Theories about the mechanism of Crossing Over

THE MECHANISM OF CROSSING OVER

Following theories have been propounded to explain the mechanism of crossing over:

1.Duplication theory. This theory was proposed by John Belling (1928)while studying meiosis in some plant species.Belling believed that crossing over might occur during duplication of homologous chromosomes and might bring about due to novel attachment formed between newly synthesized genes.He visualized genes as beads,connected by non-genic linking elements the inner chromomeric regions.

2.Copy choice hypothesis or Switch model. To explain recombination in micro-organisms,J.Laderberg (1955)proposed a modified version of Belling’s hypothesis.This is called copy choice mechanism of recombination or crossing over.The copy choice model to explain the mechanism of recombination has been criticized for the following two reasons:1.Copy choice model relies on the conservative mode of DNA replication but all experimental evidence suggest that DNA replication occurs in a semi-conservative manner. The copy choice model proposed the involvement of only two chromatids in the process of crossing over,while cytological investigations have made it clear that crossing over occurs at a four-strand stage.

3.Break and exchange theory. The break and exchange theory is the most accepted theory to explain the process of crossing over.This theory states that in the crossing over breaks occur in the non-sister chromatids of the tetrad and the exchange of the chromosomal segments occurs between the non-sister chromatids.

1.Berezovsky's contact first theory .According to this theory, two chromatids of a tetrad first touch and cross each other and then first touch and cross each other and then at the point of cross tie breakage of chromatids occur.The chromosomes segments then unite to form new combinations.

2.Muller’s breakage first theory. This theory holds that before the occurrence of chiasma and crossing over two chromatids or chromosomes break into two fragments and then the union of chromosomal segments takes place.

3.Darlington’s strain or torsion theory. Darlington (1935)propounded that chromosomal breakage occurs as a result of strain during pairing.According to this theory during the late pachytene of meiosis,the homologous chromosomes pair and become spirally twisted around each other.

4.Hybrid DNA models. In the last three decades, several hybrid DNA models have been proposed to explain the mechanism of recombination.In these models,only one strand in each of the two DNA duplexes belonging to non-sister homologous chromatin breaks.

TETRAD ANALYSIS

Genetical proof that crossing over occurs during the tetrad stage has been provided by red bread mold and other fungi and algae.In a haploid such as Neurospora,study of linkage,crossing over and chromosome mapping is facilitated due to following reasons:

1.Since a single allele per locus is present in a haploid,products of recombination can be studied without any difficulty due to dominance.

2.The life cycle of certain fungi enables us to analyse the producers of a single meiosis.

3.In Neurospora meiotic produces are linearly arranged in the form of ordered tetrad,because of which a distinction can be made between ‘first division segregation and ‘second division segregation which is used in detecting the crossing over between centromeres and a gene.

Tetrad analysis is the technique of using each of the individual spores of a tetrad for analysis to acquire information on linkage and recombination. Thus,for tetrad analysis in Neurospora, each ascospore is removed in order from ascus and germinated to determine the physiological phenotype or examined visually for such morphological traits as colour, the cross pattern of growth etc..

Lindergren(1933)studied the inheritance of two pairs of contrasting characters in Neurocrassand presented the first direct evidence to prove that crossing over occurs at a four –strand stage and involves only two of the four strands at any one place.

CYTOLOGICAL DETECTION OF CROSSING OVER

Creighton and MeClintock're the experiment.Creighton and MeClintock and Stern made the convincing correlation between cytological between cytological evidence and genetical results of crossing over in maize.They made the use of knob of 9^th chromosome of maize which in two different strains might have one allele for either coloured aleurone (C) or colourless aleurone ©and one allele for either starchy endosperm (Wx)or waxy endosperm (wax).

KINDS OF CROSSING OVER

According to the number of chiasma following types of crossing over have been described:

1.Single crossing over. When the chiasma occurs only at one point of the chromosome pair then the crossing over is known as single crossing over. The single crossing over produces two cross over chromatids and two non-cross over chromatids.

2.Double crossing over.When the chiasmata occur at two points in the same chromosome,the phenomenon is known as double crossing over.In the double crossing over following types of chiasma may be formed:

1. Reciprocal chiasma.In the reciprocal chiasma, the same two chromatids are involved in the second chiasma as in the first.Thus,the second chiasma restores the order which was changed by the first chiasma and it produces two non-cross over chromatids.
2. 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 called complimentary chiasma.

• Multiple crossing over. The multiple crossing over is that in which crossing over takes place at more than two places in the identical chromosomes pair in the same chromosome pair.

SIGNIFICANCE OF CROSSING OVER

The phenomenon of crossing over is universal in its occurrence and it occurs in viruses,bacteria moulds plants and animals.It is necessary for the natural selection because due to the chances of variation increase.

References

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Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.