The process by which parts of bivalent homologous chromosomes are exchanged between two non-sister chromatids is called crossing over. This is called genetic recombination. In 1909 scientist Thomas Hunt Morgan first observed the crossing over at Bhutra. He won the Nobel Prize in 1933.
White House Model of Crossover
According to the White House model, crossing over is done in four steps. These are-
1. Synapsis: The process by which homologous chromosomes join together is called synapsis. At this stage attraction is observed between homologous chromosomes. Homologous chromosomes come close to each other and pair along their length. The process of pairing of homologous chromosomes is called synapsis. Each pair of chromosomes is called bivalent.
2. Duplication of Chromosome: Each chromosome of bivalent divides longitudinally without centromere to form two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Siddique Publications
3. Interchange of body parts: Two non-sister chromatids of bivalents come close to each other and form X-shaped zygoma. Chromatids are broken by the action of endonuclease enzyme at the chiasma segment. A broken chromatid joins another non-sister chromatid with the help of ligase enzyme. In this way parts are exchanged between two non-sister chromatids. It is called crossing over. Dr. Siddique Publications
4. Marginalization: Repulsion begins between the bivalent chromosomes after crossing over is completed. Chromosomes move away from each other. The kaiazmas gradually move towards the edge. This is called terminalization.
Importance of crossing over
1. Rearrangement of genes: During crossing over, segments are exchanged between two non-sister chromatids. As a result, new arrangements of genes occur in the chromosomes.
2. Genetic Variation: Crossing over results in genetic variation in organisms. New varieties are created in crop plants through crossing over.
3. Genetic modification: Genetic modification can be done by artificial crossing over. This is why crossing over plays an important role in breeding.
4. Creation of Biodiversity: Biodiversity is the characteristic and characteristic difference between one organism and another organism. Differences between two plants or animals of the same species belong to biodiversity. Biodiversity is created due to changes in the position and arrangement of genes. Siddique Publications
5. Survival in new environments: Characteristic changes occur in organisms due to crossing over. In this the organism acquires the ability to survive in the new environment and survive.
6. Preparation of genetic map: By determining the percentage of crossing over, the position of the gene on the chromosome is determined and the chromosome map is prepared.
7. Genetic research: Genetic research has created a great stir worldwide. Crossing over is an interesting topic in gene theory research.
8. Aid in evolution: Crossing over results in trait changes, biodiversity and increased ability to survive in new environments. As a result, evolution is possible.
9. Breeding of crop plants: Crossover can be used to produce desirable traits in crop plants. Improved varieties of crops can be created.
10. In breeding: Alteration in heredity by artificial crossing over. Hence it has a wide role in reproductive science.
11. Linear arrangement of chromosomes: Linear arrangement of genes in chromosomes occurs due to crossing over.
12. Preparation of Chromosome Map: First the percentage of crossing over is determined. Then the location of the gene on the chromosome is determined and a map of the chromosome is made.
Crossing over is a very important process in the living world. Rearrangement of hereditary traits in chromosomes occurs through crossing over. The result is variation. The early beginnings of evolution in species. Dr. Siddique Publications
Catalyst of Crossing Over
1. High temperature: High temperature and radiation increase the tendency of crossover occurrence. Siddique Publications
2. Aging: As the organism ages, the incidence of crossing over decreases.
3. Mutation: The level of crossing over is reduced due to mutation.
4. Inversion: Inversion stops the crossing over process.
5. Interference: Interference with the formation of another chiasm near the site of chromosomal formation. This phenomenon is called interference.
6. Radiomimetic substances: Chemical substances that increase the rate of somatic crossing over are called radiomimetic substances. Eg- ethylmethane sulphonate. Dr. Siddiq Publications
7. Chemicals: Various chemicals reduce crossover. For example, colchicine, selenium, etc. Dr. Siddiq Publications