Crossing over begins very early in the prophase I stage of meiosis. During prophase I, pairs of homologous chromosomes exchange lengths of their genetic material. Crossing over leads to recombinant chromosomes and is a key factor in genetic recombination. This produces genotypes in offspring that are new and different combinations of the parental alleles. Crossing over, along with independent assortment, is the basis for Gregor Mendel’s laws of genetics.
Mendel’s laws of genetics characterize inheritance patterns of genetic material from an organism to its offspring, explains Biology Online. During early prophase I, chromosomes begin to condense, and homologous chromosomes pair loosely along their lengths. The homologous chromosomes are composed of paternal sister chromatids, matched with maternal sister chromatids. Once paired, synapsis occurs as homologous chromosomes are zippered together along their lengths by the formation of synaptonemal protein complexes. Each gene on a paternal chromosome is precisely aligned with the corresponding gene on the matching maternal chromosome. While synapsis is underway, crossing over takes place. In a single crossover event, the DNA of two non-sister chromatids of a homologous pair are broken apart at precisely corresponding points, and the two DNA segments beyond the crossover point are swapped to the opposite chromatid. This swapping of genetic material produces chromosomes with new combinations of paternal and maternal alleles. Once in metaphase II, the recombinant chromosomes of one haploid cell can orient in two alternative ways, with respect to the non-identical sister chromatid in the other haploid cell. This further increases the number of genetic types of daughter cells that can be produced during meiosis.