One of the principal mechanisms of nuclear division in living organisms, resulting in the formation of gametes (in animals) or sexual spores (in plants). During meiosis a diploid nucleus (ie one possessing a double set of chromosomes) undergoes two successive divisions. This results in the production of four cells, each receiving only one member of each chromosome pair. The halving of chromosome numbers compensates for the doubling that occurs when two haploid gametes (ie each possessing a single set of chromosomes) unite to form a zygote during sexual reproduction. The phases of meiosis are leptotene (the appearance of chromosomes as threads in the nucleus), zygotene (the pairing of chromosomes to form bivalents), pachytene (the separation of bivalents), and diplotene (the moving apart of chromosomes). Meiosis is an important process in sexual reproduction, providing the opportunity for recombination to occur, as genetic material can be exchanged by crossing over between homologous chromosomes during the pachytene phase.
In biology, meiosis is the process that allows one diploid cell to divide in a special way to generate haploid cells in eukaryotes. The word "meiosis" comes from the Greek meioun, meaning "to make smaller," since it results in a reduction in chromosome number.
Meiosis is essential for sexual reproduction. Meiosis does not occur in archaea or prokaryotes, which reproduce by asexual cell division processes.
During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA called chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes. Each gamete contains one complete set of chromosomes, or half of the genetic content of the original cell. These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating type during fertilization to create a new diploid cell, or zygote. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA.
Biochemically, meiosis uses some of the same mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes, which enable them to separate from each other.
History
Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German biologist Oscar Hertwig (1849-1922). The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann (1834-1914), who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911 the American geneticist Thomas Hunt Morgan (1866-1945) observed crossover in Drosophila melanogaster meiosis and provided the first true genetic interpretation of meiosis.
Occurrence of meiosis in eukaryotic life cycles
Meiosis occurs in all eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. The organism will then produce the germ cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it.
The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition. Some species are diploid, grown from a diploid cell called the zygote. Others are haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Human stem cells undergo meiosis to create haploid gametes, which are sperm cells for males or ova for females. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism.
Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. The diploid organism's germ-line cells undergo meiosis to produce gametes. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote.
Process
Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle.
Interphase is divided into three phases:
Gap 1 (G1) phase: Characterized by increase in cell size due to accelerated manufacture of organelles, proteins, and other cellular matter. The cell thereby transforms from diploid to tetraploid.Interphase is immediately followed by meiosis I and meiosis II. Meiosis I consists of segregating the homologous chromosomes from each other, then dividing the tetraploid cell into two diploid cells each containing one of the segregates. Meiosis II consists of decoupling each chromosome's sister strands (chromatids), segregating the DNA into two sets of strands (each set containing one of each homolog), and dividing both diploid cells to produce four haploid cells. Meiosis I and II are both divided into prophase, metaphase, anaphase, and telophase subphases, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase (G1, S, G2), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
Meiosis I
Prophase I
In the prophase stage, the cell's genetic material, which is normally in a loosely arranged pile known as chromatin, condenses into visible threadlike structures.
The first stage of Prophase I is the leptotene stage, during which individual chromosomes begin to condense into long strands within the nucleus.
The zygotene stage then occurs as the chromosomes approximately line up with each other into homologous chromosomes.
Chromosomes recondense during the diakinesis stage.
During these stages, centrioles are migrating to the two poles of the cell.
Metaphase I
As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous counterbalancing forces exerted by the two kinetochores of the bivalent.
Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.
Cells enter a period of rest known as interkinesis or interphase II.
Meiosis II
Prophase II takes an inversely proportional time compared to telophase I.
The process ends with telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
Significance of meiosis
Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring.
Nondisjunction
The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction. Nondisjunction can occur in the meiosis I or meiosis II phases of cellular reproduction, or during mitosis.
This is a cause of several medical conditions in humans, including:
Down Syndrome - trisomy of chromosome 21 Patau Syndrome - trisomy of chromosome 13 Edward Syndrome - trisomy of chromosome 18 Klinefelter Syndrome - an extra X chromosome in males Turner Syndrome - only one X chromosome present in females XYY Syndrome - an extra Y chromosome in males Triple X Syndrome - an extra X chromosome in femalesMeiosis in humans
In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females is called oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the Dictyate stage and lacks the assistance of centrosomes.
In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles.
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