Crossing over:
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 ce;; division.
Mechanism of crossing over:
The process of crossing over involves the following stages:
1.Synapsis
2.Duplication of chromosomes
3.Crossing over
4.chiasmata formation
5.Terminalisation
1.Synapsis: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.
2.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.
3.Crossing over: 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.
4.Chiasmata Formation: 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.
5.Terminalisation: 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.
Yogita Ingle 5 years, 2 months ago
Crossing over:
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 ce;; division.
Mechanism of crossing over:
The process of crossing over involves the following stages:
1.Synapsis
2.Duplication of chromosomes
3.Crossing over
4.chiasmata formation
5.Terminalisation
1.Synapsis: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.
2.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.
3.Crossing over: 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.
4.Chiasmata Formation: 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.
5.Terminalisation: 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.
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