Crossing Over

Crossing Over

The process which is generally known as crossing over in which recombination in turns accomplishes. In this process, the non-sister chromatids of homologous chromosomes exchange the chromosomal parts or segments.In another word,”the crossing over is a process that produces new combinations of genes by interchanging of corresponding segments,between non-sister chromatids of homologous chromosomes”.The chromatids resulting from such interchanges of chromosomal resulting from such interchanges of chromosomal parts are known as crossovers.The term crossing over was coined by Morgan.No crossing over occurs in male Drosophila and female silkworm, Bombyx mori.Certain external agents such as heat shock ,chemicals,radiations,etc.,have the profound effect on crossing over.

Characteristics of Crossing

1. Crossing over or recombination occurs at two levels
2. At gross chromosomal level,called chromosomal crossing over
3. At DNA level,called genetic recombination
4. A reciprocal exchange of material between homologous chromosomes in heterozygotes is reflected in crossing over.
5. The crossing over results basically from an exchange of genetic material between non-sister chromatids by break and exchange following replication.
6. The frequency of crossing over appears to be closely related to the physical distance between genes on chromosomes and serves as a tool in constructing genetic maps of chromosomes.

TYPES OF CROSSING OVER

According to its occurrence in the somatic or germ cells following two types of crossing over have been recognized:

1. Somatic or Mitotic Crossing Over

When the process of crossing over occurs in the chromosomes of the body or somatic cells of organisms during the mitotic cell division it is known as somatic or mitotic crossing over. The somatic crossing over is rare in its occurrence and it has no genetical significance.The somatic or mitotic crossing over has been reported in the body or somatic cells of Drosophila by Curt Stern and in the fungus Aspergillus nodules by G.Pontecorvo.

2. Germinal or Meiotic Crossing Over

Usually, the crossing over occurs in germinal cells during the gametogenesis in which the meiotic cell division take place.This type of crossing over is known as germinal or meiotic crossing over.The meiotic crossing over is universal in its occurrence and it's of great genetic significance.

MECHANISM OF MEIOTIC CROSSING OVER

The process of crossing over includes following stages in it ,viz.,synapsis,duplication of chromosomes,crossing over and femininization. The chromosomes that tend to undergo recombination due to meiotic crossing over necessarily complete two functions:1.99.7 percent replication of DNA and 75 percent synthesis of histones,both of which take place prior to onset of prophase 1, and 2.attachment of each chromosomes by its both ends to the nuclear envelope via to the specialized structure ,called attachment plaques.This event occurs during the leptotene stage of prophase I and though each chromosome at this stage are visually long and thin thread,but contains material of two sister chromatids(i.e.,two DNA molecules plus almost duplicated amount of histones).

• Synapsis

Synopsis or intimate pairing between the two homologous chromosomes (one maternal and another paternal )is initiated during zygotene stage of prophase I of meiosis I.Synapsis often starts when the homologous ends of the two chromosomes are brought together on the nuclear envelope and it continues inward in a zipper-like manner from both ends,aligning the two homologous chromosome side by side.In other cases synopsis may begin in internal regions of the chromosomes and proceed towards the ends,producing the same type of alignment.The phenomenon of synapsis has always intrigued cytogeneticists.Moses identified a factor in the formation of the synaptonemal complex which aids in synapsis.

1. Priority theory.To explain the question that why do homologous chromosomes during synapsis approach each other from a considerable distance and become closely associated,a British cytologist C.D.Darlington in 1937,proposed the precocity theory of meiosis which embraces well the cause of synapsis in it.
2. Synaptonemal complex.Montrose J.Moses (1956)has revealed a highly organized structure of filaments called synaptonemal complex in between the paired chromosomes of zygotene and pachytene stages in crayfish by electron microscopy. Though the morphology of lateral and central elements may vary from species to species,but the basic structure but the basic structure and the spacing of the synaptonemal complex are constant within the species.

Functions of the synaptonemal complex.The synaptonemal complex is found to be the concomitant of both chiasma formation and crossing over. For instance,the synaptonemal complex may serve to cross over by facilitating effective synopsis in one or more following ways:

1. To maintain pairing in fixed state for an extended period
2. To provide a structural framework within which molecular recombination may occur and
3. To segregate recombination DNA from the bulk of the chromosomal DNA.

Robert King (1970)suggested that the synaptonemal complex may orient the non-sister chromatids of homologs in a manner to facilitate enzymatically induced exchanges between their DNA molecules.More recently,D.A.Comings and T.A.Okada (1971),have shown electron microscopically that synapsis occurs at two levels one at chromosomal level and the other at the molecular level.

2.Duplication of Chromosomes.

The synopsis is followed by duplication of chromosomes.During this stage,each homologous chromosomes of bivalents split longitudinally and form two identical sister chromatids which remain together by an unsolicited centromere.

3.Crossing Over by Breakage and Union

It is well evident crossing over occurs in the homologous chromosomes only during the four stranded or tetrad stage.Homologous continues to stay in synopsis for days during pachytene stage and chromosomal crossing over occurs due to an exchange of chromosomal material between non-sister chromatids of each tetrad.In pachytene,the recombination nodules become visible between synapsed chromosomes.

4.Terminilisation

After the occurrence of the process of crossing over the non-sister chromatids start to repel each other because the force of synapsis attraction between them decreases.During diplotene,desynapsis begins,synaptonemal complex dissolves and two homologous chromosomes in a bivalent are pulled away from each other.The movement of chiasma is called termini limation

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.

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