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Understanding Crossing Over in Genetics: A Detailed Overview

Crossing over is the recombination of linked genes occurring during meiosis, resulting in new gene combinations. It occurs in germinal cells (meiotic crossing over) and somatic cells (mitotic crossing over). The process involves synapsis, chromosome duplication, actual crossing over at chiasmata, and terminalization. Key features include the exchange of segments between non-sister chromatids and the relationship between gene distance and crossing over frequency.

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Understanding Crossing Over in Genetics: A Detailed Overview

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  1. CROSSING OVER B.SC. I PAPER II ZOOLOGY (CBCS) DR.SHASHIKANR RAMRAO SITRE ASSISTANT PROFESSOR DEPARTMENT OF ZOOLOGY N.S.SCIENCEAND ARTS COLLEGE, BHADRAWATI

  2. Crossing Over may be defined as….. the recombination of linked genes brought about as a result of interchange of corresponding parts between the chromatids of a homologous pair of chromosomes , so as to produce new combinations of old genes.

  3. The chromatids in which crossing over has occurred, have new combinations of genes and are called cross overs. According to its occurrence in the germinal or somatic cells, following two types of cross over occurs – 1. Germinal or meiotic cross over 2. Somatic or mitotic crossing over

  4. 1. Germinal Crossing Over (Meiotic Crossing Over) ◦ Commonly crossing over occurs only in the germinal cells of reproductive organs during the process of gametogenesis which includes meiosis. This type of crossing over is called germinal or meiotic crossing over. ◦ It is universal in the occurrence and has great genetic significance.

  5. 2. Somatic Crossing Over (Mitotic Crossing Over) ◦Sometimes crossing over may occur during mitosis of somatic cells. ◦This type of crossing over occurs in rare cases, has no genetic significance and is called somatic or meiotic crossing over.

  6. Mechanism of Crossing Over 1. Synapsis – During Prophase-I of meiosis the maternal and paternal chromosomes of a homologous pair come close together and pair at Synaptotene or Zygotene substage (Synapsis).this pairing is remarkably precise and point to point. The homologous chromosomes come close together at one or more points by mutual attraction between the allele genes. At the end of zygotene, they lie inside by side in close approximation all along their lengths. The pairing is exact and point to point. The paired chromosomes are known as bivalents.

  7. 2. Duplication of Chromosomes During diplotene stage, each of the homologous chromosomes in a bivalent splits longitudinally into two sister chromatids. Thus, the bivalents now consist of four chromatids and is known as tetrad. The longitudinal splitting of chromosomes is achieved by the separation of already duplicated DNA molecules.

  8. 3. Crossing Over During Diplotene stage, when the paired chromopsomes start separating the chromatids remain at one or more points and thus establish one or more exchanges per bivalent. These points of contact are known as Chiasmata. At each chiasma two daughter non sister chromatids of the bivalent break at the corresponding points and then re-join with the exchange of segments. The new chromatids formed as a result of exchange of segments are formed of segments derived from two sister chromatids of the bivalent.

  9. 4. Terminalization ◦ After crossing over, the non sister chromatids start repelling each other because the forces of attraction keeping them together lapse. ◦ The chromatids separate from the centromere towards the tip in so doing, the chiasmata also start moving in zipper like fashion towards the ends. ◦ The movement of chiasmata Simultaneously the chromatids condense and get shortened and in diakinesis the homologous chromosomes becomes separated except at their ends. is known as terminalisation.

  10. Features of Crossing Over ◦ Only two chromatids cross over out of four, while the other two chromatids preserve their original constitution. ◦ The crossing over occurs between non-sister chromatids of the same chromosomes. The two chromatids of the same chromosomes never exchange parts. ◦ The number of chiasmata per set of chromosomes depends upon the length of chromosomes. The longer are the chromosomes, the greater is the number of chiasmata. ◦ Chances of crossing over are more if the genes are located further apart. The more is the distance between the genes, the greater is the opportunity of chiasmata formation.

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