Meiosis is a highly regulated cell division process that occurs in germ cells and leads to the formation of gametes with half the number of chromosomes of the mother cells. This process takes place in two main phases: meiosis I and meiosis II, each composed of specific subphases that ensure the precise redistribution of genetic information. Below, we will explore the eight phases of meiosis and how this crucial process for genetic diversity and sexual reproduction develops.
Phase 1: Prophase I
The first phase of meiosis, prophase I, is the longest and most complex stage of the process. During this phase, homologous chromosomes pair in a process known as synapses and form structures called bivalents or tetrads. Each bivalent is composed of two sister chromatids joined by a centromere. In addition, the phenomenon of crossing over or crossing-over occurs, where segments of non-sister chromatids are exchanged, increasing genetic variability.
Subphases of Prophase I:
1. Leptotene:
In this initial stage of prophase I, the chromosomes begin to condense and become visible under the microscope as thin filaments.
2. Zygotene:
In the zygotene, homologous chromosomes begin to pair, forming bivalent chromosomes. During this process, synapsis is established between non-sister chromatids.
3. Pachytene:
In this phase, the synapse is completed and the characteristic T-shaped structure of bivalents is formed. In addition, crossing over takes place between non-homologous chromatids, promoting genetic variability in the offspring.
4. Diplotene:
In diplotene, homologous chromosomes begin to separate, but remain joined at the points where crossing over occurred. These joining points are known as chiasmata.
5. Diakinesis:
In the last subphase of prophase I, the chromosomes continue to condense and the chiasmata are clearly visible under the microscope. The nuclear envelope begins to disintegrate, setting the stage for the next phase of meiosis.
Phase 2: Metaphase I
In metaphase I, the bivalents line up at the equatorial plate of the cell, each pair of homologous chromosomes is attached to different cell poles through spindle fibers that attach to the kinetochores of each chromosome. The arrangement of the bivalents at this stage is random, which further contributes to the genetic diversity of the resulting gametes.
Phase 3: Anaphase I
During anaphase I, the Homologous chromosomes separate and are pulled to opposite poles of the cell by spindle fibers. This process ensures that each daughter cell receives one copy of each homologous chromosome, but not a copy of both sister chromatids, which contributes to the reduction of chromosome number in the resulting cells.
Phase 4: Telophase I and Cytokinesis I
In telophase I, the chromosomes reach the opposite poles of the cell and begin to decondense. A new nuclear envelope appears around each chromosome set and cytokinesis begins, dividing the cell into two haploid cells called secondary cells or spermatocytes I in the case of men and oocytes I in women.
Phase 5: Prophase II
Prophase II is similar to the prophase of mitosis, but in this case the daughter cells already have half the number of chromosomes. During this phase, the centromeres of each sister chromatid divide, allowing the chromatids to separate independently in the next stage.
Subphases of Prophase II:
1. Early prophase II:
The chromosomes begin to condense again and the nuclear envelope disintegrates to allow the passage of the mitotic spindle to the chromosomes.
2. Late prophase II:
At this stage, the mitotic spindle is established and sister chromatids prepare to separate in the next phase.
Phase 6: Metaphase II
In metaphase II, sister chromatids align on the equatorial plate of each daughter cell. The kinetochores of the chromatids attach to the spindle fibers in preparation for their separation in the next phase.
Phase 7: Anaphase II
In anaphase II, sister chromatids are separated. They separate and are dragged to opposite poles of the daughter cells. This process ensures that each daughter cell receives a copy of each sister chromatid, which will lead to the formation of haploid gametes with unique genetic material.
Phase 8: Telophase II and Cytokinesis II
Finally, in telophase II, the chromatids reach the poles of the daughter cells, they decondense, forming chromatin, and a new nuclear envelope surrounds each chromosome set. Additionally, cytokinesis II takes place, dividing the cells into a total of four haploid cells, each with a unique set of chromosomes.
In summary, meiosis is a vital process for sexual reproduction that allows the generation of haploid gametes with genetic variability. Through the eight phases described, the correct distribution of chromosomes and genetic shuffling are guaranteed, which promotes genetic diversity in offspring, contributing to the evolution and adaptation of species over time.