Chromosomal Crossing Over.pptx

Chromosomal crossover, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes.

1. Chromosomal Crossing Over Name- Soumili Paul 1st year, M.Sc Marine Biology Department of Marine and Coastal Studies Madurai Kamaraj University

2. Crossing Over Crossing over, as related to Genetics refers to the exchange of DNA between paired homologous chromosomes ( one from each parent) that occurs during the development of eggs and sperm cells i.e Meiosis. It produces new combinations (re-combinations) of genes by inter- chanching and exchanging of corresponding segments between non-sister chromatids of homologous chromosomes. It occurs between prophase-1 and metaphase-1.

3. Frequency of crossing over of a given pair of genes is not constant. Crossing over frequencies are higher in the female sex than in males. Temperature, nutrition, sex, age, etc. influence crossing over. It is a part of meiotic mechanism during Gametogenesis . Brings about variation and leads to evolution through natural selection.

4. Mechanism of Crossing Over Crossing over, leading to recombination of linked genes, is due to interchange of sections of homologous chromosomes. At meiosis, the homologous chromosomes come together and pair, or synapse, during prophase. The pairing is remarkably precise and is evidently brought about by mutual attraction of the parts of the chromosomes that are similar or homologous because they contain allellic genes.

5. Types of Crossing Over Somatic crossing over Meiotic crossing over Pairing of homologous chromosomes occurs in meiotic cells but some times in somatic cells. Somatic crossing over is reported in Drosophilla by Curt Stern(1935). It occurs rarely. This type of crossing over takes place in germinal cells during gametogenesis .

6. K inds of Crossing Over Single Crossing Over: In this type of crossing over only one chiasma is formed all along the length of a chromosome pair. Gametes formed by this type of crossing over are called Single Crossing Over gametes. * chiasma- the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover in meiosis. Double Crossing Over: In this type two chiasmata are formed along the entire length of the chromosome leading to breakage and rejoin of chromatids at two point. The gametes produced produced are Double Crossing Over gametes.

7. Multiple Crossing Over: In this type more than two chiasmata are formed and thus crossing over occurs at more than two points on the same chromosome pair. It is a rare phenomenon.

8. What is the function of Crossing Over? In species that reproduce sexually, offsprings are genetically distinct from their parents because they inherit genetic materials from both. Such genetic diversity is the product of meiosis, a type of cellular division that creates reproductive cells from the parent cells. The paired chromosomes can exchange segments of DNA via, a mechanism called Crossing over.

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