L>Mitosis and also Meiosis: A Review

Cell Division: A ReviewCells have the right to reproduce one of two people asexually via mitosis or sexually via meiosisIn mitosis, asexual cell division, one diploid (2n) parental cell provides rise to two diploid daughter cells that room genetically identical to the initial cell and to every other.

You are watching: Meiosis occurs in specialized animal organs known as gonads.

Where walk mitosis occur?In multicell organisms, somatic (body) cells undergo mitosis to provide brand-new cells for expansion or to replace cells that have actually been damaged and died.Some species reproduce via asexually (sometimes referred to as parthenogenesis indigenous the Greek parthen, "virgin" and genesis, "origin". In together species, progenitor cell are produced via mitosis.A clone is a team of geneticallyidentical organisms. Mito - Greek because that "thread"(referring come the threadlike figure of the chromosomes throughout division)sis - Greek for "theact of"In meiosis, sexual cell division, one diploid (2n) meiocyte (a.k.a. Germline cell) divides to produce four haploid (n) daughter cells. this are more processed to become sex cell (gametes).meio - "less"Meiosis is "the plot of making less"Where does meiosis occur?In sexually reproducing organisms, a meiocyte (a.k.a. Germline cell) experience meiosis to develop gametes. In animals this occurs in the gonads (ovaries in females; testes in males). In plants this wake up in the archegonia in females and also in the antheridia in males. In fungi this occurs in devoted structures top top fruiting bodies referred to as sporangia. Protists deserve to undergo meiosis to produce haploid version of us which then have the right to fuse through other individuals who have actually done the very same thing. Prokaryotes (bacteria and also archaea) don"t undergo mitosis or meiosis because they space haploid, and also don"t have linear chromosomes.
Mitosis: Asexual cell DivisionA cabinet undergoes mitosis to create two (daughter) cell that room genetically identical come the initial (parent) cell.Cellular structures appropriate to mitosis or meiosis: plasma membrane - "gateway" that the cell cytosol - proteinaceous matrix containing the organelles mitochondria and chloroplasts - power transduction organelles save circular DNA separate from the cell nucleus mitochondrial DNA - mtDNA chloroplasts DNA - cpDNA DNA is circular, not direct resembles bacterial DNA mtDNA, happen on only via maternal parent, is sometimes referred to as "Eve"s DNA" nuclear membrane - twin membrane neighboring the DNA and forming the cell core nucleoplasm - proteinaceous procession inside the nucleus nucleolus - dark-staining an ar within the nucleus locatedat the Nucleolar Organizer an ar (NOR) that the DNA. It is the site of ribosome assembly. centromere - place of the kinetochore,the physical structure to which spindle yarn attach.
Chromosomes can be classified on the basis of centromere position: metacentric - in ~ the midpoint that the chromosome submetacentric - slightly balance out from the midpoint of the chromosome acrocentric - close come the end of the chromosome telocentric - at the telomere p arm = brief arm of a chromosome q arm = lengthy arm that a chromosomeBy convention, chromosomes are shown with the quick arms pointing upward.

Phases the Mitosis

The phases permit us come separate significant events of cabinet division.They are separated somewhat arbitrarily. However you require to understand them, anyway. As soon as a cabinet is not proactively dividing, it may be in interphase - typical state of the cabinet Gap 1 - proteins required for cell department are produced Synthesis (S) phase - DNA is replicated Gap 2 - short duration between DNA synthetic completion and start of mitosis Prophase euchromatin condenses into heterochromatin chromosomes are now visible together joined sisters chromatids core source disappears (no more ribosome synthesis!) mitotic spindle forms Metaphase spindle microtubules have attached to the kinetochores of the sisters chromatids replicated chromosomes line up in ~ the metaphase bowl Anaphase spindle pull the sisters chromatids apart, illustration them come opposite poles that the cell. Telophase chromosome sets rally at the contrary poles that the cabinet nuclear envelope forms about each chromosome setelophase is (usually) followed by cytokinesis, department of the cytoplasm to kind two new cells. Each new daughter cells is genetically the same to the parental cell.Meiosis: sex-related Cell DivisionA cell undergoes meiosis to produce four (daughter) cells (gametes) that room genetically different native the initial (parent) cell (germline cell).Meiosis produce haploid cell from a diploid cell in preparation for sex-related reproduction.Why Sex?The word comes from the Latin secare, which method to cutor divide something the was once whole. during meiosis, the make of sex cells, the genetic enhance of a parental cell is divided into two tantamount halves.Meiosis I is reduction division - the cabinet goes from having two homologous chromosomes every pair to having only one member of every homologous pair.Meiosis II is equational division - the cabinet divides again, in basically the exact same manner as mitosis: the sisters chromatids segregate to two brand-new daughter cells.

Stages the Mitosis: Meiosis I

Prophase IA. leptonema (adjective=leptotene) indigenous the Greeklepto, meaning "thin" 1. Nuclear envelope and nucleoli disappear 2. Spindle fibers begin to kind 3. In animals, centrioles begin migration to oppositepoles. 4. Chromosomes begin to supercoil 5. "loose" or "rough" pairing that homologs (synapsis is just starting)B. zygonema (adjective = zygotene) native the Greekzygo, definition "yoke" 1. Synaptonemal complex forms: 2 paired homologs are joined through a "ladderlike" complex of synaptonemal proteins. Once this is complete, the pair is knownas a bivalent. 2. Synapsis is proceeding to developC. pachynema (adjective = pachytene) native the Greekpachy, meaning "thick" 1. Chromosomes become shorter and thicker (moresupercoiling) 2. Sisters chromatids begin to unwind, coming to be visibleas two chromosomes joined at the centromere 3. At this point, the bivalent is recognized as a tetrad4. Crossing end takes place(Note: homologous pairs are analogous to a "husbandand wife", and the sister chromatids space analogous come identicaltwins--at least prior to crossing over.)D. diplonema (adj = diplotene) from the Greek diplo,meaning "double" 1. Synaptonemal complicated starts come disintegrate 2. Chiasmata (crossover points) become visible,sometimes together a facility mesh, due to the fact that there may be many crossover points. 3. Note that sister chromatids space no longer identical,as they have actually undergone cross over.(Note: some animals stop here, including humans. Meiosis walk not proceed until fertilization or ovulation.)E. diakinesis indigenous the Greek dia, meaning"across" and kinesis, definition "movement." 1. Chiasmata relocate tothe advice of the chromatids, whereby they continue to be attached. Thisprocess is known as terminalization. 2. Spindle fibers connect to kinetochores. Metaphase I - spindle fibers arrange homologs alongthe metaphase plate at the cell"s equator. Anaphase I- spindle fibers different homologs, carryingthem come opposite poles, but sister chromatids are still connectedat the centromere. In ~ this point, every two-part member that the former tetrad is recognized as a dyad Telophase I, if it wake up (some types skip this step), is a backwards progression to interphase-like conditions. In Meiosis, the is recognized as interkinesis.Meiosis II: the equational division is physicallythe very same as mitosis, despite the hereditary composition in the nuclei are different because of cross over and also recombination.After meiosis, gametogenesis wake up to do the newhaploid cells right into gametes.
The generalized animal scenario:Male: Testes save on computer 2n spermatogonial cells, whichconstantly renew us via mitosis. At some point, somewill mature and also enter into meiosis to become... primary spermatocytes <2n --> 2(n + n)>. These diploid cells undergo meosis ns to become two... secondary spermatocytes (n + n). Haploid spermatocytes experience meiosis II to become four spermatids (n). More spermatogenesis outcomes in the typicalflagellated spermatozoa.Female: Ovaries contain 2n oogonial cells, which usually perform notrenew themselves. At some point in their cellular lifespan, these get in into meiosis ns tobecome primary oocytes <2n --> 2(n + n)>, which undergo meosisI. One main oocyte will come to be a polar body The polar body will divide again to provide rise come a 2nd polar body. Meanwhile, the other primary oocyte will end up being a... secondary oocyte (n + n), which will certainly undergo meiosis IIto become one ootid and also another polar body (n).4. More oogenesis outcomes in the typical cytoplasm-richovum.Plant gametogenesis to add an extra generation. Oversimplifying a bit, us have...Male: within the microsporangium that a sporophyte plant, microsporocyte (2n) experience meiosis I and also II to create haploid microspores (n). every microspore grows into a haploid male gametophyte (n) The gametophyte produce sperm (n) via mitosis.Female: within the megasporangium of a sporophyte plant, a megasporocyte (2n) undergoes meiosis I and II to produce asingle haploid megaspore (n) the three polar bodies created degenerate, together in pets The megaspore grows right into the female gametophyte (n), i beg your pardon produces ova (n) via mitosis. polyspermy: combination of much more than one sperm through a single egg.In animals, this is practically always lethal, together polyploidy (more than 2 sets that chromosomes) will certainly not create a viable pet embryoIn plants, however, polyspermy is one method that polyploidy can result in speciation.

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Chromosomes: Pairs and Sets

euploidy - the normal variety of chromosome sets members of all homologous pairs...expected in a provided cell (of the species).(from the Greek eu, an interpretation "true") aneuploidy- an ext or fewer 보다 the normal variety of chromosomes per homologous pair (from the Greek an - "away" ) nullisomy - a homolgous pair is entirely absent monosomy - just one member of a homologous pair is present trisomy - three duplicates of a homologous chromosome are present tetrasomy - four duplicates of a homologous chromosome are present etc.Aneuploidies can result from nondisjunction - three copies of a homologous chromosome space present- 2 homologs move to the same brand-new gamete, leaving the various other one "blank" lagging chromosome - three duplicates of a homologous chromosome are current - one member the a homologous pair is no separated right into the newly creating gamete in ~ the same rate as the rest, and is left the end of the nucleus when the nuclear membrane forms.Autosomal aneuploidies space far more devastating than sex c"some abnormalities; the last are regularly survivable, and some persons through sex c"some abnormalities deserve to lead totally normal lives and produce regular offspring. ploidy - the variety of complete chromosome set in a cell haploid - one complete set of chromosomes (in humans, one collection comprises 23 chromosomes) diploid - two finish sets of chromosomes (in humans, two sets that 23 chromosomes) triploid - three complete sets that chromosomes tetraploid - three finish sets that chromosomes etc.Because most eukaryotic organisms room diploid, the problem of having more than two finish sets that chromosomes is known, generically, as polyploidy