Crossing over may be defined as an exchange of genetic material between non- sister chromatids of homologous chromosomes resulting in a new combination of genes. The crossing over takes place during the early stage of prophase first of meiosis cell division. Mechanism of crossing over:
During zygotene substage of prophase I, the maternal and paternal homologous chromosomes come close to each other and start pairing along their length. The pairing of homologous chromosomes is called synapsis. They paired homologous chromosomes are called bivalents. It is mechanical basis of crossing over.
Duplication of chromosomes
The synapsis is followed by duplication of chromosomes. During pachytene substage of prophase I , the chromotids of each homologous chromosome splits lengthwise and forms two identical sister chromatids.Thus each bivalent contains four chromatids so it is known as tetrad.
The non-sister chromatids of homologous pair twist over each other at one or more points. The chromatid segments break at the corresponding points and the segment of one side fuses with the segment of the opposite side due to the action of enzyme. Thus the crossing over includes breaking of chromatid segments, their transposition and fusion.
Chiasmata are the points of attachment between two homologous chromosomes, where the crossing over occurs. The number of chiasmata depends on the length of the chromosomes; greater the length greater is the number.The crossing over may take place at one or several points in one tetrad and may result in the formation of one or more chiasma.
After the process of crossing over, the non- sister chromatids start to repel each other due to lack of attraction force between them. The repulsion of chromatids starts from the centromere towards the chiasma and the chiasma itself moves in a zipper fashion towards the end of the tetrad. The movement of chiasma is known as terminalization. Due to terminalisation the hlomologous are separated completely.
Significance of Crossing over
It affords a proof for the linear arrangement of genes in the chromosomes. Result of crossing over, new gene combination are produced play an important role in evolution. It is necessary for natural selection due to changes of variation increase. Useful combination can be formed , which may be used in breeding.