CELL DIVISION: Meiosis... (2024)

Comparison ofMitosis and Meiosis | Gametogenesis| Links

Meiosis | Backto Top

Sexual reproduction occurs only ineukaryotes.During the formation of gametes,the number of chromosomesis reduced by half, and returned to the full amount when the twogametesfuse during fertilization.

The above image is from http://www.biosci.uga.edu/almanac/bio_103/notes/apr_3.html.

Ploidy | Back toTop

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. Diploidorganisms 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. Haploidorganisms/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 termedhom*ologouschromosomes. The alleleson hom*ologous chromosomes may differ, as in the case ofheterozygousindividuals. Organisms (normally) receive one set of hom*ologouschromosomes from each parent.

Meiosisis a special type of nuclear division which segregates one copy ofeach hom*ologous 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 (fertilization)occurs the ploidy of the parents will be reestablished.

Most cells in the human body are produced bymitosis. These are the somatic(or vegetative) line cells. Cells that become gametes are referred toas germline cells. The vast majority of celldivisions in the human body are mitotic, with meiosis beingrestricted to the gonads.

Life Cycles | Backto Top

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.

The above image is from http://www.biosci.uga.edu/almanac/bio_103/notes/apr_4.html.

Plant life cycles have two sequential phases thatare termed alternationof generations. The sporophytephase is "diploid", and is that part of the life cycle in whichmeiosis occurs. However, many plant species are thought to arise bypolyploidy,and the use of "diploid" in the last sentence was meant to indicatethat the greater number of chromosome sets occur in this phase. Thegametophytephase is "haploid", and is the part of the life cycle in whichgametes are produced (by mitosis of haploid cells). In floweringplants (angiosperms)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.

The above is from http://www.biosci.uga.edu/almanac/bio_104/notes/apr_3.html.

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 | Backto Top

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.

Prophase I | Backto Top

Prophase I has a unique event -- the pairing (byan as yet undiscovered mechanism) of hom*ologouschromosomes. Synapsisis the process of linking of the replicated hom*ologous chromosomes.The resulting chromosome is termed a tetrad,being composed of two chromatidsfrom each chromosome, forming a thick (4-strand) structure.Crossing-overmay occur at this point. During crossing-over chromatids break andmay be reattached to a different hom*ologouschromosome.

The alleles on this tetrad:

A B C D E F G

a b c d e f g

will produce the following chromosomes if there isa crossing-over event between the 2nd and 3rd chromosomes from thetop:

A B C D E F G

A B c d e f g

a b C D E F G

a b c d e f g

Thus, 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 chiasma(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 hom*ologous chromosomes begin to separate slightly,although they remain attached at chiasmata.

Metaphase I | Backto Top

Metaphase I is when tetrads line-up along theequator of the spindle. Spindle fibers attach to the centromereregion of each hom*ologous chromosome pair. Other metaphase events asin mitosis.

Anaphase I | Backto Top

Anaphase I is when the tetrads separate, and aredrawn to opposite poles by the spindle fibers. The centromeres inAnaphase I remain intact.