<P> In prokaryotic sex, DNA from one prokaryote is released into the surrounding medium, is then taken up by another prokaryote and its information integrated into the DNA of the recipient prokaryote . In modern prokaryotes the donor DNA is transferred by conjugation, transduction or transformation . Transformation is hypothesized to be the ancestral mechanism, in which DNA from one prokaryote is released into the surrounding medium and then taken up by another prokaryotic cell . One theory on how meiosis arose is that it evolved from transformation . According to this view, the evolutionary transition from prokaryotic sex to eukaryotic sex was continuous . </P> <P> Transformation, like meiosis, is a complex process requiring the function of numerous gene products . A key similarity between prokaryotic sex and eukaryotic sex is that DNA originating from two different individuals (parents) join up so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information (a process called genetic recombination). After the new recombinant chromosome is formed it is passed on to progeny . </P> <P> When genetic recombination occurs between DNA molecules originating from different parents, the recombination process is catalyzed in prokaryotes and eukaryotes by enzymes that have similar functions and that are evolutionarily related . One of the most important enzymes catalyzing this process in bacteria is referred to as RecA, and this enzyme has two functionally similar counterparts that act in eukaryotic meiosis, RAD51 and DMC1 . </P> <P> The evolution of meiosis from transformation happened as eukaryotes evolved through endosymbiosis between an anaerobic host cell, most likely to be an archaeon and an internalized aerobic bacterium, most likely to be a descendant of α - proteobacterium, a likely ancestor of extant mitochondria . A genomic analysis on 630 orthologous groups hints a close evolutionary relationship between alpha - proteobacterial and eukaryotic proteins, suggesting there was indeed genetic transfer and, therefore, transformation between the two cells . Furthermore, in extant organisms, the RecA gene orthologs which are crucial for meiotic recombination have a high sequence similarity to that of the RecA gene in proteobacteria or cyanobacteria . This indicates a gene flow involving RecA gene or its orthologs from pre-mitochondrial bacteria to ancestral eukaryotes . Since the ancestors of eukaryotes were then homogenized to be able to undergo transformation, the genes needed for transformation, including RecA found in eubacteria and Rad51 family found in archaea, were likely descended to early eukaryotes as well and mutated into the ancestral meiotic genes . </P>

What is the function of meiosis in eukaryotic cells