This first phase of meiosis is when a developing egg or sperm cell is most likely to make mistakes in distributing one of each chromosome to its two daughter cells. In humans, these errors usually result in spontaneous abortion because the developing embryo cannot survive. But some errors are not lethal. The resulting child will have Down syndrome. In synapsis, a structure called the synaptonemal complex solidifies this pairing, forming a scaffold between the two chromosomes.
Recombination takes advantage of the chromosome alignment.
With the DNA of one chromosome lined up next to the similar DNA of its partner, recombination proteins are able to recognize the similarity and do some cutting and pasting. The result is a chromosome that is a mixture of DNA from both parents, so any child resulting from the egg or sperm will carry traits from all grandparents. After the synaptonemal complex dissolves, recombination also serves to link the chromosome pairs together, so the cell can select one chromosome from each pair for the two daughter cells. In the new work, Villeneuve and her colleagues found a gene from the worm Caenorhabditis elegans that looked like the yeast gene for Spo11 and then deleted it to test whether things worked the same way in the worm. They observed that mutant worms lacking the gene could survive, but that fewer than 1 percent of their offspring survived.