L>Mitosis and Meiosis: A Review

Cell Division: A ReviewCells can reproduce either asexually via mitosis or sexually via meiosisIn mitosis, asexual cell division, one diploid (2n) parent cell gives rise to two diploid daughter cells that are genetically identical to the original cell and to each other.

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Where does mitosis occur?In multicellular organisms, somatic (body) cells undergo mitosis to provide new cells for growth or to replace cells that have been damaged and died.Some species reproduce via asexually (sometimes called parthenogenesis from the Greek parthen, "virgin" and genesis, "origin". In such species, progenitor cells are produced via mitosis.A clone is a group of geneticallyidentical organisms. mito - Greek for "thread"(referring to the threadlike appearance of the chromosomes during 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. These are further processed to become sex cells (gametes).meio - "less"Meiosis is "the act of making less"Where does meiosis occur?In sexually reproducing organisms, a meiocyte (a.k.a. germline cell) undergoes meiosis to produce gametes. In animals this occurs in the gonads (ovaries in females; testes in males). In plants this occurs in the archegonia in females and in the antheridia in males. In fungi this occurs in specialized structures on fruiting bodies called sporangia. Protists can undergo meiosis to produce haploid versions of themselves which then can fuse with other individuals who have done the same thing. Prokaryotes (bacteria and archaea) don"t undergo mitosis or meiosis because they are haploid, and don"t have linear chromosomes.
Mitosis: Asexual Cell DivisionA cell undergoes mitosis to produce two (daughter) cells that are genetically identical to the original (parent) cell.Cellular structures relevant to mitosis or meiosis: plasma membrane - "gateway" of the cell cytosol - proteinaceous matrix containing the organelles mitochondria and chloroplasts - energy transduction organelles contain circular DNA separate from the nucleus mitochondrial DNA - mtDNA chloroplast DNA - cpDNA DNA is circular, not linear resembles bacterial DNA mtDNA, passed on only via maternal parent, is sometimes called "Eve"s DNA" nuclear membrane - double membrane surrounding the DNA and forming the nucleus nucleoplasm - proteinaceous matrix inside the nucleus nucleolus - dark-staining region within the nucleus locatedat the Nucleolar Organizer Region (NOR) of the DNA. It is the site of ribosome assembly. centromere - location of the kinetochore,the physical structure to which spindle fibers attach.
Chromosomes can be classified on the basis of centromere position: metacentric - at the midpoint of the chromosome submetacentric - slightly offset from the midpoint of the chromosome acrocentric - close to the end of the chromosome telocentric - at the telomere p arm = short arm of a chromosome q arm = long arm of a chromosomeBy convention, chromosomes are depicted with the short arms pointing upward.

Phases of Mitosis

The phases allow us to separate major events of cell division.They are divided somewhat arbitrarily. But you need to know them, anyway. When a cell is not actively dividing, it may be in interphase - normal state of the cell Gap 1 - proteins needed for cell division are manufactured Synthesis (S) phase - DNA is replicated Gap 2 - short period between DNA synthesis completion and start of mitosis Prophase euchromatin condenses into heterochromatin chromosomes are now visible as joined sister chromatids nucleolus disappears (no more ribosome synthesis!) mitotic spindle forms Metaphase spindle microtubules have attached to the kinetochores of the sister chromatids duplicated chromosomes line up at the metaphase plate Anaphase spindle pull the sister chromatids apart, drawing them to opposite poles of the cell. Telophase chromosome sets assemble at opposite poles of the cell nuclear envelope forms around each chromosome setelophase is (usually) followed by cytokinesis, division of the cytoplasm to form two new cells. Each new daughter cells is genetically identical to the parent cell.Meiosis: Sexual Cell DivisionA cell undergoes meiosis to produce four (daughter) cells (gametes) that are genetically different from the original (parent) cell (germline cell).Meiosis produces haploid cells from a diploid cell in preparation for sexual reproduction.Why Sex?The word comes from the Latin secare, which means to cutor divide something that was once whole. During meiosis, the making of sex cells, the genetic complement of a parent cell is divided into two equivalent halves.Meiosis I is reduction division - the cell goes from having two homologous chromosomes per pair to having only one member of each homologous pair.Meiosis II is equational division - the cell divides again, in essentially the same manner as mitosis: the sister chromatids segregate to two new daughter cells.

Stages of Mitosis: Meiosis I

Prophase IA. leptonema (adjective=leptotene) from the Greeklepto, meaning "thin" 1. nuclear envelope and nucleoli disappear 2. spindle fibers begin to form 3. in animals, centrioles begin migration to oppositepoles. 4. chromosomes begin to supercoil 5. "loose" or "rough" pairing of homologs (synapsis is just starting)B. zygonema (adjective = zygotene) from the Greekzygo, meaning "yoke" 1. synaptonemal complex forms: two paired homologs are joined by a "ladderlike" complex of synaptonemal proteins. Once this is complete, the pair is knownas a bivalent. 2. synapsis is continuing to developC. pachynema (adjective = pachytene) from the Greekpachy, meaning "thick" 1. chromosomes become shorter and thicker (moresupercoiling) 2. sister chromatids begin to unwind, becoming visibleas two chromosomes joined at the centromere 3. at this point, the bivalent is known as a tetrad4. crossing over takes place(Note: homologous pairs are analogous to a "husbandand wife", and the sister chromatids are analogous to identicaltwins--at least before crossing over.)D. diplonema (adj = diplotene) from the Greek diplo,meaning "double" 1. synaptonemal complex starts to disintegrate 2. chiasmata (crossover points) become visible,sometimes as a complex mesh, since there may be multiple crossover points. 3. note that sister chromatids are no longer identical,as they have undergone crossing over.(Note: some animals stop here, including humans. Meiosis does not continue until fertilization or ovulation.)E. diakinesis from the Greek dia, meaning"across" and kinesis, meaning "movement." 1. chiasmata move tothe tips of the chromatids, where they remain attached. Thisprocess is known as terminalization. 2. spindle fibers attach to kinetochores. Metaphase I - spindle fibers arrange homologs alongthe metaphase plate at the cell"s equator. Anaphase I- spindle fibers separate homologs, carryingthem to opposite poles, but sister chromatids are still connectedat the centromere. At this point, each two-part member of the former tetrad is 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 known as interkinesis.Meiosis II: the equational division is physicallythe same as mitosis, though the genetic composition in the nuclei are different because of crossing over and recombination.After meiosis, gametogenesis occurs to make the newhaploid cells into gametes.
The generalized animal scenario:Male: Testes contain 2n spermatogonial cells, whichconstantly renew themselves via mitosis. At some point, somewill mature and enter into meiosis to become... primary spermatocytes <2n --> 2(n + n)>. These diploid cells undergo meosis I to become two... secondary spermatocytes (n + n). Haploid spermatocytes undergo meiosis II to become four spermatids (n). Further spermatogenesis results in the typicalflagellated spermatozoa.Female: Ovaries contain 2n oogonial cells, which usually do notrenew themselves. At some point in their cellular lifespan, these enter into meiosis I tobecome primary oocytes <2n --> 2(n + n)>, which undergo meosisI. One primary oocyte will become a polar body The polar body will divide again to give rise to a second polar body. Meanwhile, the other primary oocyte will become a... secondary oocyte (n + n), which will undergo meiosis IIto become one ootid and another polar body (n).4. Further oogenesis results in the typical cytoplasm-richovum.Plant gametogenesis adds an extra generation. Oversimplifying a bit, we have...Male: Inside the microsporangium of a sporophyte plant, microsporocyte (2n) undergoes meiosis I and II to produce haploid microspores (n). Each microspore grows into a haploid male gametophyte (n) The gametophyte produces sperm (n) via mitosis.Female: Inside the megasporangium of a sporophyte plant, a megasporocyte (2n) undergoes meiosis I and II to produce asingle haploid megaspore (n) the three polar bodies produced degenerate, as in animals The megaspore grows into the female gametophyte (n), which produces ova (n) via mitosis. polyspermy: fusion of more than one sperm with a single egg.In animals, this is almost always lethal, as polyploidy (more than two sets of chromosomes) will not produce a viable animal embryoIn plants, however, polyspermy is one way that polyploidy can result in speciation.

See more: Are The Bonds In Each Of The Following Substances Ionic, Nonpolar Covalent, Or Polar Covalent?

Chromosomes: Pairs and Sets

euploidy - the normal number of chromosome sets members of all homologous pairs...expected in a given cell (of its species).(from the Greek eu, meaning "true") aneuploidy- more or fewer than the normal number of chromosomes per homologous pair (from the Greek an - "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 present tetrasomy - four copies of a homologous chromosome are present etc.Aneuploidies can result from nondisjunction - three copies of a homologous chromosome are present- two homologs migrate to the same new gamete, leaving the other one "blank" lagging chromosome - three copies of a homologous chromosome are present - one member of a homologous pair is not separated into the newly forming gamete at the same rate as the rest, and is left out of the nucleus when the nuclear membrane forms.Autosomal aneuploidies are far more devastating than sex c"some abnormalities; the latter are often survivable, and some persons with sex c"some abnormalities can lead totally normal lives and produce normal offspring. ploidy - the number 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, two sets of 23 chromosomes) triploid - three complete sets of chromosomes tetraploid - three complete sets of chromosomes etc.Because most eukaryotic organisms are diploid, the condition of having more than two complete sets of chromosomes is known, generically, as polyploidy