of yeast [17].) Ring chromosomes which occur occasionally in eukaryotes were also destroyed in nuclear division due to preceding intrachromosomal recombination (odd numbers of sister chromatid exchanges [18]). 3) Episomic DNA occurs in bacteria which could occasionally evolve linearity by means of stable and replicative terminal sequences (e.g., palindromes) [15]. 4) Suffering from a double-strand break, circular DNA may become integrated into linear DNA at homologous sequences [16]. 5) In yeast, for example, linear plasmids with telomeric and centromeric sequences may become mitotically more stable when enlarged with repetitive DNA [19]. 6) Migration of DNA from circular genomes of organelles into the linear genome of nucleus is proven [20]. 7) Endosymbiosis leading to mutual loss of independence of both partners is widespread and well established and can be observed experimentally [2].
Thus, all prerequisites for an endosymbiontic origin of the nucleus as the first step in evolution of eukaryotes are provided by nature and make the present hypothesis an alternative to the idea of compartmentation or endocytotic origin of the nucleus.