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 cell division, one diploid (2n) parent cell offers rise to two diploid daughter cells that room genetically the same to the original cell and also to each other.

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

Where walk mitosis occur?In multicellular organisms, somatic (body) cells experience mitosis to provide new cells for development or to replace cells that have actually been damaged and also died.Some types reproduce via asexually (sometimes dubbed parthenogenesis native the Greek parthen, "virgin" and genesis, "origin". In together species, progenitor cells are developed via mitosis.A clone is a group of geneticallyidentical organisms. Mito - Greek because that "thread"(referring come the threadlike appearance of the chromosomes throughout division)sis - Greek because that "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 additional processed to become 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 produce gametes. In animals this wake up in the gonads (ovaries in females; testes in males). In plants this occurs in the archegonia in females and also in the antheridia in males. In fungi this occurs in committed structures ~ above fruiting bodies called sporangia. Protists can undergo meiosis to produce haploid versions of us which then can fuse v other people who have done the same thing. Prokaryotes (bacteria and also archaea) don"t experience mitosis or meiosis because they space haploid, and also don"t have actually linear chromosomes.
Mitosis: Asexual cell DivisionA cabinet undergoes mitosis to develop two (daughter) cell that are genetically identical to the original (parent) cell.Cellular structures pertinent to mitosis or meiosis: plasma membrane - "gateway" the the cabinet cytosol - proteinaceous matrix containing the oribel mitochondria and chloroplasts - energy transduction organelles save circular DNA separate from the cell core mitochondrial DNA - mtDNA chloroplast DNA - cpDNA DNA is circular, not straight resembles bacterial DNA mtDNA, happen on only via maternal parent, is sometimes referred to as "Eve"s DNA" nuclear membrane - dual membrane bordering the DNA and also forming the cell core nucleoplasm - proteinaceous procession inside the nucleus nucleolus - dark-staining region within the cell core locatedat the Nucleolar Organizer an ar (NOR) that the DNA. The is the site of ribosom assembly. centromere - location of the kinetochore,the physical structure to i m sorry spindle fibers attach.
Chromosomes can be classified on the communication of centromere position: metacentric - in ~ the midpoint of the chromosome submetacentric - slightly counter from the midpoint that the chromosome acrocentric - close to the end of the chromosome telocentric - in ~ the telomere p arm = short arm that a chromosome q arm = long arm that a chromosomeBy convention, chromosomes are depicted with the short arms pointing upward.

Phases of Mitosis

The phases allow us to separate significant events of cabinet division.They are separated somewhat arbitrarily. However you require to recognize them, anyway. As soon as a cabinet is not proactively dividing, it might be in interphase - normal state of the cell Gap 1 - proteins necessary for cell department are manufactured Synthesis (S) phase - DNA is replicated Gap 2 - short period between DNA synthetic completion and start of mitosis Prophase euchromatin condenses right into heterochromatin chromosomes are currently visible together joined sister chromatids core source disappears (no much more ribosome synthesis!) mitotic spindle creates Metaphase spindle microtubules have attached come the kinetochores that the sisters chromatids duplicated chromosomes heat up at the metaphase key Anaphase spindle traction the sister chromatids apart, illustration them come opposite poles of the cell. Telophase chromosome sets assemble at the contrary poles of the cell nuclear envelope forms about each chromosome setelophase is (usually) complied with by cytokinesis, division of the cytoplasm to type two brand-new cells. Each new daughter cell is genetically the same to the parent cell.Meiosis: sex-related Cell DivisionA cell undergoes meiosis to create four (daughter) cell (gametes) that space genetically different from the original (parent) cabinet (germline cell).Meiosis to produce haploid cell from a diploid cabinet in preparation for sex-related reproduction.Why Sex?The word comes from the Latin secare, which way to cutor division something the was as soon as whole. during meiosis, the do of sex cells, the genetic match of a parental cell is divided into two identical halves.Meiosis I is reduction division - the cell goes from having actually two homologous chromosomes every pair to having actually only one member of every homologous pair.Meiosis II is equational division - the cabinet divides again, in essentially the very same manner together mitosis: the sister chromatids segregate come two brand-new daughter cells.

Stages the Mitosis: Meiosis I

Prophase IA. leptonema (adjective=leptotene) from the Greeklepto, definition "thin" 1. Atom envelope and nucleoli disappear 2. Spindle fibers start to kind 3. In animals, centrioles start migration to oppositepoles. 4. Chromosomes begin to supercoil 5. "loose" or "rough" pairing that homologs (synapsis is just starting)B. zygonema (adjective = zygotene) from the Greekzygo, an interpretation "yoke" 1. Synaptonemal facility forms: 2 paired homologs room joined by a "ladderlike" complex of synaptonemal proteins. When 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 more thickness (moresupercoiling) 2. Sister chromatids begin to unwind, becoming visibleas 2 chromosomes joined at the centromere 3. At this point, the bivalent is well-known as a tetrad4. Crossing end takes place(Note: homologous pairs room analogous to a "husbandand wife", and the sister chromatids are analogous come identicaltwins--at least before crossing over.)D. diplonema (adj = diplotene) native the Greek diplo,meaning "double" 1. Synaptonemal complicated starts to disintegrate 2. Chiasmata (crossover points) come to be visible,sometimes as a complex mesh, due to the fact that there might be multiple crossover points. 3. Note that sister chromatids are no longer identical,as they have undergone cross over.(Note: some animals stop here, consisting of humans. Meiosis walk not proceed until fertilization or ovulation.)E. diakinesis indigenous the Greek dia, meaning"across" and kinesis, meaning "movement." 1. Chiasmata relocate tothe advice of the chromatids, wherein they remain attached. Thisprocess is well-known as terminalization. 2. Spindle fibers connect to kinetochores. Metaphase I - spindle yarn arrange homologs alongthe metaphase plate in ~ the cell"s equator. Anaphase I- spindle fibers different homologs, carryingthem come opposite poles, yet sister chromatids space still connectedat the centromere. In ~ this point, each two-part member the the previous tetrad is well-known as a dyad Telophase I, if it occurs (some species skip this step), is a backwards progression to interphase-like conditions. In Meiosis, it is well-known as interkinesis.Meiosis II: the equational division is physicallythe very same as mitosis, though the hereditary composition in the nuclei are different due to the fact that of cross over and recombination.After meiosis, gametogenesis occurs to do the newhaploid cells right into gametes.
The generalized pet scenario:Male: Testes save 2n spermatogonial cells, whichconstantly renew us via mitosis. At some 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 undergo meiosis II to become four spermatids (n). Further spermatogenesis results in the typicalflagellated spermatozoa.Female: Ovaries save 2n oogonial cells, i m sorry usually do notrenew themselves. In ~ some suggest in your cellular lifespan, these go into into meiosis ns tobecome primary oocytes <2n --> 2(n + n)>, which experience meosisI. One major oocyte will become a polar body The polar body will certainly divide again to give rise come a second polar body. Meanwhile, the other main oocyte will come to be a... secondary oocyte (n + n), which will certainly undergo meiosis IIto come to be one ootid and also another polar body (n).4. Additional oogenesis outcomes in the usual cytoplasm-richovum.Plant gametogenesis add to an extra generation. Oversimplifying a bit, us have...Male: inside the microsporangium the a sporophyte plant, microsporocyte (2n) undergoes meiosis I and II to create haploid microspores (n). each microspore grows into a haploid male gametophyte (n) The gametophyte produce sperm (n) via mitosis.Female: within the megasporangium the a sporophyte plant, a megasporocyte (2n) undergoes meiosis I and also II to create asingle haploid megaspore (n) the three polar bodies produced degenerate, as 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 with a single egg.In animals, this is almost always lethal, as polyploidy (more than two sets that chromosomes) will not develop a viable animal embryoIn plants, however, polyspermy is one way that polyploidy can result in speciation.

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Chromosomes: Pairs and also Sets

euploidy - the normal variety of chromosome sets members of all homologous pairs...expected in a provided cell (of that species).(from the Greek eu, meaning "true") aneuploidy- more or fewer 보다 the normal variety of chromosomes per homologous pair (from the Greek one - "away" ) nullisomy - a homolgous pair is entirely missing monosomy - only one member of a homologous pair is present trisomy - three copies of a homologous chromosome are existing tetrasomy - four copies of a homologous chromosome are existing etc.Aneuploidies can an outcome from nondisjunction - three copies of a homologous chromosome are present- two homologs move to the same brand-new gamete, leave the other one "blank" lagging chromosome - three duplicates of a homologous chromosome are current - one member that a homologous pair is not separated right into the newly developing gamete at the same rate as the rest, and is left out of the nucleus as soon as the nuclear membrane forms.Autosomal aneuploidies space far an ext devastating 보다 sex c"some abnormalities; the latter are often survivable, and some persons through sex c"some abnormalities have the right to lead entirely normal lives and produce common offspring. ploidy - the variety of complete chromosome set in a cabinet haploid - one complete collection of chromosomes (in humans, one set comprises 23 chromosomes) diploid - two finish sets of chromosomes (in humans, 2 sets of 23 chromosomes) triploid - three complete sets that chromosomes tetraploid - three complete sets the chromosomes etc.Because most eukaryotic organisms room diploid, the problem of having more than two complete sets that chromosomes is known, generically, together polyploidy