MeiosisMeiosis

The Continuation of LifeThe Continuation of Life

Meiosis: The Continuation of Meiosis: The Continuation of LifeLife

Homologous ChromosomesHomologous Chromosomes

Paired chromosomesPaired chromosomes

One from each One from each parentparent

Genes for same traitGenes for same trait– AllelesAlleles– Same order on Same order on

chromosomechromosome

Homologous ChromosomesHomologous Chromosomes

Diploid vs. HaploidDiploid vs. Haploid Diploid Diploid

– Cell that has TWO Cell that has TWO of each kind of of each kind of chromosome chromosome (parent)(parent)

– 2n2n

HaploidHaploid– Cell that has ONE Cell that has ONE

of each kind of of each kind of chromosomechromosome

– 1n1n

QuestionQuestion

What would happen if Mitosis was the What would happen if Mitosis was the only way our cells could divide? only way our cells could divide?

23pairs 23pairs 46pairs 46pairs 96pairs…etc. 96pairs…etc.

We need a way to divide the We need a way to divide the Chromosome # in half! What is this Chromosome # in half! What is this called??called??

Meiosis!Meiosis!

MeiosisMeiosis

Purpose – Purpose –

make haploid cellsmake haploid cells

GametesGametes– EggEgg– SpermSperm

Form ZygoteForm Zygote

Phases of MeiosisPhases of Meiosis

Meiosis I Meiosis I

• Prophase IProphase I• Metaphase IMetaphase I• Anaphase IAnaphase I• Telophase ITelophase I

Meiosis IIMeiosis II

• Prophase IIProphase II• Metaphase IIMetaphase II• Anaphase IIAnaphase II• Telophase IITelophase II• CytokinesisCytokinesis

Prophase IProphase I

Same as in Mitosis…Same as in Mitosis…

But…But…

Homologous Homologous Chromosomes find Chromosomes find each othereach other

Synapsis Synapsis – TetradsTetrads

Prophase IProphase I

SynapsisSynapsis

Crossing over Crossing over occursoccurs

Exchange of Exchange of Genetic materialGenetic material

Crossing OverCrossing Over

Crossing Over and Crossing Over and its products!its products!

Metaphase IMetaphase I

Homologous pairs Homologous pairs line upline up

Tetrads still Tetrads still togethertogether

Anaphase IAnaphase I

Homologous Homologous Chromosomes Chromosomes separateseparate

Independent Independent AssortmentAssortment

Telophase ITelophase I

Just like Telophase Just like Telophase in Mitosisin Mitosis

Chromosomes Chromosomes unwindunwind

Nuclear membrane Nuclear membrane formsforms

Meiosis IIMeiosis II

Just like Mitosis!!Just like Mitosis!!

Meiosis IIMeiosis II– Prophase IIProphase II– Metaphase IIMetaphase II– Anaphase IIAnaphase II– Telophase IITelophase II

Prophase IIProphase II

Chromosomes appearChromosomes appear Centrioles migrateCentrioles migrate Spindle fibersSpindle fibers

Metaphase IIMetaphase II

Chromosomes line upChromosomes line up– Like mitosisLike mitosis

Anaphase IIAnaphase II

Chromosomes separate like in Chromosomes separate like in mitosismitosis

Move to ends of cellMove to ends of cell

Telophase IITelophase II

Nuclear membrane formsNuclear membrane forms Chromatids unwind Chromatids unwind Chromatin Chromatin

Finally…Cytokinesis!Finally…Cytokinesis!

ProductsProducts

From single diploid (2N)From single diploid (2N)

4 Haploid (1N)4 Haploid (1N)

Cellsalive!Cellsalive!

Meiosis: An Interactive Animation

Web Lesson Meiosis

Mitosis vs. MeiosisMitosis vs. Meiosis

GametesGametes

Why are some of the chromosomes Why are some of the chromosomes different colors?different colors?

Crossing Over!Crossing Over!

VariationVariation

Genetic Variation is provided by Genetic Variation is provided by meiosismeiosis

Because of assortment 70 trillion Because of assortment 70 trillion possibilitiespossibilities

Crossing Over increases that numberCrossing Over increases that number– Reason for great variety of peopleReason for great variety of people

Mistakes in MeiosisMistakes in Meiosis

NondisjunctionNondisjunction – the failure of homologous chromosomes the failure of homologous chromosomes

to separate properlyto separate properly

TrisomyTrisomy – 1 extra chromosome – 1 extra chromosome MonosomyMonosomy – 1 less chromosome – 1 less chromosome PolyploidyPolyploidy – extra set of all – extra set of all

chromosomeschromosomes

TrisomyTrisomy

Trisomy 21Trisomy 21 Down’s SyndromeDown’s Syndrome

– Flattened faceFlattened face

– Shorter than normalShorter than normal

– Heart defectsHeart defects

MonosomyMonosomy

Usually lethal but, Usually lethal but, but not in…but not in…

Turner’s SyndromeTurner’s Syndrome

Look at KaryotypeLook at Karyotype

What is wrong here?What is wrong here?

Turner’s SyndromeTurner’s Syndrome

PolyploidyPolyploidy

Whole extra set of chromosomesWhole extra set of chromosomes– (3n) or (4n) (3n) or (4n)

Death – animal zygotesDeath – animal zygotes

Often occurs in plantsOften occurs in plants– Bigger and HealthierBigger and Healthier

Phase? #1Phase? #1

QuestionQuestion

What separates during Meiosis I?What separates during Meiosis I?

– Homologous ChromosomesHomologous Chromosomes

QuestionQuestion

What type of cell results from What type of cell results from meiosis I?meiosis I?

– Haploid!Haploid!

QuestionQuestion

What separates during Meiosis II?What separates during Meiosis II?

– Sister ChromatidsSister Chromatids

Questions?Questions?