Crossing Over

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Gene exchange is an important chromosomal process that occurs in the cells of organisms. New traits or genetic variations resulting from gene exchange play an important role in biological evolution.

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Crossing Over

Gene exchange takes place at the pachytene stage of the first prophase of meiosis. In this condition, there is mutual exchange of chromosomal segments at one or more places between chromatids of Homologous Chromosomes, this exchange is called gene exchange or crossing over.

After gene exchange between homologous chromosomes, new recombined DNA is formed, which is the carrier of new characteristics. Through these, variations arise in the progeny generation, which are helpful in biological evolution.

Mechanism Of Gene Exchange

The process of gene exchange mainly occurs in the pachytene stage, but its action starts from the zygotene stage and continues till the diplotene stage. The mechanisms of gene exchange are as follows –

1. Formation of Paired Chromosomes or Synapsis

In the zygotene stage of the first prophase of meiosis, two homologous chromosomes (one from the mother and the other from the father) are attracted to each other and come very close to each other and join each other. This pairing of chromosomes is called synapsis and these paired pairs are called Bivalents.

2. Duplication of Chromosomes

The duplication and crossing over of chromosomes mainly occurs in the pachytene stage only. In this phase, the chromosomes become more and more short and thick. Both the homologous chromosomes of the bivalents get wrapped around each other, due to which the chromatids of both become visible separately, that is, four chromatids become visible. Now it is called Tetrad.

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3. Crossing Over

After the formation of tetrad in the pachytene state, tension arises in the chromatids of the two homologous chromosomes which are joined together, which is called chiasma or chiasmata, and the two inner chromatids, which remain attached to each other, begin to exchange segments. While there is no change in the outer two chromatids. Basically, the exchange of segments takes place only at the chysmata. This exchange of segments is called transit or crossing over. Thus, one chromatid of each chromosome remains in its original state, which carries the paternal characters, while crossing over in the other chromatids results in new characters or non-paternal characters.

4. Termination

The crossing over ends at the diplotene stage. In this phase, repulsion occurs in the homologous chromosomes, due to which the centromeres start moving away from each other. Due to the movement of the centromere, the chromosomes start moving away and now the chromatids remain connected only at the chiasma, which due to tension move towards the edge. starts slipping. This action is called termination.

Types of Gene Exchange

There are mainly three types of gene exchange, whose brief explanation is as follows –
  1. Single Crossing Over
  2. Double Crossing Over
  3. Multiple Crossing Over
Genetic Terminology - Genetics

1. Single Crossing Over

In this type of gene exchange, chyme is formed at only one place. Thus genes are exchanged at only one place. In this, two recombined chromatids are formed, while the two parental chromatids remain intact.

2. Double Crossing Over

In this type, the chromatids of the homologous chromosomes meet at two places i.e. two chiasma are formed. In this way fragments are exchanged at two places. Due to the formation of two chyme in biexchange, they form a ring-like structure.

3. Multiple Crossing Over

When chromatids of homologous chromosomes meet at more than two places and form more than two chiasma. In this way, exchange of segments takes place at more than two places. This process is very rare in nature.

Importance of Gene Exchange

  • In gene exchange, new characteristics are formed by the exchange of fragments, which appear in the progeny generation.
  • New gene combinations formed by gene exchange play an important role in biological evolution.
  • Gene exchange ensures that the genes are arranged in a linear order on the chromosome.

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