Leptotene, Zygotene, Pachytene, Diplotene, & Diakinesis are the 5 phases that make up Prophase 1, the first stage of Meiosis (in that order). Genetic material between non-sister chromatids crosses across and recombinates during prophase 1. As a result, haploid daughter chromatid cells that are genetically distinct are produced.
The final phase of meiosis prophase-1 is diakinesis (a five-stage process). Chiasmata terminalisation is a characteristic of the diakinesis stage. The cell that is dividing enters metaphase after diakinesis.
Meiosis
Meiosis is a unique form of cell division that occurs in germ cells. It occurs in species that reproduce sexually. When creating gametes like sperm and egg cells, meiosis takes place. German researcher Oscar Hertwig initially explained the meiosis mechanism in 1876. Meiosis I & meiosis II are the two primary stages of meiosis. Prophase I, Metaphase I, Anaphase I, & Telophase I are the four phases of Meiosis I. Prophase I consists of five stages: diplotene, zygotene, pachytene, & leptotene. Prophase II, Metaphase II, Anaphase II, & Telophase II are the other 4 phases of Meiosis II.
Stages of Prophase I
Leptotene
The first of Prophase 1’s five stages, Leptotene, involves condensing already replicated chromosomes; this process continues throughout the Prophase 1. Electron microscopy, which can discriminate between sister chromatids, makes the chromosomes visible. At this point in Prophase 1, the chromosomes resemble “a necklace of beads”; the beads are known as chromomeres. The sister chromatids are so near to one another and connected to the nuclear envelope that they could be mistaken for just one chromosome. This Prophase 1 stage is the shortest.
Zygotene
The substage known as zygotene in which synapsis between the homologous chromosomes starts. It also goes by the name zygonema. Due to the coils of chromatin, these synapses, also known as synaptonemal complexes, can form up and down the chromosomes, allowing several points of interaction. The aligned chromosomes are held together by the synaptonemal complex, which promotes synapsis. The homologous pairings are either referred to as tetrads or bivalents once they synaptise. Due to similar nomenclature for similar states, bivalent is more frequently used at an advanced level.
Pachytene
Once a tetrad has developed, the process of crossing over and the ensuing recombination, in which some genetic information from parental DNA sequences is swapped over to boost gene variety, can proceed. The chromosomes continue to be linked as a pair, but the chromatid sisters start to split from one another. Under an electron microscope, they become considerably easier to distinguish as a result. The illustration below depicts the exchange of the genetic material between 2 non – sister chromatids that are part of the same homologous chromosome pair. The chiasma, which joins two chromatids that are not sisters, enables the interchange of alleles.
Diplotene
The chromosomal pairs start to separate when the synaptonemal complex starts to degrade, as it happens during the diplotene stage. Since they are still connected by the chiasmata, they are unable to travel too far apart from one another. The two chromosomes’ repulsive properties cause a preliminary movement of the incomplete meiosis I spindle machinery, which will be finished during the prometaphase 1 that immediately follows Prophase I, towards the opposing poles.
Diakinesis
The 5th stage of prophase I of the meiosis I cell division is diakinesis. The Greek word “diakinesis” means “moving through.” Chromosomes continue to condense at this time. The four components of the tetrads usually become apparent for the 1st time during meiosis. Through efficient overlapping, the sites of crossing over become intertwined. Chiasmata are readily evident as a result.
The chiasmata connections of diakinesis are located at the ends of chromatid arms of chromosome. Termination is the term for this entry. Chromosomes can no longer migrate toward the poles of unfinished spindle construction because they are highly condensed and yet joined by chiasmata.
Other structural alterations take place during meiosis I to get ready for the following stage. The nuclear envelope and nucleolus disintegrate. As a result, spindle remnants from mitotic cell division can travel along with centrioles (centrosome-forming microtubules), which are important for spindle formation. The primary components of spindle construction are microtubules found in the cytoplasm of the cell.
Diakinesis Stage
The elimination of chromosomal condensing during Diakinesis, the final stage of Prophase 1, makes the chiasmata & bivalent structure more obvious under an electron microscope. During diakinesis, the chromosomes are in their most robust state. In a bivalent, there is still at least one chiasm connecting the homologous chromosomes. As a result, the centrosome moves to the equator, the nucleolus dissolves, the nuclear envelope collapses, and the mitotic spindles move.
Diakinesis represents transition to metaphase.
The changes in this sub-phase are as follows:
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It is accomplished that the nucleolus completely vanishes
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There is a loss of nuclear membrane
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Spindle apparatus forms toward the conclusion of this subphase
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Chromosome chiasmata travel to the ends of chromosomes as homologous chromosomes begin to split
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Chromosomes becomes recondense, transcription halts, and the transition to metaphase takes place as prophase I comes to a conclusion
Conclusion
The final phase of meiosis prophase-1 is diakinesis (a 5 – stage process). Chiasmata terminalisation is a characteristic of the diakinesis stage. The cell that is dividing enters metaphase after diakinesis.
Bivalents currently distribute them equally across the nucleus. The nucleolus is lost as a result of the nuclear membrane disintegrating. Chiasma terminalization is the movement of the chiasma toward the terminus. Chromatids reach the metaphase stage and only remain connected at the terminal chiasmata.
There is chromosomal recondensation. The development of spindles is started as the tetrad moves near the equatorial metaphase plate.
Prophase-1 ends with diakinesis, and the cell then enters metaphase 1 of the meiotic process.