Steps Involved In Gamete Formation
Meiosis is a reductional division which reduces the chromosomes number to half. Gametes should have haploid number of chromosomes so that the diploid status can be regained after fertilization. When meiosis would take place in the life cycle depends upon the ploidy of the mother cells of the gametes. If the mother cell is already haploid like in ferns, where the plant body bearing gametes is a gametophyte, gametes are produced by mitosis instead of meiosis. In such cases meiosis occurs in the spote mother cells at the time of spore formation. While in certain and many other cases meiosis occurs during gametogenesis, for example ib all angiosperms, the egg mother cell or the pollen mother cell which are part of the sporophytic plant body undergo meiosis to produce gametes.
CELL DIVISION: Meiosis. CELL DIVISION: MEIOSIS AND SEXUALREPRODUCTION Table of Contents Meiosis Sexual reproduction occurs only in.During the formation of,the number ofis reduced by half, and returned to the full amount when the twofuse during. Ploidy Haploid and diploid are terms referring to thenumber of sets of chromosomes in a cell. Gregor Mendel determined hispeas had two sets of alleles, one from each parent.organisms are those with two (di) sets. Human beings (except fortheir gametes), most animals and many plants are diploid. Weabbreviate diploid as 2n.
Ploidy is a term referring to the number ofsets of chromosomes.organisms/cells have only one set of chromosomes, abbreviated as n.Organisms with more than two sets of chromosomes are termedpolyploid. Chromosomes that carry the same genes are termed. Theon homologous chromosomes may differ, as in the case ofindividuals. Organisms (normally) receive one set of homologouschromosomes from each parent.is a special type of nuclear division which segregates one copy ofeach homologous chromosome into each new 'gamete'.
Mitosis maintainsthe cell's original ploidy level (for example, one diploid 2n cellproducing two diploid 2n cells; one haploid n cell producing twohaploid n cells; etc.). Meiosis, on the other hand, reduces thenumber of sets of chromosomes by half, so that when gameticrecombination ( )occurs the ploidy of the parents will be reestablished.Most cells in the human body are produced bymitosis. These are the(or vegetative) line cells.
Cells that become gametes are referred toas. The vast majority of celldivisions in the human body are mitotic, with meiosis beingrestricted to the. Life Cycles Life cycles are a diagrammatic representation ofthe events in the organism's development and reproduction. Wheninterpreting life cycles, pay close attention to the ploidy level ofparticular parts of the cycle and where in the life cycle meiosisoccurs. For example, animal life cycles have a dominant diploidphase, with the gametic (haploid) phase being a relative few cells.Most of the cells in your body are diploid, germ line diploid cellswill undergo meiosis to produce gametes, with fertilization closelyfollowing meiosis.Plant life cycles have two sequential phases thatare termed.
Thephase is 'diploid', and is that part of the life cycle in whichmeiosis occurs. However, many plant species are thought to arise by,and the use of 'diploid' in the last sentence was meant to indicatethat the greater number of chromosome sets occur in this phase. Thephase is 'haploid', and is the part of the life cycle in whichgametes are produced (by mitosis of haploid cells).
Formation Of Gametes Human
In floweringplants ( )the multicelled visible plant (leaf, stem, etc.) is sporophyte, whilepollen and ovaries contain the male and female gametophytes,respectively. Plant life cycles differ from animal ones by adding aphase (the haploid gametophyte) after meiosis and before theproduction of gametes.Many protists and fungi have a haploid dominatedlife cycle. The dominant phase is haploid, while the diploid phase isonly a few cells (often only the single celled zygote, as inChlamydomonas ).
Many protists reproduce by mitosis untiltheir environment deteriorates, then they undergo sexual reproductionto produce a resting zygotic cyst. Phases of Meiosis Two successive nuclear divisions occur, Meiosis I(Reduction) and Meiosis II (Division). Meiosis produces 4 haploidcells.
Mitosis produces 2 diploid cells. The old name for meiosis wasreduction/ division. Meiosis I reduces the ploidy level from 2n to n(reduction) while Meiosis II divides the remaining set of chromosomesin a mitosis-like process (division). Most of the differences betweenthe processes occur during Meiosis I.The above image is from Prophase I Prophase I has a unique event - the pairing (byan as yet undiscovered mechanism) of.is the process of linking of the replicated homologous chromosomes.The resulting chromosome is termed a,being composed of twofrom each chromosome, forming a thick (4-strand) structure.may occur at this point. During crossing-over chromatids break andmay be reattached to a different homologouschromosome.The alleles on this tetrad:A B C D E F GA B C D E F Ga b c d e f ga b c d e f gwill produce the following chromosomes if there isa crossing-over event between the 2nd and 3rd chromosomes from thetop:A B C D E F GA B c d e f ga b C D E F Ga b c d e f gThus, instead of producing only two types ofchromosome (all capital or all lower case), four differentchromosomes are produced.
This doubles the variability of gametegenotypes. The occurrence of a crossing-over is indicated by aspecial structure, a(plural chiasmata) since the recombined inner alleles will align morewith others of the same type (e.g.
A with a, B with B). Near the endof Prophase I, the homologous chromosomes begin to separate slightly,although they remain attached at chiasmata.Crossing-over between homologous chromosomesproduces chromosomes with new associations of genes andalleles. Image from Purves et al., Life: The Science ofBiology, 4th Edition, by Sinauer Associates and WH Freeman ,used with permission.Events of Prophase I (save for synapsis andcrossing over) are similar to those in Prophase of mitosis: chromatincondenses into chromosomes, thedissolves, nuclear membrane is disassembled, and the forms.Major events in Prophase I. Image fromPurves et al., Life: The Science of Biology, 4th Edition, bySinauer Associates and WH Freeman ,used with permission.
Metaphase I Metaphase I is when tetrads line-up along theequator of the spindle. Spindle fibers attach to the centromereregion of each homologous chromosome pair. Other metaphase events asin mitosis. Anaphase I Anaphase I is when the tetrads separate, and aredrawn to opposite poles by the spindle fibers. The centromeres inAnaphase I remain intact.Events in prophase and metaphse I. Imagefrom Purves et al., Life: The Science of Biology, 4th Edition,by Sinauer Associates and WH Freeman ,used with permission.
Gamete Formation In Plants
Telophase I Telophase I is similar to Telophase of mitosis,except that only one set of (replicated) chromosomes is in each'cell'. Depending on species, new nuclear envelopes may or may notform. Some animal cells may have division of the centrioles duringthis phase.The events of Telophase I. Image fromPurves et al., Life: The Science of Biology, 4th Edition, bySinauer Associates and WH Freeman ,used with permission.
Prophase II During Prophase II, nuclear envelopes (if theyformed during Telophase I) dissolve, and spindle fibers reform. Allelse is as in Prophase of mitosis. Indeed Meiosis II is very similarto mitosis.The events of Prophase II. Image fromPurves et al., Life: The Science of Biology, 4th Edition, bySinauer Associates and WH Freeman ,used with permission.
Metaphase II Metaphase II is similar to mitosis, with spindlesmoving chromosomes into equatorial area and attaching to the oppositesides of the centromeres in the kinetochore region. Anaphase II During Anaphase II, the centromeres split and theformer chromatids (now chromosomes) are segregated into oppositesides of the cell.The events of Metaphase II and Anaphase II.Image from Purves et al., Life: The Science of Biology, 4thEdition, by Sinauer Associates and WH Freeman ,used with permission. Telophase II Telophase II is identical to Telophase of mitosis.Cytokinesis separates the cells.The events of Telophase II.
Image fromPurves et al., Life: The Science of Biology, 4th Edition, bySinauer Associates and WH Freeman ,used with permission. Comparison ofMitosis and Meiosis Mitosis maintains ploidy level, while meiosisreduces it.
Meiosis may be considered a reduction phase followed by aslightly altered mitosis. Meiosis occurs in a relative few cells of amulticellular organism, while mitosis is more common.Comparison of the events in Mitosis andMeiosis. Images from Purves et al., Life: The Science ofBiology, 4th Edition, by Sinauer Associates and WH Freeman ,used with permission.