MEIOSISMEIOSIS

What is it?What is it? Making of gametesMaking of gametes

Sex CellsSex Cells Sperm (pollen) in malesSperm (pollen) in males Eggs (ova) in femalesEggs (ova) in females

Diploid Diploid Haploid Haploid Involves:Involves:

Cell divisionCell division 2 nuclear divisions2 nuclear divisions

Reduction of chromosome number by halfReduction of chromosome number by half

The human life cycleThe human life cycle

Egg cell

Sperm cellFERTILIZATIONMEIOSIS

Mitosis anddevelopment

Diploidzygote (2n = 46)

Multicellulardiploid adults (2n = 46)

Haploid gametes (n = 23)

Chromosomes

Centromere

Sister chromatids

•Somatic cells of each species contain a specific number of chromosomes

•Human cells have 46, making up 23 pairs of homologous chromosomes

•In each homologous pair - ~ 1 comes from “Mom” ~ 1 comes from “Dad”

Homologous Chromosomes

InterphaseInterphase

Just like “Interphase” of MitosisJust like “Interphase” of Mitosis GG1 1 ~ ‘Gap’ 1 = growth and preparation~ ‘Gap’ 1 = growth and preparation

S ~ ‘Synthesis’ = DNA replication to makeS ~ ‘Synthesis’ = DNA replication to make

exact copies of the chromosomesexact copies of the chromosomes

GG22 ~ ‘Gap’ 2 = growth and shape change ~ ‘Gap’ 2 = growth and shape change

Meiosis I ~ Reduction Meiosis I ~ Reduction DivisionDivision

Prophase IProphase I Chromatin thickens into chromosomesChromatin thickens into chromosomes

You can now see them under a light microscopeYou can now see them under a light microscope Nuclear membrane begins to disintegrateNuclear membrane begins to disintegrate Centrioles migrate to opposite polesCentrioles migrate to opposite poles Homologues do something unique to meiosisHomologues do something unique to meiosis

Prophase I - continuedProphase I - continued

Homologous Pairs line up next to each otherHomologous Pairs line up next to each other SynapsisSynapsis forms a tetradforms a tetrad

Crossing OverCrossing Over pieces of the chromosomes ‘switch’ placespieces of the chromosomes ‘switch’ places

Stay connected at Stay connected at Chiasma(ta)Chiasma(ta)

MOST IMPORTANT ~MOST IMPORTANT ~ for genetic diversity for genetic diversity

Metaphase IMetaphase I Attached homologues line up on metaphase Attached homologues line up on metaphase

plateplate Anaphase IAnaphase I

Tetrads separate at chiasma(ta)Tetrads separate at chiasma(ta) ** sister chromatids are STILL together** sister chromatids are STILL together

Telophase ITelophase I Homologues move to polesHomologues move to poles 2 nuclear membranes form (in some cells)2 nuclear membranes form (in some cells)

** Chromosome number is now ** Chromosome number is now HAPLOIDHAPLOID

~~ there is now half the number of original there is now half the number of original chromosomeschromosomes

from the parent.from the parent.

InterkinesisInterkinesis

Stage between Meiosis I and IIStage between Meiosis I and II Resting stageResting stage NOT another InterphaseNOT another Interphase

NO DNA replication NO DNA replication Strictly a “recovery” phase orStrictly a “recovery” phase or

rest period before moving on.rest period before moving on.

Meiosis II – like MitosisMeiosis II – like Mitosis Prophase IIProphase II

Just like Meiosis I – but NO crossing overJust like Meiosis I – but NO crossing over Metaphase IIMetaphase II

Sister chromatids line up on metaphase Sister chromatids line up on metaphase plateplate

Usually at right angles to meiosis IUsually at right angles to meiosis I Anaphase IIAnaphase II

Sister chromatids separate at centromere Sister chromatids separate at centromere andand

move to opposite polesmove to opposite poles Telophase II and CytokinesisTelophase II and Cytokinesis

4 new nuclei are formed 4 new nuclei are formed Each are haploidEach are haploid

Crossing Over = Genetic Diversity

Mitosis and Meiosis Compared

WEB LINKSWEB LINKS

http://www.lewport.wnyric.org/jwanamahttp://www.lewport.wnyric.org/jwanamaker/animations/meiosis.htmlker/animations/meiosis.html

http://www3.telus.net/byngscience/sc1http://www3.telus.net/byngscience/sc10/Sc10~Biology/s_tan_division.swf0/Sc10~Biology/s_tan_division.swf

http://www.pbs.org/wgbh/nova/baby/dihttp://www.pbs.org/wgbh/nova/baby/divi_flash.htmlvi_flash.html