<Li> Vibrio--comma - shaped </Li> <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>

Cells without a membrane-bound nucleus and membrane systems in the cytoplasm are called