Corssing over is the exchange of genetic material between homologus chromosomes that results in recombinant choromosomes sexual reproduction.
Mechanism: During the zygotene stage of the first prophase of meiosis the homologous material and paternal Chromosomes start pairing and lie closly side by side. This phemomenon is called synapsis. This pairing of homologous chromosomes is brought about by the mutual attraction between the allelic genes. The paried chromosomes are known as bivalent. A recent study reveal that synapsis and chiasma formation is faciliated by a highly organised structure at filaments called synaptonemal complex synapsis is follows by the duplication of chromosomes. Which change the bivalent nature of chromosome pair into tetravalent. In crossing over two or three chromatide are involved and accordingly two or more chiasmate are formed. At each chiasma are formed. At each chiasma the chromatide breaks and the broken segment rejoin a new chromatid. Thus exchange of parts of chromatids brings about alternation of original sequence of genes in the chromosome.
