Name: Sex Determination (Drosophila melanogaster)
Uploaded: 2019-07-19
Duration: 7 min 20 s
Description: Drosophila melanogaster or the common fruit fly has contributed a lot to genetics. One of them is its sex determination system with more than five types of s...

Question

Define crossing over. Explain crossing over with a diagram.

Accepted Answer

Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is the swapping of genetic material that occurs in the germline.

Crossing over takes place during the meiotic prophase, i.e., during the pachytene stage. It involves the following steps :

Synapsis: Crossing over occurs between non-sister chromatids. Thus, one chromatid from each of the two homologous chromosomes is involved in crossing over. The pairing phenomenon of the homologous chromosome is called synapsis.

Tetrad formation: Each crossing over involves only two of the four chromatids of two homologous chromosomes. At this stage, each bivalent has four chromatids. This stage is called the tetrad stage.

Cross Over: The point of exchange of segments between non-sister chromatids of homologous chromosomes during meiotic prophase is called chiasma (pleural chiasmata). It is thought to be the place where crossing over takes place. Depending on the position, chiasma is of two types, terminal and interstitial. When the chiasma is located at the end of the pairing chromatids, it is known as terminal chiasma, and when it is located in the middle part of non-sister chromatids, it is referred to as interstitial chiasma.

Terminalisation: The movement of chiasma away from the centromere and towards the end of tetrads is called terminalisation. The chiasma terminalisation occurs between diplotene and metaphase I.

Define crossing over. Explain crossing over with a diagram.

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