Moses in 1956 first discovered synaptonemal complex (SC), a feature of meiotic prophase. Synaptonemal complex is a tripartite structure usually found between the two paired homologous chromosomes of each bivalent in all animal and plant nuclei undergoing meiosis. This is considered to be a physical structure which is associated with synapsis of homologous chromosomes. Complete synaptonemal complexes are seen at zygonema in the region of pairing. At pachynema these complexes are even more conspicuous.
Structure of Synaptonemal Complex
The synaptonemal complex is a configuration in which the 10nm fibres of the chromosomes are arranged into a superstructure that can be viewed under the electron microscope. It is composed of three parallel, electron dense elements that are separated by less dense areas. The two lateral elements seen to be composed of fibres that are slightly wider than 10nm and are called synaptomeres. They vary in structure at different stages of meiotic prophase I within a species. The central element is a ladder like configuration in the centre of the SC. In some species it is comparatively more pronounced. The transverse elements are electron-dense filament that interconnect the central element with the lateral elements. The lateral elements may be spaced at a distance ‘ranging’ from 20nm to 30nm to as much as 100nm to 125nm. Cytochemical studies have demonstrated that the lateral elements are rich in DNA, RNA and proteins, but that the central element contains mainly RNA, protein and little DNA.
Sometimes synaptonemal complex like structures are also found which are not associated with synapsed chromosomes. They show some similarities with the true synaptonemal complex. The lateral elements of these anomalous complexes may differ in size and density from those in the true synaptonemal complexes of that species. However, the true autosomal synaptonemal complex is always a unit tripartite complex but anomalous complexes are often multiple stacks of alternating lateral elements and central elements.
Formation of synaptonemal complex and meiotic pairing of chromosomes. In 1970. King presented a hypothesis for the formation of synaptonemal complex. The hypothesis is known as ‘The Synaptomere Zygosome Hypothesis.’ According to this hypothesis, there are structures called synaptomeres which are coiled polynucleotide segments scattered along the length of a pair of synapsed chromosomes lying in close proximity to one another. Each synaptomere is composed of three segments: A, B and C. The lateral segments of the synaptomeres (A and C) pair with the respective segments of the adjacent synaptomeres. The B segments are directed towards the central element and the sites where the so-called zygosomes are attached. Zygosomes are rod shaped sub-units, assembled in the nucleoplasm and are each visualized as protein molecules having a folded head by which they can attach to the central segment (B) of. the synaptomere. The tail ends of the zygosomes contain charged sites that are represented by four dots in These charges allow the zygosomes to bind laterally with adjacent zygosomes in a ladder-like fashion_
The exact nature of events that lead to synapsis is still a subject of debate. A strong group of investigators believe that homologous chromosomes are prepared for synaptic pairing by the attachement of their telomeres to so-called ‘attachment sites’ on the nuclear envelope. This is believed to be followed by attachment of zygosomes to central segments of synaptomeres and to adjacent zygosomes. When each synaptomere has a zygosome, pegs extend from chromosome folds and pairing extends in zipper like action.
Function of Synaptonemal Complex
The appearance and disappearance of the synaptonemal complex coincide with the stages of meiosis in which pairing and recombination occur. This has led to the interpretation that they are functionally related. In leptonema, before pairing, single elements of synaptonemal complex are observed. Complete synaptonemal complexes are seen at zygonema in the region of pairing.
Several pieces of evidence Indicate that the synaptonemal complex is more directly related to the process of recombination. Some evidence, for example, is provided by the action of inhibitors of DNA synthesis at meiotic prophase, there is a small amount of DNA synthesis which, if inhibited, can arrest the function of the synaptonemal complexes.
The synaptonemal complex has been interpreted as protein framework that permits the proper alignment of the homologous chromosomes. However, since recombination by crossing over occurs at molecular level, it is necessary to assume that DNA fibres of the paired chromatids should reach the central component of SC within a distance of at least 1.0m for the recombination to take place. At diplonema the synaptonemal complex is shed from the bivalents with the exceptions of the regions in which the repelling homologues are held together each by a chiasma. Thus, a chiasma contains a piece of synaptonemal complex that will ultimately disappear and will be replaced by a chromatin bridge.
Recombination Nodules and their role in Meiotic Recombination
The synaptonemal complex (SC), described in the preceding section, is though associated with all paired chromosomes at pachytene, its most likely function is perhaps restricted to meiotic synapsis and/or maintenance of the synaptic state. Another class of important structures, associated with paired pachytene chromosomes are ‘recomination.nodules’, which are believed to be involved meiotic recombination.
A correspondence between meiotic exchange events and the numbers and locations of recombination nodules has been observed in a variety of materials suggesting the possible role of these structures in recombination. In Drosophila females, two types of recombination nodules, spherical (larger in size) and ellipsoidal (smaller in size) have been reported.
A number of Drosophila mutants, which are defective in recombination, were found to have normal synaptonemal complex, but were found to have changes in the number and morphology of one or both the types of recombination nodules. These observations suggested a positive role of recombination nodules in exchange of chromosome segment during pachytene leading to recombination.
Although data from many organisms firmly establish the correlations between recombination nodules and meiotic recombination (in number/nucleus, number/bivalent arm, etc), their exact role in recombination is not clearly understood. A study of mutants defective for recombination suggested that role of these nodules cannot be trivial. These nodules in some way perform the following two distinct roles: (i) they help in choice of number and location of recombination sites and (ii) also help in the recombination itself, by performing enzymatic and supporting functions.
Significance of Meiosis
It is obvious from the study of mitosis that daughter cells resulting from a mitotic division have same chromosome number as the parent cell had. This is necessary for growth and reproduction, if only asexual method is present. However, in sexual method of reproduction, fusion of sex cells (male and female) is required. If sex cells have the same chromosome number as the somatic cell have, the zygote will have double the chromosome number.
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- Molecular Structure of Chromosome (Nucleosome Structure)
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