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What is Crossing Over?

Concept of crossing Over

Concept of Crossing Over

The crossing over is the process that produces new combinations (recombination) of genes by inter-changing of corresponding segments between non-sister chromatids of homologous chromosomes. The chromatids resulting from such interchanges of chromosomal parts are known as cross-overs. According to its occurrence in the somatic or germ cells, crossing over is of two types:

  1. Somatic or mitotic crossing over :

    When the process of crossing over occurs in the chromosomes of body or somatic cells of an organism during the mitotic cell division it is known as somatic or mitotic crossing over.

  2. Germinal crossing over:

    Usually, the crossing over occurs in germinal cells during gametogenesis in which the meiotic cell division takes place. This type of crossing over is known as germinal or meiotic crossing over.

Mechanism of Crossing Over

  1. Belting’s hypothesis:

    According to him, chromosome duplication consists of two stages – (a) formulation of new genes and (b) the formation of new connections between these newly synthesized genes. This results in the formation of recombinants or cross overs.

  2. Copy choice theory:

    J. Laderberg (1955) proposed that a newly synthesized daughter chromatid is derived due to copying of one homologous chromosome up to a certain distance and then switching to the other homologous chromosome for copying the remaining distance or region of the chromosome.

  3. Break and exchange theory:

    This theory states that in the crossing over breaks occur in the non-sister chromatids of the tetrad and between the non-sister chromatids the exchange of chromosomal segments occurs. The endonuclease enzyme helps in the breaking of the chromatids and ligase restores the broken segments, to the broken non-sister chromatids.

Kinds of Crossing Over

  1. Single crossing over:

    When the chiasma occurs only at on point of the chromosome pair. The single crossing over produces two cross-over chromatids and two non-cross over chromatids.

  2. Double crossing over:

    When the chiasma occurs at two points in the same chromosome. In the double crossing over, the formation of each chiasma is independent of the other and it is possible classes of recombination occurs. In the double crossing over the following two types of chiasma may be formed:

  • Reciprocal chiasma: Same two chromatids are involved in the second chiasma as in the first. Thus, the second chiasma restores the other which was changed by the first chiasma and it produces two non-cross over chromatids. The reciprocal chiasma occurs in two strand double crossing over in which out of four chromatids only two are involved in the double over.
  • Complimentary chiasma: When both the chromatids taking part in the second chiasma are different from those chromatids involved in the first chiasma, the chiasma is known as complimentary. It produces four single cross overs but no non-cross overs.
  1. Multiple crossing over-

    When crossing over takes place at more than two places in the same chromosome then such crossing over is known as multiple crossing over.

Significance of Crossing over

  1. As a result of crossing over new combinations are produced. Thus in an organism new characters are developed.
  2. By crossing over it is proved that genes are linearly arranged.
  3. The behaviour of chromosomes can be studied and chromosomal maps can be drawn.
  4. Thorough study of linkage and crossing over may reveal the nature of genes.
  5. Crossing over supports the definition of gene that “gene is the smallest section of chromosome within which crossing over do not takes place. It means that minimum cross over unit is the whole gene and not a part of it.
  6. It is very important from breeding and hybridization point of view as it is helpful to improve the varieties of plants and animals.
  7. The non-desirable characters may be removed by this process.
  8. It increases the chances of variations which is necessary for evolution.

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