At this point, while still associated at the chiasmata, the sister chromatids start to part from each other although they are still firmly bound at the centromere; this creates the X-shape commonly associated with condensed chromosomes. The nuclear membrane starts to dissolve by the end of diplonema and the chromosomes complete their condensation in preparation for the last substage of prophase I, diakinesis.
During this part, the chiasmata terminalize move toward the ends of their respective chromatids and drift further apart, with each chromatid now bearing some newly-acquired genetic material as the result of crossing over. Simultaneously, the centrioles, pairs of cylindrical microtubular organelles, move to opposite poles and the region containing them becomes the source for spindle fibers. These spindle fibers anchor onto the kinetochore, a macromolecule that regulates the interaction between them and the chromosome during the next stages of meiosis.
The kinetochores are attached to the centromere of each chromosome and help move the chromosomes to position along a three-dimensional plane at the middle of the cell, called the metaphase plate. The cell now prepares for metaphase I, the next step after prophase I.
During metaphase I, the tetrads finish aligning along the metaphase plate, although the orientation of the chromosomes making them up is random. The chromosomes have fully condensed by the point and are firmly associated with the spindle fibers in preparation for the next step, anaphase I. During this third stage of meiosis I, the tetrads are pulled apart by the spindle fibers, each half becoming a dyad in effect, a chromosome or two sister chromatids attached at the centromere.
Assuming that nondisjunction failure of chromosomes to separate does not occur, half of the chromosomes in the cell will be maneuvered to one pole while the rest will be pulled to the opposite pole. This migration of the chromosomes is followed by the final and brief step of meiosis I, telophase I, which, coupled with cytokinesis physical separation of the entire mother cell , produces two daughter cells.
Each of these daughter cells contains 23 dyads, which sum up to 46 monads or single-stranded chromosomes. Meiosis II follows with no further replication of the genetic material. The chromosomes briefly unravel at the end of meiosis I, and at the beginning of meiosis II they must reform into chromosomes in their newly-created cells. This brief prophase II stage [isEmbeddedIn] is followed by metaphase II, during which the chromosomes migrate toward the metaphase plate. During anaphase II, the spindle fibers again pull the chromosomes apart to opposite poles of the cell; however, this time it is the sister chromatids that are being split apart, instead of the pairs of homologous chromosomes as in the first meiotic step.
A second round of telophase this time called telophase II and cytokinesis splits each daughter cell further into two new cells. Each of these cells has 23 single-stranded chromosomes, making each cell haploid possessing 1N chromosomes. As mentioned, sperm and egg cells follow roughly the same pattern during meiosis , albeit a number of important differences. Spermatogenesis follows the pattern of meiosis more closely than oogenesis, primarily because once it begins human males start producing sperm at the onset of puberty in their early teens , it is a continuous process that produces four gametes per spermatocyte the male germ cell that enters meiosis.
A total of four haploid daughter cells are produced during the course of meiosis II. The four stages of meiosis I are as follows, according to " Molecular Biology of the Cell.
Prophase I : At this stage, chromosomes become compact, dense structures and are easily visible under the microscope. The homologous chromosomes pair together. The two sets of sister chromatids resemble two X's lined up next to each other. Each set exchanges bits of DNA with the other and recombines, thus creating genetic variation. This process is known as crossing over, or recombination. Even though in humans the male sex chromosomes X and Y are not exact homologs, they can still pair together and exchange DNA.
Crossing over occurs within only a small region of the two chromosomes. Metaphase I : The meiotic spindle, a network of protein filaments, emerges from two structures called the centrioles, positioned at either end of the cell. The meiotic spindle latches onto the fused sister chromatids. By the end of metaphase I, all the fused sister chromatids are tethered at their centromeres and line up in the middle of the cell.
The homologs still look like two X's sitting close together. Anaphase I : The spindle fibers start to contract, pulling the fused sister chromatids with them.
Each X-shaped complex moves away from the other, toward opposite ends of the cell. Telophase I : The fused sister chromatids reach either end of the cell, and the cell body splits into two. Meiosis I results in two daughter cells, each of which contains a set of fused sister chromatids.
The genetic makeup of each daughter cell is distinct because of the DNA exchange between homologs during the crossing-over process. In other words, by the end of the process, the chromosome number is unchanged between the cells that enter meiosis II and the resulting daughter cells.
Prophase II : The nuclear membrane disintegrates, and meiotic spindles begin to form once again. The meiotic spindle forms again. Metaphase II: In each of the two daughter cells the chromosomes pair of sister chromatids line up end-to-end along the equator of the cell.
The centrioles are now at opposites poles in each of the daughter cells. Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.
Anaphase II: The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle. The separated chromatids are now individual chromosomes. Telophase II and cytokinesis: The chromosomes complete their move to the opposite poles of the cell. A membrane forms around each set of chromosomes to create two new cell nuclei. This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis. Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes haploid : in males, these four cells are all sperm cells in females, one of the cells is an egg cell while the other three are polar bodies small cells that do not develop into eggs.
Related Content:. What is a cell? What is a chromosome? What is mitosis? What is DNA? Mitosis versus meiosis. What is a genetic disorder? How helpful was this page? Citation: O'Connor, C. Nature Education 1 1 How is the same process responsible for genetic recombination and diversity also the cause of aneuploidy?
Understanding the steps of meiosis is essential to learning how errors occur. Aa Aa Aa. Figure 1. Figure Detail.
Meiosis Is a Highly Regulated Process. Figure 2. Meiosis I. Figure 3. Meiosis II. Figure 4. Figure 5. Figure 6: Visualization of chromosomal bridges in Allium fistulosum and Allium cepa plant meiocytes. The sites of double-stranded break DSB dependent homologue interaction can be seen as approximately nm bridges between chromosome axes.
These bridges, which probably contain a DSB that is already engaged in a nascent interaction with its partner DNA, occur in large numbers. Their formation depends on the RecA recombination protein homologues that are expressed in this species. In the next phase of homologue interaction, these nascent interactions are converted to stable strand-invasion events.
This nucleates the formation of the synaptonemal complex SC. Homologous chromosome interactions in meiosis: diversity amidst conservation. Nature Reviews Genetics 6, All rights reserved. References and Recommended Reading Gerton, J. Science , — Petes, T.
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