Observations under microscope have disclosed the behavior of chromosomes during meiosis and the mechanism of crossing over and the correlation between crossing over and chiasma formation. The various events associated with crossing over and recombination are:
Mechanism of Crossing Over and Recombination:
Synapsis:
Synapsis is an event of great importance. It provides the mechanical basis of heredity and variations. During synaptotene or zygotene (synapsis) of prophase-1 of meiosis the material and paternal chromosomes of a homologous pair come close together and pair. This pairing is remarkably precise and point to point.
The homologous chromosomes come close together at their centromere by mutual attraction and then pairing extends both ways between the allelic genes. At the end of zygote they lie side by side in close approximation all along their lengths. The pairing is exact and point to point . The paired chromosomes are known as bivalent.
Cytochemistry of Synapsis:
To explain why do homologous chromosomes during synpasis approach each other and become closely associated, following theories have been put forth:
i. Precocity Theory: C.D. Darlington (1937) proposed that chromosomes enter meiotic prophase as unreplicated structures consisting of a single chromatid. The chromosomes with single chromatid represent an unsaturated and unbalanced state. In order to become saturated they must pair. The force which binds the chromatids together during mitosis. It also keeps the homologous chromosomes paired during meiosis upto pachytene subtage. As soon as the chromsomes duplicate themselves in pachytene, and their pairing need is satisfied, the separation of chromosomes starts.
ii. Synthesis of DNA and Protein: Hotta, Ito and Stern (1966) reported in Lilium about 3% of total DNA is sythesized during zygotene or pachytene. Yuri Bogdanov (1968) has reported that in Crillyus Domesticus about 25% histone synthesis occurs in Zygotene. It is commpleted by pachytene. It was, therefore, concluded that shortage of DNA and histone in chromosomes during zygotene creates a condition for homologous pairing. It is found that if DNA synthesis is inhabited, the chormosomal synapsis fails to occur and meiosis stops. It means DNA synthesis plays an important role in chromosomal alignment during meiosis pairing. Therefore, crossing over involves physical exchange between double helices of DNA of nonsister chromatids.
Ultrastructure of Synapsis:
Synaptonemal Complex: The Synaptonemal Complex was described by Montrose J. Moses (1955). It occurs as a highly organized structure of filaments between the paired chromosomes in zygoten and pachytene stages of meiosis in all eukaryotic cells undergoing meiosis. It is formed during prophase of meiosis. DNA synthesis during zygotene is related to the formation of synaptonemal complex between homologous. In male Drosophila and in females of silkmoth no crossing over occurs and synaptonemal complex is not formed.
The electron micro-graphs of synaptonemal complex show three parallel dense line equally spaced in one plane and flanked by chromatin. The central element is of variable prominence, whereas the two with the central element. The lateral elements may show subdivision into two longitudinal components. The central element may also appear as a long tripartite bar with ladder-like transverse connections. The protein composing synaptonemal complex is called synaptonemal complex material.
Functions of Synaptonemal Complex:
1. Moses considered that synaptonemal complex is a per-requisite to chiasma formation and crossing over but alone it is not sufficient. It is neither responsible nor necessarily a consequence of pairing, though chromosome pairing precedes its formation.
2. It may facilitate effective synapsis:
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i. By maintaining pairing in fixed state.
ii. By providing a structural framework within which molecular recombination may occur
iii. By segregating recombination of DNA from bulk of the chromosomal DNA.
3. Robert King (1970) suggested that the synaptonemal complex may orient the non-sister chromatids of homologous in a manner that facilitates enzymatically induced exchanges between the DNA molecules.
4. Comings and Okada (1971) reported that synaptonemal complex pulls homologous chromosomes into approximate association with each other but plays no role in molecular pairing of DNA strands.