Prophase
The prophase is the first phase of the cellular division (mitotic like meiotic) at the living beings Eucaryotes.
At the time of this phase, the genetic material (DNA), which in normal weather is present in the core in the form of Chromatine, condenses of structures very ordered and individualized called Chromosome S. the Nucléole S disaggregate. As the genetic material was duplicated before the beginning of the Mitose, there are then two identical copies of the genome in each cell. During this phase, the chromosomes thus consist of two Chromatide S indicated under the name of chromatides sisters and carrying both same genetic information. It also contain each one an element of DNA called Centromère, which plays a big role in the segregation of the chromosomes. Both chromatides of the same chromosome are connected to the level of the centromeric area. A named protein cohésine plays the part of adhesive and links both chromatides of the same chromosome.
The important second Organelle of the prophase is the Centrosome. Each Centrosome includes/understands two Centriole S. As for the chromosomes, the centrosome was duplicated before the beginning of the Prophase. During the prophase, the centrosome which contains 4 then centrioles is divided into two and each of both centrosomes migrates towards a pole of the cell. The cytosquelette of Microtubule S reorganizes to form the Fuseau mitotic bipolar structure which extends between the two centrosomes but remains outside the core. By the growth of the microtubules, the spindle mitotic lengthens, which stretches the cellular Noyau.
One can represent the microtubules like poles or rails, in the cell. Certain cells Eucaryote S, in particular the vegetable cells, are deprived of Centriole. ----
The prophase of the Méiose is divided into 5 different stages: ; Leptotene: (of the Greek: leptos = thin and taenia = the ribbon) One sees appearing the individualized chromosomes whose Télomère S start to stick inside the nuclear membrane. The Chromatide S paired sisters start to condense in the form of loops bordering a dense axial proteinic element.
; Zygotène: (Zygos = yoke and ten = filament): The counterparts approach according to a process called synapsis , which is closely related to the assembly of the Complexe synaptonémal. At the beginning of the stage zygotene, the Télomère S of the chromosomes are gathered on the level of a point on the nuclear envelope what reveals arrangement in bouquet chromosomes.
; Pachytene: (Packhus = thick): The Synapsis is finished, the Homolog S are connected between them via the complex synaptonémal.
; Diplotène: (diploos = double): At the beginning of this stage, the Complexe synaptonémal disintegrates and the chromosomes are décondensent, later, they condense again, the chromatides-sisters remain closely associated while the homologous chromosomes tend to separate from/to each other and are retained more together only via the chiasma S. This stage can last of the days or even of the years, according to the kind of organization and its type (Ovocyte S of 1st order in the ovary blocked in the stage diplotene until their transformation into ovocytes of 2nd order). The transcription of the chromosomes is very active during the stage diplotene, in particular in the females because the egg is devoted actively to the storage of the material which will be used in very first divisions of the embryonic development.
; Diakinesis: (dia = through and kinesis = movement): During this stage, the homologous chromosomes shorten and condense once more.
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