The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis. Meiosis II 6. Prophase II: Now there are two daughter cells, each with 23 chromosomes 23 pairs of chromatids. In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.
The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes. The centrioles duplicate. 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? Metaphase I : Instead of all chromosomes pairing up along the midline of the cell as in mitosis, homologous chromosome pairs line up next to each other.
This is called synapsis. Homologous chromosomes contain the matching alleles donated from mother and father. This is also when meiotic recombination , also know as "crossing over" see below occurs.
This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations. See the close-up diagram below. Anaphase I : Instead of chromatids splitting at the centromere, homologous chromosome pairs now shuffled by crossing over move along the spindle fibers to opposite poles.
Anaphase II : The chromatids split at the centromere and migrate along the spindle fibers to opposite poles. Telophase II : The cells pinch in the center and divide again. The final outcome is four cells, each with half of the genetic material found in the original. In the case of males, each cell becomes a sperm. In addition, in meiosis I, the chromosomal number is reduced from diploid 2 n to haploid n during this process.
During prophase I, chromosomal condensation allows chromosomes to be viewed under the microscope. In late prophase I, homologous chromosomes also called bivalent chromosomes, or bivalents pair laterally, or side-by-side. At this time they are said to be in synapsis. During synapsis, crossovers — cross-connections that form from breakage and rejoining between sister chromatids — can occur between the paired bivalents, leading to genetic recombination exchange of genetic material between the strands involved.
The point where a crossover occurs is called a chiasma plural chiasmata see below figure. In figure below, following crossing over, the blue and red chromosomes, which originally carried AA and aa alleles, respectively, now carry Aa alleles in both chromosomes at the end of prophase I.
Note that these bivalents have two chromosomes and four chromatids, with one chromosome originating from each parent. In metaphase I, each pair of bivalents two chromosomes, four chromatids total align on the metaphase plate.
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