plasmid may integrate into the host bacterial chromosome, and subsequently transfer part of the host bacterial DNA to another bacterium . Plasmid mediated transfer of host bacterial DNA (conjugation) also appears to be an accidental process rather than a bacterial adaptation . </P> <P> Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the intervening medium . Unlike transduction and conjugation, transformation is clearly a bacterial adaptation for DNA transfer, because it depends on numerous bacterial gene products that specifically interact to perform this complex process . For a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state called competence . About 40 genes are required in Bacillus subtilis for the development of competence . The length of DNA transferred during B. subtilis transformation can be as much as a third to the whole chromosome . Transformation is a common mode of DNA transfer, and 67 prokaryotic species are thus far known to be naturally competent for transformation . </P>

What eukaryote is often studied with the prokaryotes