The process of meiosis happens in the male and female reproductive organs. Just like in mitosis , a cell starts meiosis in interphase. In interphase, the DNA is copied, the cell grows and the organelles are copied too. After interphase:. All gametes are genetically different from each other. Two parents are needed in sexual reproduction. During this process the nuclei of the male and female gametes are fused in order to create a zygote.
This process is known as fertilisation. The gametes in:. In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II. Telophase I and cytokinesis: The chromosomes complete their move to the opposite poles of the cell. At each pole of the cell a full set of chromosomes gather together. A membrane forms around each set of chromosomes to create two new nuclei. 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. Excluding mutation and mistakes, these sperm are identical except for their individual, unique genetic load.
They each contain the same amount of cytoplasm and are propelled by whip-like flagella. In females, oogenesis and meiosis begin while the individual is still in the womb. The primary oocytes, analogous to the spermatocyte in the male, undergo meiosis I up to diplonema in the womb , and then their progress is arrested. Once the female reaches puberty, small clutches of these arrested oocytes will proceed up to metaphase II and await fertilization so that they may complete the entire meiotic process; however, one oocyte will only produce one egg instead of four like the sperm.
This can be explained by the placement of the metaphase plate in the dividing female germ cell. Instead of lying across the middle of the cell like in spermatogenesis, the metaphase plate is tucked in the margin of the dividing cell, although equal distribution of the genetic material still occurs. This results in a grossly unequal distribution of the cytoplasm and associated organelles once the cell undergoes cytokinesis.
This first division produces a large cell and a small cell. The large cell, the secondary oocyte , contains the vast majority of the cytoplasm of the parent cell, and holds half of the genetic material of that cell as well. The small cell, called the first polar body, contains almost no cytoplasm, but still sequesters the other half of the genetic material. This process repeats in meiosis II, giving rise to the egg and to an additional polar body.
These differences in meiosis reflect the roles of each of the sex cells. Sperm must be agile and highly motile in order to have the opportunity to fertilize the egg—and this is their sole purpose. For this reason, they hardly carry any cellular organelles excluding packs of mitochondria which fuel their rapid motion , mostly just DNA.
For this reason, only a single, well-fortified egg is produced by each round of meiosis. Meiosis is a process that is conserved, in one form or another, across all sexually-reproducing organisms. This means that the process appears to drive reproductive abilities in a variety of organisms and points to the common evolutionary pathway for those organisms that reproduce sexually.
It is vitally important for the maintenance of genetic integrity and enhancement of diversity. Since humans are diploid 2N organisms, failure to halve the ploidy before fertilization can have disastrous effects.
For this reason, only very select types of abnormal ploidy survive and do so with noticeable defects ; most combinations containing abnormal ploidy never make it into the world. During prophase II, the chromosomes condense, and a new set of spindle fibers forms.
The chromosomes begin moving toward the equator of the cell. During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells. Then in anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell. Finally, during telophase II, the chromosomes are enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of the two cells.
At the conclusion of meiosis, there are four haploid daughter cells that go on to develop into either sperm or egg cells. Further Exploration Concept Links for further exploration cell division replication metaphase anaphase telophase linkage chromosome cytokinesis haploid prometaphase principle of segregation principle of independent assortment spindle fibers gamete DNA chromatin nucleus cytoplasm eukaryote prophase recombination principle of segregation Principles of Inheritance.
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