Crossing over - Definition, Types, Significance, Mechanism, Factors Affecting Crossing Over

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Crossing Over

Definition:

Crossing over is the interchange of chromosomal parts be tween non-sister chromatids of a homologous pair of chromosomes resulting in recombination of genes. Crossing over was discovered by Morgan. Crossing over is the interchange of chromosomal segments between the homologous chromosomes. It occurs during meiosis or gametogenesis. The crossing over occurs only between non-sister chromatids of the homologous chromosomes. The number of crossing over depends upon the length of the chromosome. The longer the length, the higher the percentage of crossing over. When the genes are located distantly, the chance for crossing over is higher. When the genes are closely located the chance for crossing over is lesser.

Percentage of crossing over:

The number of crossing over between two genes is represented by percentage of crossing over. The percentage of crossing over is directly proportional to the distance between two genes. The percentage of crossing over is the expression of the number of recombination’s in percentage to the total number of offspring.

Formula:

Crossing over =  Percentage of total number of offspring in F₂ / Total number of recombination’s X 100

The percentage of crossing over is also called frequency of crossing over. Thus percentage of crossing over= Frequency of crossing over = Percentage of recombination.

Generally crossing over is less frequent near the centromere and the tip of the chromosome. Crossing over at one point prevents another crossing over nearby.

Types of crossing over:

Single crossing over:

When there is a single crossing over in a homologous chromo some, it is called single crossing over.

Double crossing over:

When there are two cross overs in a homologous chromosome, it is called double crossing over.

Multiple crossing over:

When there are many cross overs in a homologous chromosome, it is called multiple crossing over.


 The crossing over may be formed between two strands or 3 strands or 4 strands. It is always formed between non-sister chromatids.

 Significance of Crossing Over :

1. Linear arrangement of genes: 

Crossing over clearly illustrates the linear arrangement of genes in the chromosomes.

2. Chromosome Maps: 

The frequency of crossing over is very useful to construct the chromosome maps.

3. Recombination: 

Crossing over produces new combination of genes.

4. Variations: 

Crossing over leads to genetic variation which is the raw material for evolution.

Mechanism of Crossing Over:

  • Crossing over is the interchange of chromosomal segments between non-sister chromatids.
  • Crossing over occurs during meiosis of gametogenesis. The homologous chromosomes move towards each other and come to lie side by side. This phenomenon of pairing of homologous chromosomes is called synapsis. The paired homologous chromosomes are called bivalents.
  • The homologous chromosomes split longitudinally. Each chromosome splits into two chromatids. Hence four chromatids are produced from two homologous chromosomes. This stage is called tetrad stage. The two chromatids of a chromosome are attached by a single centromere. These are called sister chromatids.
  • The non-sister chromatids of homologous chromosomes twist over each other. At certain points, the non-sister chromatids are connected with each other. The points of contact between non-sister chromatids are called chiasmata (chiasma-singular). Chiasma means a cross.
  • At the chiasma the chromatids break. This is brought about an enzyme called endonuclease. The broken segment of one chromatid is fused with the other chromatid and vice versa. This fusion brought about by another enzyme called ligase. The process of fusion is called ligation. The exchange of chromosomal segments tween non-sister chromatids is called crossing over.
  • After crossing over, the non-sister chromatids repel each other They separate from each other from twisting. The separation starts from the centromere towards the ends like a zip. This separation called terminalization. This leads to the separation of homologous chromosomes.

 Factors Affecting Crossing Over:

 1. High temperature increases the frequency of crossing over.

2. X-ray increases the frequency of crossing over.

3. The frequency of crossing over decreases with increasing age in female Drosophila.

4. Some genic mutation decreases the frequency of crossing over.

5. Crossing over is less frequent near centromeres and the tips of the chromosomes.

6. Inversion of chromosome segments suppress the crossing over.

7. Chiasma formation at one point prevents the chiasma formation in the vicinity. This phenomenon is known as interference. 





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