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Meiosis I consists of prophase I, metaphase I, anaphase I, and telophase I. Prophase I is also a long stage of the meiosis process.
A sister chromatid is an identical half of a replicated chromosome that is still attached at the centromere. Sister chromatids remain held together at the centromere, a region of chromosomes where the chromatids are joined. Since all of the genetic material is being replicated, sex cells spend most of their time in this stage.
It is used mostly as a resting stage for the cells. During the resting stage, no chromosome duplication happens. This step is unique to meiosis. Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II.
This event occurs during the S phase. After this, each chromosome has two sister chromatids which are the cloned DNA after replication. The centromere connects both of them in the chromosome at the center for storage. They may also get separated into individual chromosomes when necessary.
MetaphaseMetaphase. During metaphase, spindle fibers attach to the centromere of each pair of sister chromatids (see Figure below). The sister chromatids line up at the equator, or center, of the cell. This is also known as the metaphase plate.
MetaphaseMetaphase. Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide.
metaphaseDuring metaphase, the kinetochore microtubules pull the sister chromatids back and forth until they align along the equator of the cell, called the equatorial plane. There is an important checkpoint in the middle of mitosis, called the metaphase checkpoint, during which the cell ensures that it is ready to divide.
During (metaphase II), all sister chromatids align themselves along the metaphase plate.
During the G2 phase the cell has already undergone DNA replication (in the S phase). Each of the chromosomes will therefore have identical sister chromatids. There will be 92 sister chromatids, but only 46 chromosomes.
anaphase IISister chromatids move towards opposite poles in anaphase II of second division.
The events of metaphase II are similar to those of mitotic metaphase — in both processes, the chromosomes line up along the cell's equatorial plate, also called the metaphase plate, in preparation for their eventual separation (Figure 5).
Metaphase. Next, chromosomes assume their most compacted state during metaphase, when the centromeres of all the cell's chromosomes line up at the equator of the spindle. Metaphase is particularly useful in cytogenetics, because chromosomes can be most easily visualized at this stage.
metaphaseDuring metaphase, all of the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to each other. At this time, the chromosomes are maximally condensed.
During metaphase, the sister chromatids are forced to the center of the cell.
Before the initiation of the process of meiosis, a very important event has to occur, which is the replication or duplication of the normal diploid DNA or chromosomes. Hence, there are now 4 copies of each of the genes, located in two full sets of DNA and each of those sets have two alleles.
This is the first step in the process of Meiosis cell division. The Prophase of Meiosis is quite long and more complex than the Prophase of Mitosis. This single substep of Prophase is further subdivided into 5 stages, namely - Leptotene, Zygotene, Pachytene Diplotene, and Diakinesis. These are discussed briefly below.
In the step of Metaphase I of Meiosis I, the alignment of the bivalent chromosomes are seen on the equatorial plate. From the opposite poles of the spindle, the microtubules emerge to attach themselves to the kinetochore of homologous chromosomes.
In the Anaphase I stage of Meiosis I, separation of the homologous chromosomes is seen, while the sister chromatids remain connected at their centromeres.
This is the last stage of Meiosis I. Here, the reappearance of the nucleolus and the nuclear membrane is seen. It is followed by Cytokinesis and is called the dyad of cells. Interkinesis is the time between two meiotic divisions. It is usually short-lived. No DNA replication takes place during interkinesis.
Prophase II is the first step in the process of Meiosis II. Prophase II is simpler than Prophase I to a great extent. It starts right after Cytokinesis, even before the full elongation of the chromosomes. Meiosis II is similar to a normal Mitosis.
Alignment of the chromosomes at The equator is seen in the Metaphase II of Meiosis II. Again, from the opposite poles of the spindle, the microtubules arise and get attached to the kinetochores of the sister chromatids.