<P> There are two popular and overlapping theories that explain the origins of crossing - over, coming from the different theories on the origin of meiosis . The first theory rests upon the idea that meiosis evolved as another method of DNA repair, and thus crossing - over is a novel way to replace possibly damaged sections of DNA . The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity . </P> <P> Crossing over and DNA repair are very similar processes, which utilize many of the same protein complexes . Recombinases and primases lay a foundation of nucleotides along the DNA sequence . One such particular protein complex that is conserved between processes is RAD51, a well conserved recombinase protein that has been shown to be crucial in DNA repair as well as cross over . Several other genes in D. melanogaster have been linked as well to both processes, by showing that mutants at these specific loci cannot undergo DNA repair or crossing over . Such genes include mei - 41, mei - 9, hdm, spnA, and brca2 . This large group of conserved genes between processes supports the theory of a close evolutionary relationship . Furthermore, DNA repair and crossover have been found to favor similar regions on chromosomes . In an experiment using radiation hybrid mapping on wheat's (Triticum aestivum L .) 3B chromosome, crossing over and DNA repair were found to occur predominantly in the same regions . Furthermore, crossing over has been correlated to occur in response to stressful, and likely DNA damaging, conditions </P> <P> The process of bacterial transformation also shares many similarities with chromosomal cross over, particularly in the formation of overhangs on the sides of the broken DNA strand, allowing for the annealing of a new strand . Bacterial transformation itself has been linked to DNA repair many times . The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating genetic diversity. . Thus, this evidence suggests that it is a question of whether cross over is linked to DNA repair or bacterial transformation, as the two do not appear to be mutually exclusive . It is likely that crossing over may have evolved from bacterial transformation, which in turn developed from DNA repair, thus explaining the links between all three processes . </P> <P> Meiotic recombination may be initiated by double - stranded breaks that are introduced into the DNA by exposure to DNA damaging agents or the Spo11 protein . One or more exonucleases then digest the 5' ends generated by the double - stranded breaks to produce 3' single - stranded DNA tails (see diagram). The meiosis - specific recombinase Dmc1 and the general recombinase Rad51 coat the single - stranded DNA to form nucleoprotein filaments . The recombinases catalyze invasion of the opposite chromatid by the single - stranded DNA from one end of the break . Next, the 3' end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single - stranded DNA generated from the other end of the initial double - stranded break . The structure that results is a cross-strand exchange, also known as a Holliday junction . The contact between two chromatids that will soon undergo crossing - over is known as a chiasma . The Holliday junction is a tetrahedral structure which can be' pulled' by other recombinases, moving it along the four - stranded structure . </P>

When do crossing over and independent assortment occur