In
Sexuality and Recombination in Bacteria and Viruses, we discussed sexuality in bacteria and viruses. In this connection, we discussed the role of sex factor in conjugation and also made a reference to
episomes , which are characterized as follows : (i) episomes are DNA molecules, but not indispensable, and therefore, may or may not be present; (ii) when absent, episomes can not originate
de novo and may be acquired only from other strains due to infection (phage) or conjugation (sex factor); (iii) when present, episomes may be found in autonomous (free) or integrated state (attached to chromosome) and may change its state from autonomous to integrated or
vice versa, and (iv) when present, episomes may be lost irreversibly.
With the help of above criteria, one may include sex factors (F), bacteriophages, and colicinogenic factors (
Col)in the class of genetic elements called episomes. One may visualize that, whenever present, these genetic elements called episomes form an accessory genetic system found in addition to the main bacterial chromosome. However, not all accessory DNA molecules can be called episomes on the basis of criteria described above. Four additional terms
plasmids, IS elements, transposons and
retroelements have recently became popular to describe some of the accessory genetic elements found both in prokaryotes and eukaryotes. These elements will be described briefly in this section.
