Meiosis
At this stage each chromosome is split into two sister chromatids, held together by the centromeres. The paired chromosomes now have four chromatids (2 sets of 'sisters') pressed together. Crossing over takes place between two of the non-sister chromatids; the other two remain uncrossed. The crossover results in the exchange of segments of each of the participating chromatids, DNA and associated chromatin protein. Genetically, the process is called recombination.