L>Mitosis and Meiosis: A Review

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

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

Where does mitosis occur?In multicell organisms, somatic (body) cells undergo mitosis to provide new cells for expansion or to change cells that have actually been damaged and died.Some varieties reproduce via asexually (sometimes dubbed parthenogenesis native the Greek parthen, "virgin" and also genesis, "origin". In together species, progenitor cell are developed via mitosis.A clone is a team of geneticallyidentical organisms. Mito - Greek for "thread"(referring come the threadlike appearance of the chromosomes throughout division)sis - Greek because that "theact of"In meiosis, sex-related cell division, one diploid (2n) meiocyte (a.k.a. Germline cell) divides to produce four haploid (n) daughter cells. this are further processed to come to be sex cells (gametes).meio - "less"Meiosis is "the plot of do less"Where does meiosis occur?In sexually reproducing organisms, a meiocyte (a.k.a. Germline cell) undergoes meiosis to develop gametes. In animals this wake up in the gonads (ovaries in females; testes in males). In plants this wake up in the archegonia in females and in the antheridia in males. In fungi this occurs in specialized structures ~ above fruiting bodies referred to as sporangia. Protists can undergo meiosis to produce haploid execution of us which then have the right to fuse v other people who have actually done the same thing. Prokaryotes (bacteria and archaea) don"t undergo mitosis or meiosis because they space haploid, and also don"t have actually linear chromosomes.
Mitosis: Asexual cabinet DivisionA cabinet undergoes mitosis to develop two (daughter) cell that room genetically identical to the initial (parent) cell.Cellular structures pertinent to mitosis or meiosis: plasma membrane - "gateway" that the cabinet cytosol - proteinaceous matrix containing the organelles mitochondria and chloroplasts - power transduction organelles contain circular DNA separate from the cell nucleus mitochondrial DNA - mtDNA chloroplastic DNA - cpDNA DNA is circular, not linear resembles bacter DNA mtDNA, happen on just via maternal parent, is sometimes called "Eve"s DNA" nuclear membrane - twin membrane bordering the DNA and forming the cell core nucleoplasm - proteinaceous matrix inside the nucleus nucleolus - dark-staining an ar within the cell nucleus locatedat the Nucleolar Organizer region (NOR) of the DNA. The is the site of ribosom assembly. centromere - location of the kinetochore,the physical structure to which spindle fibers attach.
Chromosomes can be share on the communication of centromere position: metacentric - in ~ the midpoint the the chromosome submetacentric - slightly offset from the midpoint the the chromosome acrocentric - close to the finish of the chromosome telocentric - in ~ the telomere p arm = brief arm that a chromosome q arm = lengthy arm the a chromosomeBy convention, chromosomes are shown with the short arms pointing upward.

Phases of Mitosis

The phases permit us come separate major events of cell division.They are split somewhat arbitrarily. But you need to know them, anyway. Once a cell is not proactively dividing, it might be in interphase - common state that the cabinet Gap 1 - proteins essential for cell division are made Synthesis (S) phase - DNA is replicated Gap 2 - short period between DNA synthesis completion and also start that mitosis Prophase euchromatin condenses right into heterochromatin chromosomes are currently visible together joined sister chromatids nucleolus disappears (no much more ribosome synthesis!) mitotic spindle creates Metaphase spindle microtubules have actually attached come the kinetochores the the sisters chromatids copied chromosomes heat up in ~ the metaphase plate Anaphase spindle traction the sisters chromatids apart, drawing them to opposite poles that the cell. Telophase chromosome sets assemble at the opposite poles the the cell nuclear envelope forms approximately each chromosome setelophase is (usually) adhered to by cytokinesis, division of the cytoplasm to type two brand-new cells. Each new daughter cell is genetically identical to the parent cell.Meiosis: sex-related Cell DivisionA cell undergoes meiosis to produce four (daughter) cells (gametes) that space genetically different from the original (parent) cabinet (germline cell).Meiosis produces haploid cell from a diploid cell in preparation for sexual reproduction.Why Sex?The word originates from the Latin secare, which method to cutor division something that was when whole. during meiosis, the make of sex cells, the genetic enhance of a parent cell is split into two tantamount halves.Meiosis I is reduction division - the cabinet goes from having actually two homologous chromosomes per 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 that Mitosis: Meiosis I

Prophase IA. leptonema (adjective=leptotene) indigenous the Greeklepto, definition "thin" 1. Atom envelope and also nucleoli disappear 2. Spindle fibers start to type 3. In animals, centrioles start migration to oppositepoles. 4. Chromosomes start to supercoil 5. "loose" or "rough" pairing of homologs (synapsis is just starting)B. zygonema (adjective = zygotene) from the Greekzygo, an interpretation "yoke" 1. Synaptonemal complex forms: 2 paired homologs space joined by a "ladderlike" facility of synaptonemal proteins. Once this is complete, the pair is knownas a bivalent. 2. Synapsis is continuing to developC. pachynema (adjective = pachytene) native the Greekpachy, meaning "thick" 1. Chromosomes become shorter and more thickness (moresupercoiling) 2. Sisters chromatids start to unwind, becoming visibleas 2 chromosomes joined at the centromere 3. In ~ this point, the bivalent is recognized as a tetrad4. Crossing over takes place(Note: homologous pairs are analogous to a "husbandand wife", and the sisters chromatids space analogous come identicaltwins--at least before crossing over.)D. diplonema (adj = diplotene) from the Greek diplo,meaning "double" 1. Synaptonemal facility starts come disintegrate 2. Chiasmata (crossover points) end up being visible,sometimes together a facility mesh, since there may be multiple crossover points. 3. Keep in mind that sisters chromatids space no much longer identical,as they have undergone crossing over.(Note: some animals stop here, consisting of humans. Meiosis go not proceed until fertilization or ovulation.)E. diakinesis from the Greek dia, meaning"across" and also kinesis, definition "movement." 1. Chiasmata relocate tothe tips of the chromatids, wherein they continue to be attached. Thisprocess is recognized 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 space still connectedat the centromere. In ~ this point, every two-part member of the former tetrad is recognized as a dyad Telophase I, if it occurs (some types skip this step), is a backwards development to interphase-like conditions. In Meiosis, it is known as interkinesis.Meiosis II: the equational division is physicallythe very same as mitosis, though the hereditary composition in the nuclei room different since of crossing over and recombination.After meiosis, gametogenesis occurs to do the newhaploid cells into gametes.
The generalized animal scenario:Male: Testes save on computer 2n spermatogonial cells, whichconstantly renew themselves via mitosis. At part point, somewill mature and enter right into meiosis come become... primary spermatocytes <2n --> 2(n + n)>. This diploid cells undergo meosis i to come to be two... secondary spermatocytes (n + n). Haploid spermatocytes experience meiosis II to end up being four spermatids (n). More spermatogenesis results in the typicalflagellated spermatozoa.Female: Ovaries save on computer 2n oogonial cells, i m sorry usually perform notrenew themselves. In ~ some point in your cellular lifespan, these go into into meiosis ns tobecome primary oocytes <2n --> 2(n + n)>, which undergo meosisI. One primary oocyte will come to be a polar body The polar body will certainly divide again to offer rise to a second polar body. Meanwhile, the other main oocyte will become a... secondary oocyte (n + n), which will certainly undergo meiosis IIto end up being one ootid and another polar body (n).4. More oogenesis results in the usual cytoplasm-richovum.Plant gametogenesis to add an extra generation. Oversimplifying a bit, we have...Male: inside the microsporangium the a sporophyte plant, microsporocyte (2n) experience meiosis I and II to produce haploid microspores (n). every microspore grows right into a haploid male gametophyte (n) The gametophyte produce sperm (n) via mitosis.Female: within the megasporangium that a sporophyte plant, a megasporocyte (2n) experience meiosis I and also II to produce asingle haploid megaspore (n) the 3 polar bodies created degenerate, as in pets The megaspore grows into the female gametophyte (n), which produces ova (n) via mitosis. polyspermy: combination of more than one sperm with a single egg.In animals, this is virtually always lethal, as polyploidy (more than two sets that chromosomes) will certainly not develop a viable pet embryoIn plants, however, polyspermy is one means that polyploidy can an outcome in speciation.

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Chromosomes: Pairs and Sets

euploidy - the normal number of chromosome to adjust members of all homologous pairs...expected in a given cell (of its species).(from the Greek eu, definition "true") aneuploidy- more or fewer 보다 the normal variety of chromosomes every homologous pair (from the Greek an - "away" ) nullisomy - a homolgous pair is entirely absent monosomy - just one member the a homologous pair is current trisomy - three duplicates of a homologous chromosome are present tetrasomy - four duplicates of a homologous chromosome are current etc.Aneuploidies can result from nondisjunction - three duplicates of a homologous chromosome space present- two homologs migrate to the same brand-new gamete, leave the various other one "blank" lagging chromosome - three copies of a homologous chromosome are current - one member the a homologous pair is not separated right into the newly developing gamete at the same price as the rest, and also is left out of the nucleus when the nuclear membrane forms.Autosomal aneuploidies are far much more devastating than sex c"some abnormalities; the last are regularly survivable, and also some persons through sex c"some abnormalities deserve to lead entirely normal lives and also produce typical offspring. ploidy - the variety of complete chromosome sets in a cabinet haploid - one complete set of chromosomes (in humans, one set comprises 23 chromosomes) diploid - two complete sets of chromosomes (in humans, 2 sets the 23 chromosomes) triploid - three complete sets that chromosomes tetraploid - three complete sets of chromosomes etc.Because most eukaryotic organisms are diploid, the condition of having much more than two complete sets the chromosomes is known, generically, as polyploidy