On the origin of meiosis and sex

The pre-karyote (the host for alpha-proteobacteria) is not assumed to have had a fusion prohibiting cell surface, and thus unlike prokaryotes could have practiced sex. A single DNA damage checkpoint control might have been the only checkpoint pathway regulating the ancient pre-karyotic cell cycle from which the modern eukaryotic cell cycle evolved. This single checkpoint would allow the cell division only when all the pre-karyote genome was completely replicated without mistakes. If the restart of DNA replication was impossible after DNA damage, the last chance to survive for this in S-phase blocked pre-karyote would be the fusion with the partner. After the fusion, recombination, and subsequent completing the replication of both haploid sets, the single DNA damage checkpoint would allow two subsequent rounds of fission. The fusion might have represented both useful repair strategy, and a mechanism of horizontal transmission of the ancestors of mitochondria. The presence/absence of these symbionts might have been a primordial form of sexual determination.