Synaptonemaler complex
In cell biology, a structure made up of proteins , RNA and DNA is referred to as a synaptonemal complex , or more rarely synaptic complex , which is present in the cell nucleus during a certain phase of meiosis and plays a role in the pairing of chromosomes .
The complex protein structure mediates the exact pairing of homologous chromosomes, called synapsis , during the prophase of meiosis I. In the zygotene of prophase I, the structure of the chromosome ends attached to the inner nuclear membrane is built up like a zipper. In the subsequent pachytan , it then forms a kind of scaffold that allows a controlled interaction of the two sister chromatids (partly connected to each other via cohesin ) of the two chromosomes in homologous segments. This facilitates the process of crossing over of non-sister chromatids, which leads to the mixing of previously paternal and maternal genetic information ( intrachromosomal recombination ). The four chromatids of a tetrad are held together at certain points by the approximately 100-300 nm wide complex and assigned to one another. At this stage, other chromatid components are still present as loosened chromatin , in which a crossover can then occur.
The synaptonemal complex, which is the same in all organisms examined so far, results from a three-part parallel arrangement of its elements, with two rows of lateral elements flanking a central row of elements at a constant distance on both sides. The linked elements of this structure show the image of a ladder with a central spar. This and the ladder rungs are formed by the so-called central element. It is connected to the two side bars, which are formed by the so-called lateral element, via the rungs . On the one hand (paternal) sister chromatids of one chromosome and on the other hand (maternal) sister chromatids of the other chromosome are attached to these side elements. In the cross section, the following sequence results: paternal sister chromatids 1 + 2 - lateral element - central element - lateral element - maternal sister chromatids 3 + 4, or vice versa.
In the central element row, recoupling nodes, also called recombination nodes, are often formed at greater distances from one another . Here the so-called recoupling takes place for the recombination after a crossover (to the chiasm ). Genome sections of non-sister chromatids are exchanged for one another, i.e. sections of a maternal chromatid over the central element area are exchanged for homologous sections of a paternal chromatid, and vice versa. After the chromosome pairing in pachytene , the synaptonemal complex dissolves again. If the complex persisted, the four chromatids of a tetrad could neither be separated nor distributed.
Origin and function
The precursor structures of the synaptonemal complex already form in the leptotene stage of the meiosis . The sister chromatids of the chromosomes condense and form what is known as a lateral element, a structure that, under the electron microscope, is reminiscent of the left or right half of a zip. Each “tooth” of this half of the zipper is called a synaptomer and consists of highly compressed chromatin and additional proteins.
During the homologous pairing (synapsis), one lateral element is placed together with its homologous partner (i.e. the paternal and maternal chromosome). The synapsis begins at the ends of the lateral elements, which, like a closing zipper, continue to come together. Here, the information on the DNA strands that come together plays a controlling role, since mostly only sections with "comparable" information come together.
If the lateral elements are together (the zipper is closed at one point), a protein band forms between the lateral elements (the central element). With the formation of the central element, the synaptonemal complex is completed. The entire structure now holds the homologous DNA segments of the paternal and maternal genetic information spatially together for a certain time and thus enables the exchange of homologous paternal and maternal gene segments.
literature
- ↑ Welsch, Bernhard: Synaptonemal complex and chromosome structure in the achiasmatic spermatogenesis of Panorpa communis (Mecoptera) Chromosoma (Berl.) 43 , 19-74 (1973)
- ↑ Klug, Cummings, Spencer "Genetics" (8th edition)
- ↑ Wilfried Janning, Elisabeth Knust: Genetics: General Genetics - Molecular Genetics - Developmental Genetics . 2nd Edition. Georg Thieme, Stuttgart 2008, ISBN 978-3-13-151422-6 , p. 34 f .
