In: Facts In the Cell. During meiosis one cell divides twice to form four daughter cells. These four daughter cells only have half the number of chromosomes of the parent cell — they are haploid. Meiosis produces our sex cells or gametes eggs in females and sperm in males.
These are divided between the first time the cell divides meiosis I and the second time it divides meiosis II : Meiosis I 1. Interphase: The DNA in the cell is copied resulting in two identical full sets of chromosomes.
Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division. During interphase, microtubules extend from these centrosomes. Prophase I: The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.
Each chromosome is composed of two sister chromatids containing identical genetic information. The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on. The pairs of chromosomes may then exchange bits of DNA in a process called recombination or crossing over. At the end of Prophase I the membrane around the nucleus in the cell dissolves away, releasing the chromosomes. The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.
Metaphase I: The chromosome pairs line up next to each other along the centre equator of the cell. The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them.
The meiotic spindle fibres attach to one chromosome of each pair. Anaphase I: The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole. 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.
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. The correct reduction of the number of chromosomes insures that once fertilization takes place, the correct amount of genetic material is established in the fertilized egg and, eventually, in the person resulting from it.
Meiosis in Humans By: Inbar Maayan. Keywords: Human development , Meiosis. Meiosis in Humans Meiosis, the process by which sexually reproducing organisms generate gametes sex cells , is an essential precondition for the normal formation of the embryo.
Gilbert, Scott F. Sunderland, MA: Sinauer, Hochwagen, Andreas. Klug, William S. Single-celled eukaryotes, such as amoeba and yeast, use mitosis to reproduce asexuallyand increase their population. Multicellular eukaryotes, like humans, use mitosis to grow or heal injured tissues. Meiosis, on the other hand, is a specialized form of cell division that occurs in organisms that reproduce sexually.
As mentioned above, it produces reproductive cells, such as sperm cells, egg cells, and spores in plants and fungi. In humans, special cells called germ cells undergo meiosis and ultimately give rise to sperm or eggs. Germ cells contain a complete set of 46 chromosomes 23 maternal chromosomes and 23 paternal chromosomes. By the end of meiosis, the resulting reproductive cells, or gametes , each have 23 genetically unique chromosomes.
The overall process of meiosis produces four daughter cells from one single parent cell. Each daughter cell is haploid, because it has half the number of chromosomes as the original parent cell. Andrew Hoyt, a biologist and professor at Johns Hopkins University. Unlike in mitosis, the daughter cells produced during meiosis are genetically diverse.
Homologous chromosomes exchange bits of DNA to create genetically unique, hybrid chromosomes destined for each daughter cell. Before meiosis begins, some important changes take place within the parent cells. First, each chromosome creates a copy of itself. These duplicated chromosomes are known as sister chromatids. They are fused together and the point where they are joined is known as the centromere.
Fused sister chromatids roughly resemble the shape of the letter "X. Meiosis occurs over the course of two rounds of nuclear divisions, called meiosis I and meiosis II, according to Nature Education's Scitable. Furthermore, meiosis I and II are each divided into four major stages: prophase, metaphase, anaphase and telophase. Meiosis I is responsible for creating genetically unique chromosomes.
Sister chromatids pair up with their homologs and exchange genetic material with one another. At the end of this division, one parent cell produces two daughter cells, each carrying one set of sister chromatids.
Meiosis II closely resembles mitosis. The two daughter cells move into this phase without any further chromosome duplication. The sister chromatids are pulled apart during this division. 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.
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