Pre-insertion preparation: After reaching the nucleus, mtDNA has to enter the nuclear genome. The rate of mtDNA incorporation into the nuclear genome can be expected to depend on the DSB number in nDNA, the activity of DSB repair systems, and the rate of mtDNA escape from organelles. MtDNA insertion comprises three main processes, shown in figure 2; first, the mtDNA has to have the proper form and sequence; in other words, the mtDNA has to be edited which gives a rise to the new edited site in the polynucleotide structure. Mitochondrial DNA is not universal and, in animals similar to plants, mitochondrial editing shows very erratic patterns of taxon-specific occurrence. As shown in figure 2, there are three possible ways that mtDNA can become prepared to be inserted into the nuclear DNA. The process mainly depends on the time mtDNA transfers into the nucleus. As shown in figure 2b, direct integration of unedited mtDNA fragments into the nuclear genomes is the most plausible and the evidence both found in plants, Arabidopsis genome, and animals with the help of different methods, including BLAST-based analysis. In this case, mtDNA is transferred into the nucleus whereby editing and introns arise in the mitochondrion later. If a gene, for instance, was transferred to the nucleus in one lineage before mitochondrial editing evolved, but remained in the organelle in other lineages where editing arose, the nuclear copy would appear more similar to an edited transcript than to the remaining mitochondrial copies at the edited sites. Another represented and less supported model, figure 2a, is the cDNA-mediated model, which intron-contained mtDNA enters the nucleus and by reverse transcription of spliced and edited mitochondrial transcript, it becomes integrated into the nDNA. The third proposed mechanism is the direct transfer and integration of intronless mtDNA into the nucleus, figure 2c, whereby editing and introns in the mitochondrion come and go during evolution. In this case, the introduction and removal of the intron, as well as, reverse transcription occur within mitochondria and the final product, the edited intronless mtDNA, will integrate into nDNA after being transferred into the nucleus.