Meiosis Totally Explained

Everything about Meiosis totally explained

In biology or life science, meiosis (pronounced mi-o-sis or me-o-sis) is a process of reduction division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes. The word "meiosis" comes from the Greek meioun, meaning "to make small," since it results in a reduction in chromosome number in the gamete cell.
   Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis doesn't occur in archaea or bacteria, which reproduce via asexual processes such as mitosis or binary fission. Each cell has half the number of chromosomes as the parent cell.
   During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contain one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. 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. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.
   In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization. In these groups, gametes are produced by mitosis.
   Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and genetic recombination between homologous chromosomes.

History

Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German biologist Oscar Hertwig (1849-1922). It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846-1910), in Ascaris worms' eggs. 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 genetics.

Evolution

Meiosis is thought to have appeared 1.4 billion years ago. The only supergroup of eukaryotes which doesn't have meiosis in all organisms is excavata. The other five major supergroups, opisthokonts, amoebozoa, rhizaria, archaeplastida and chromalveolates all seem to have genes for meiosis universally present, even if not always functional. Some excavata species do have meiosis which is consistent with the hypothesis that excavata is an ancient, paraphyletic grade. An example of eukaryotic organism in which meiosis doesn't exist is euglenoid.

Occurrence of meiosis in eukaryotic life cycles

sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there's an intermediary step between the diploid and haploid transition where the organism grows. 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.
   Cycling meiosis and fertilisation events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (gametic life cycle), or during the haploid state (zygotic life cycle), or both (sporic life cycle, in which there two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense, there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organisms phase(s). In the gametic life cycle, the species is diploid, grown from a diploid cell called the zygote. In the zygotic life cycle the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are spermatozoa for males or ova for females. These gametes then fertilize in the Fallopian tubes of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and differentiation, turns into a blastocyst and then gets implanted in the uterus endometrium to create an embryo.
   In the gametic life cycle, of which humans are a part, the living organism is diploid in nature. Here, we'll generalize the example of human reproduction stated previously. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. 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.
   In the zygotic life cycle, the living organism is haploid. 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. Many fungi and many protozoa are members of the zygotic life cycle.
   Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles.

Process

Because meiosis is a "one-way" process, it can't 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:

Growth 1 (G₁) phase: Immediately follows cytokinesis. This is a very active period, where the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it'll need for growth. In G1 stage each of the 46 human chromosomes consists of a single (very long) molecule of DNA. At this point cells are 46,2N, identical to somatic cells.
Synthesis (S) phase: The genetic material is replicated: each of its chromosomes duplicates (46,2N). The cell is still diploid, however, because it still contains the same number of centromeres. However, the identical sister chromatids are in the chromatin form because spiralisation and condensation into denser chromosomes have not taken place yet. It will take place in prophase I in meiosis.
Growth 2 (G₂) phase: G2 phase is absent in Meiosis

Interphase is immediately followed by meiosis I and meiosis II. Meiosis I consists of segregating the homologous chromosomes from each other, then dividing the diploid cell into two haploid 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 homologue), and dividing both haploid, duplicated cells to produce four haploid, unduplicated 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 generates genetic diversity in two ways: (1) independent assortment of chromosomes at both of the meiotic divisions allows genetic differences among gametes; and (2) physical exchange of chromosomal regions by homologous recombination during prophase I results in new genetic combinations within chromosomes.

Meiosis I

In meiosis I, the chromosomes in a diploid cell separate, producing two diploid cells (23, N).

Prophase I

Homologous chromosomes pair and form synapsis, a step unique to meiosis. The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata.

Leptotene

The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads." During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they're indistinguishable from one another. The chromosomes in the leptotene stage show a specific arrangement where the telomeres are oriented towards the nuclear membrane. Hence this stage is called, "bouquet stage".

Zygotene

The zygotene stage, also known as zygonema, from Greek words meaning "paired threads,"
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are also nonkinetochore microtubules.

Meiosis-phases

Metaphase I

Homologous pairs move together along the phase plate:
as kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate.

Anaphase I

Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one functional unit of a pair of kinetochores. Polyploidy is poorly tolerated in animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. Recombination and independent assortment allow for a greater diversity of genotypes in the population. As a system of creating diversity, meiosis allows a species to maintain stability under environmental changes.

Nondisjunction

The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation isn't normal, it's called nondisjunction. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy. 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:

Down Syndrome - trisomy of chromosome 21

Patau Syndrome - trisomy of chromosome 13

Edward Syndrome - trisomy of chromosome 18

Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY

Turner Syndrome - atypical X chromosome dosage in females - ie XO, XXX, XXXX

XYY Syndrome Supermale - an extra Y chromosome in males

Meiosis in humans

In females, meiosis occurs in cells known as oogonia (singular: oogonium). Each oogonium that initiates meiosis will divide twice to form a single oocyte and three polar bodies. However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females occurs during 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. Sperm is produced at a steady pace. The process of meiosis in males occurs during spermatogenesis.

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