MEIOSIS /maɪˈoʊsᵻs/ ( listen ) is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells , each genetically distinct from the parent cell that gave rise to them. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes , including animals , plants , and fungi . Errors in meiosis resulting in aneuploidy are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.
Because the number of chromosomes is halved during meiosis, gametes
can fuse (i.e. fertilization ) to form a diploid zygote that contains
two copies of each chromosome, one from each parent. Thus, alternating
cycles of meiosis and fertilization enable sexual reproduction , with
successive generations maintaining the same number of chromosomes. For
example, diploid human cells contain 23 pairs of chromosomes including
1 pair of sex chromosomes (46 total), half of maternal origin and half
of paternal origin.
* 1 Overview * 2 History * 3 Occurrence in eukaryotic life cycles * 4 Process
* 5 Phases
* 5.1.1 Prophase I
* 188.8.131.52 Leptotene * 184.108.40.206 Zygotene * 220.127.116.11 Pachytene * 18.104.22.168 Diplotene * 22.214.171.124 Diakinesis * 126.96.36.199 Synchronous processes
* 6 Origin and function * 7 Nondisjunction * 8 In plants and animals * 9 In mammals * 10 Compared to mitosis * 11 See also
* 12 References
* 12.1 Cited texts
* 13 External links
While the process of meiosis is related to the more general cell division process of mitosis , it differs in two important respects:
RECOMBINATION meiosis shuffles the genes between the two chromosomes in each pair (one received from each parent), producing recombinant chromosomes with unique genetic combinations in every gamete
mitosis occurs only if needed to repair DNA damage;
usually occurs between identical sister chromatids and does not result in genetic changes
CHROMOSOME NUMBER (PLOIDY) meiosis produces four genetically unique cells, each with half the number of chromosomes as in the parent
mitosis produces two genetically identical cells, each with the same number of chromosomes as in the parent
The term meiosis (originally spelled "maiosis") was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905:
We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming .
It is derived from the Greek word μείωσις, meaning 'lessening'.
OCCURRENCE IN EUKARYOTIC LIFE CYCLES
Gametic life cycle. Zygotic life cycle. Main article:
Biological life cycle
Cycling meiosis and fertilization 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 or diploid life cycle), during the haploid state (zygotic or haploid life cycle), or both (sporic or haplodiploid life cycle, in which there are 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 organism phase(s).
In the gametic life cycle or " diplontic life cycle", of which humans are a part, the organism is diploid, grown from a diploid cell called the zygote . The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.
In the zygotic life cycle the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete . Two organisms of opposing sex contribute their haploid gametes 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 utilize 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 spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles.
The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle.
Interphase is divided into three phases:
* Growth 1 (G1) phase : In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA. * Synthesis (S) phase : The genetic material is replicated; each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis. * Growth 2 (G2) phase : G2 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G2 phase of the mitotic cell cycle.
Interphase is followed by meiosis I and then meiosis II.
During meiosis, specific genes are more highly transcribed . In addition to strong meiotic stage-specific expression of mRNA , there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.
Diagram of the meiotic phases
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". :27In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids—become "individualized" to form visible strands within the nucleus. :27 :353 The two sister chromatids closely associate and are visually indistinguishable from one another. During leptotene, lateral elements of the synaptonemal complex assemble. Leptotene is of very short duration and progressive condensation and coiling of chromosome fibers takes place.
The zygotene stage, also known as zygonema, from Greek words meaning "paired threads", :27 occurs as the chromosomes approximately line up with each other into homologous chromosome pairs. In some organisms, this is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place, facilitated by assembly of central element of the synaptonemal complex . Pairing is brought about in a zipper-like fashion and may start at the centromere (procentric), at the chromosome ends (proterminal), or at any other portion (intermediate). Individuals of a pair are equal in length and in position of the centromere. Thus pairing is highly specific and exact. The paired chromosomes are called bivalent or tetrad chromosomes.
The pachytene (pronounced /ˈpækᵻtiːn/ PAK-ə-teen ) stage, also known as pachynema, from Greek words meaning "thick threads",. :27 At this point a tetrad of the chromosomes has formed known as a bivalent. This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. Nonsister chromatids of homologous chromosomes may exchange segments over regions of homology. Sex chromosomes , however, are not wholly identical, and only exchange information over a small region of homology. At the sites where exchange happens, chiasmata form. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope, and chiasmata are not visible until the next stage.
During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", :30 the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I.
In mammalian and human fetal oogenesis all developing oocytes develop to this stage and are arrested before birth. This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.
Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through". :30 This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
During these stages, two centrosomes , containing a pair of centrioles in animal cells, migrate to the two poles of the cell. These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, which are essentially cellular ropes and poles. The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore . The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centrosome, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I.
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centrosome: these are called nonkinetochore microtubules or polar microtubules. A third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton.
Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrosomes attach to their respective kinetochores, the paired 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. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids.
The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. 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. Sister chromatids remain attached during telophase I.
Cells may enter a period of rest known as interkinesis or interphase
In PROPHASE II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division.
In METAPHASE II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
This is followed by ANAPHASE II, in which the remaining centromeric cohesin is cleaved allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
The process ends with TELOPHASE II, which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes reform and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
ORIGIN AND FUNCTION
Main article: Origin and function of meiosis
The ORIGIN AND FUNCTION OF MEIOSIS are fundamental to understanding the evolution of sexual reproduction in Eukaryotes . There is no current consensus among biologists on the questions of how sex in Eukaryotes arose in evolution , what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex . It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendents of the original sexually reproducing species are still sexual reproducers, including plants , fungi , and animals .
Main article: Nondisjunction
The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, 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 .
Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include but are not limited to:
Down syndrome – trisomy of chromosome 21
Patau syndrome – trisomy of chromosome 13
Edwards syndrome – trisomy of chromosome 18
Klinefelter syndrome – extra X chromosomes in males – i.e.
XXY, XXXY, XXXXY, etc.
Turner syndrome – lacking of one
The probability of nondisjunction in human oocytes increases with increasing maternal age, presumably due to loss of cohesin over time.
IN PLANTS AND ANIMALS
Overview of chromatides' and chromosomes' distribution within the mitotic and meiotic cycle of a male human cell
In females, meiosis occurs in cells known as oocytes (singular:
oocyte). Each oocyte that initiates meiosis divides twice, unequally
in each case. The first division produces a daughter cell that will
undergo a second division, and a much smaller "polar body" that is
extruded from the surface of the cell and does not divide further.
In males, meiosis occurs during spermatogenesis in the seminiferous
tubules of the testicles .
In female mammals , meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. It is retinoic acid, derived from the primitive kidney (mesonephros) that stimulates meiosis in ovarian oogonia. Tissues of the male testis suppress meiosis by degrading retinoic acid, a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid.
COMPARED TO MITOSIS
In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis.
End result Normally four cells, each with half the number of chromosomes as the parent Two cells, having the same number of chromosomes as the parent
Function Production of gametes (sex cells) in sexually reproducing eukaryotes Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Reproductive cells of almost all eukaryotes (animals, plants, fungi, and protists ) All proliferating cells in all eukaryotes
Genetically same as parent? No Yes
Crossing over happens? Yes, normally occurs between each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Yes No
* ^ Freeman, Scott (2011). Biological Science (6th ed.). Hoboken,
NY: Pearson. p. 210.
* ^ Letunic, I; Bork, P (2006). "Interactive Tree of Life".
Retrieved 23 July 2011.
* ^ Bernstein H, Bernstein C, Michod RE (2011). “
* Freeman, Scott (2005). Biological Science (3rd ed.). Upper Saddle River, NJ: Pearson Prentice Hall.
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