Nature Reviews Molecular Cell Biology 5 , — doi Page, S. Chromosome choreography: The meiotic ballet. Science , — Petes, T. Meiotic recombination hot spots and cold spots. Zickler, D. Meiotic chromosomes: Integrating structure and function.
Annual Review of Genetics 33 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities.
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Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. 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. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. After replication, each chromosome becomes a structure comprising 2 identical chromatids. The chromosomes condense into visible X shaped structures that can be easily seen under a microscope, and homologous chromosomes pair up.
Recombination occurs as homologous chromosomes exchange DNA. At the end of this phase, the nuclear membrane dissolves. The pairs of chromosomes separate and move to opposing poles. Either one of each pair can go to either pole. Nuclear membranes reform. Cell divides and 2 daughter cells are formed, each with 23 chromosomes. There are 4 new haploid daughter cells. In males, 4 sperm cells are produced.
In females, 1 egg cell and 3 polar bodies are produced. Polar bodies do not function as sex cells. During fertilisation, 1 gamete from each parent combines to form a zygote. Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA.
This produces a unique combination of genes in the resulting zygote. Recombination or crossing over occurs during prophase I. 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.
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.
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