Ch.11.4 - Meiosis

Formation of Gametes

(egg & sperm)

Our Chromosomes•46 Chromosomes (23 pairs)• #1-22 Pairs are Autosomes (body cell chromosomes):

• Determine all traits except gender

• #23 pair are Sex chromosomes referred to as X & Y

• male (XY)• female (XX)

X - X - ChromosomeChromosome

Y-Y-ChromosomeChromosome

Chromosome NumbersVary in organisms

A closer look at Chromosome Pairs

KaryotypeKaryotype

Picture of chromosomes arranged by size

Pairs 1-22 = autosomesPair 23 = sex chromo

Detects abnormalities &XX female or XY male

Homologous Chromosomes (Buddy-Buddy)

• Matching pair = homologous chromosomes

• Homologous chromosomes: 2 chromosomes (one from mom and one from dad) that are alike in:• size, • location of centromere, • dark/light banding pattern of genes

Remember: only non matching pair are sex chromosomes

Homologous Chromosome Pairs

Mom’s Blue eye gene

Dad’s Brown eye gene

*Allele – different versions of the same gene (traits)*

Homologous Pair

• If a cell has all 46 chromo (23 pairs), it’s called a diploid cell• Shorthand: 2N

Somatic Cell Gamete • Body Cell• Skin cell, etc.

• Mitosis

• 2 sets of DNA• 46 total chromo• 1 set from each parent

• DIPLOID (2N)

• Sex Cell• Egg/sperm

• Meiosis

• 1 set of DNA• 23 total chromo• ½ set from each parent

• HAPLOID (N)

Important Vocab• Somatic Cell vs. Gamete• Body cell vs. Sex cell

• Diploid (2N) vs. Haploid (1N)• 2 sets of DNA vs. 1 set of DNA

• Homologous Chromosomes• Matching pairs of chromo in 2N cell

• Alleles• Different version of the same trait

• Fertilization• When sperm meets egg and combines DNA

• Zygote• Cell in growth state following fertilization

• Tetrad• Structure containing 4 chromatids

Meiosis Info…Similar but different from Mitosis:

1. Sex cell division only2. Involves 2 cell divisions3. Results in 4 cells with half the normal

genetic info• Produces gametes (egg/sperm)• Male Testes (spermatogenesis)• Female Ovaries (oogenesis)

Why Do we Need Why Do we Need Meiosis?Meiosis?

Basis of sexual reproductionBasis of sexual reproduction• Accounts for individual genetic diversity• You are unique! You look a little bit like your mom

and a little like your dad!Two haploid (1n) gametes are brought Two haploid (1n) gametes are brought

together through fertilization to form a together through fertilization to form a diploid (2n) zygote (fertilized egg) diploid (2n) zygote (fertilized egg)

1N

1N

2N

Fertilization

HereHere’’s the key to your s the key to your ““uniquenessuniqueness””

Homologous PairHomologous Pair same genes, same genes, different allelesdifferent alleles

SisterSisterChromatidsChromatids(same genes,(same genes,same alleles)same alleles)

Gene XGene X

Homologous pairs separate in meiosis and Homologous pairs separate in meiosis and therefore different alleles (versions of therefore different alleles (versions of

traits) separate. So many combos of traits traits) separate. So many combos of traits are possible!are possible!

Meiosis Forms Haploid Meiosis Forms Haploid GametesGametesMeiosis must reduce the chromosome # by half

Fertilization then restores the 2n number 23 chromo from egg + 23 chromo from sperm = you 46!

from mom from dad child

meiosis reducesgenetic content

toomuch!

The right number!

EGG

EGG

SPERM

SPERM YOU

YOU

Meiosis: 2-Part Cell DivisionMeiosis: 2-Part Cell Division

HomologHomologousousPairPairseparateseparate

Sister Sister chromatidschromatidsseparateseparate

Diploid (46)Diploid (46)

MeiosisMeiosisII

MeiosMeiosisisIIII

Diploid Diploid (2x46 = (2x46 = 92)92) HaploiHaploi

d (23)d (23)1 replication of chromosomes is followed by 2 cell divisions (aka Interphase only happens once!)

46Interphase

Meiosis: Reduction Division

• 2 part cell division• Meiosis I• Interphase I• PMAT I

• Meiosis II• PMAT II

• End result: 4 genetically different haploid cells• 4 sperm or 1 egg

Meiosis I

Meiosis II

Interphase

Meiosis I: Reduction DivisionMeiosis I: Reduction Division

NucleusNucleus SpindleSpindlefibersfibers NuclearNuclear

envelopeenvelopeEarly Early Prophase IProphase I(Chromoso(Chromosome number me number

doubled)doubled)

Late Late ProphasProphas

e Ie I

MetaphaMetaphase Ise I AnaphasAnaphas

e Ie ITelophase Telophase I (diploid)I (diploid)

Chromo Chromo pair uppair up

Meiosis II: Reducing Chromo #Meiosis II: Reducing Chromo #

Prophase Prophase IIII

MetaphaMetaphase IIse II

AnaphasAnaphase IIe II

TelophasTelophase IIe II 4 4

genetically genetically different different haploid haploid

cellscells

Interphase I

• SAME as MITOSIS• Chromosomes will double•G1, S, G2

Prophase IProphase I •Homologs pair up and Homologs pair up and form form tetradtetrad (a pair of (a pair of homologous homologous chromosomeschromosomes

Chromosomes Chromosomes condense.condense.Spindle forms.Spindle forms.Nuclear envelope Nuclear envelope disappears.disappears.Crossing overCrossing over occurs occurs

Tetrads Form in Prophase Tetrads Form in Prophase II

Homologous Homologous chromosomeschromosomes

Pair upPair up(each with sister chromatids) (each with sister chromatids)

  

Join to form a Join to form a TETRADTETRAD

Called SynapsisCalled Synapsis

Crossing-Over occurs in Crossing-Over occurs in Prophase IProphase I

Tetrad FormsTetrad Forms

DefinitionDefinition: Pieces : Pieces of chromosomes of chromosomes or genes are or genes are exchangedexchanged

Advantage of Advantage of sexual sexual reproduction = reproduction = genetic variation!genetic variation!

It’s hard to predict what traits you’ll get from mom and dad because there is so many possible combinations!

**THIS IS ONE SOURCE OF GENETIC VARIABILITY!**

Crossing-OverCrossing-Over

Genetic Variability is due to…

1. Crossing over

2. Independent Assortment

3. Random Segregation

4. Random Fertilization

Meaning: You are unique for these 4 reasons!

• Independent Assortment

• The way a pair of chromosomes lines up during metaphase is not dependent on other pairs.

• Aka Mom’s chromosomes don’t necessarily all line up on one side

•Random Segregation

• Random chromatids separate into the newly forming eggs/sperm

• Aka if you inherit mom’s hair color, you might get her brown hair trait OR her blonde hair trait

Independent Independent AssortmentAssortment

Random Random SegregationSegregation

Metaphase I and Anaphase Metaphase I and Anaphase IIMetaphase IMetaphase I

Homologous pairs Homologous pairs of chromosomes of chromosomes align along the align along the equatorequator

Anaphase IAnaphase I-Homologs separate -Homologs separate and move to opposite and move to opposite poles.poles.-Sister chromatids remain -Sister chromatids remain attached at their attached at their centromerescentromeres.

Telophase I and Telophase I and CytokinesisCytokinesis

Nuclear envelopes Nuclear envelopes reassemble.reassemble.

Spindle disappears.Spindle disappears.

Cytokinesis Cytokinesis divides cell divides cell into two new diploid into two new diploid cellscells..

Meiosis II-occurs in 2 cellsMeiosis II-occurs in 2 cells

Only one homolog of each Only one homolog of each chromosome is present in the chromosome is present in the cellcell.

Meiosis II produces gametes Meiosis II produces gametes with with

one copy of each one copy of each chromosome/gene.chromosome/gene.

Sister chromatids carry Sister chromatids carry identical genetic identical genetic

informationinformation. .

Gene XGene X

Meiosis II: Reducing Chromo #Meiosis II: Reducing Chromo #

Prophase Prophase IIII

MetaphaMetaphase IIse II

AnaphasAnaphase IIe II

TelophasTelophase IIe II 4 4

genetically genetically different different haploid haploid

cellscells

• Prophase II• Nuclear envelope disappears• Spindle fibers form• No Crossing Over

Metaphase II Chromosomes align

along the equator

Anaphase IIAnaphase II

Sister Sister chromatidschromatids separate and separate and move to move to opposite polesopposite poles.

EquatorEquator

PolePole

Telophase IITelophase II

Nuclear envelope Nuclear envelope reforms.reforms.

Chromosomes Chromosomes loosen into loosen into chromatin.chromatin.

Spindle Spindle breakdown.breakdown.

Cytokinesis Cytokinesis breaks the cells breaks the cells into 2 new into 2 new daughter cellsdaughter cells

Results of MeiosisResults of Meiosis

GametesGametes (egg & sperm) (egg & sperm) formform

Four haploid cellsFour haploid cells with with one copy of each one copy of each chromosome chromosome

One allele of each geneOne allele of each gene

Different combinations Different combinations of alleles for different of alleles for different genes along the genes along the chromosomechromosome

Prophase I

Metaphase I

Anaphase I

Telophase I

Cytokinesis

Prophase II

Metaphase II

Anaphase II

Telophase II

Cytokinesis

Meiosis AnimationMeiosis Animation

Overview of MeiosisOverview of Meiosis

THE PROCESS OF MAKING THE PROCESS OF MAKING

EGG AND SPERMEGG AND SPERM

Oogenesis & Oogenesis & SpermatogenesisSpermatogenesis

SpermatogenesisSpermatogenesis““Creation of spermCreation of sperm””TestesTestes

2 divisions produce 2 divisions produce

4 viable haploid 4 viable haploid spermatidsspermatids

Spermatids mature into Spermatids mature into spermsperm

Men produce about Men produce about 250,000,000 sperm per 250,000,000 sperm per dayday

Spermatogenesis in the TestesSpermatogenesis in the Testes

SpermatiSpermatidd

SpermatogenesisSpermatogenesis

OogenesisOogenesis

““Creation of EggsCreation of Eggs””OvaryOvary

2 divisions produce 2 divisions produce 3 polar bodies3 polar bodies that die + that die + 1 viable egg1 viable egg

Polar bodies die because of Polar bodies die because of unequal unequal division of cytoplasmdivision of cytoplasm

Starting at puberty, if unfertilized, Starting at puberty, if unfertilized, one immature oocyte matures into an one immature oocyte matures into an ovum (egg)ovum (egg) every 28 days every 28 days MENSTRUAL CYCLEMENSTRUAL CYCLE

Oogenesis in the Oogenesis in the OvariesOvaries

**Egg cells are special…only one egg is **Egg cells are special…only one egg is made every time meiosis occurs; the other 3 made every time meiosis occurs; the other 3 cells (polar bodies) that are made are much cells (polar bodies) that are made are much smaller & are discarded (Remember: the smaller & are discarded (Remember: the egg cell is the largest cell youegg cell is the largest cell you’’ll come ll come across)across)

Oogenesis

OogoniumOogonium(diploid)(diploid)

MitosisMitosis

PrimaryPrimaryoocyteoocyte(diploid)(diploid)

Meiosis IMeiosis I

SecondarySecondaryoocyteoocyte(haploid)(haploid)

Meiosis IIMeiosis II

11stst polar body polar bodymay divide may divide (haploid)(haploid)

PolarPolarbodiesbodiesdiedie

OocyteOocyte

polar bodypolar body(dies)(dies)

a

A

X

X

a

X

A X

a

X

a

X

MatureMatureEgg (ovum)Egg (ovum)

A

X

A

X

Meiosis is IMPORTANT… Genetic Variability

1. Independent assortment (late pro/early meta I&II)

-chromosomes line up randomly2. Law of Segregation

(Late meta/ana I&II)-spindle fibers attach randomly to chromosomes and

separate.3. Crossing-over

(Pro I) pieces of sister chromatids are switched

4. Random fertilization Random sperm + random egg are combined

What are the possibilities?

***Total possible chromosome combinations due to independent assortment = 2n [for humans = 223 = 8,388,608]

***Total possible chromosomally different zygotes due to fertilization = (223)2 = 70,368,744,000,000

***Possible genetically different zygotes per couple if crossing-over occurs only once = (423)2 = 4,951,760,200,000,000,000,000,000,000

--advantageous b/c variability needed for evolution

MitosisMitosis MeiosisMeiosis

# of divisions# of divisions 1122

Number of Number of daughter cellsdaughter cells 22 44

Genetically Genetically identical?identical? YesYes NoNo

Chromosome #Chromosome # Same as parentSame as parent Half of parentHalf of parent

WhereWhere Somatic cellsSomatic cells Gamete cellsGamete cells

WhenWhen Throughout lifeThroughout life At sexual maturityAt sexual maturity

RoleRole Growth and Growth and repairrepair

Sexual Sexual reproductionreproduction

Comparison of DivisionsComparison of Divisions

92Single

Chromosomes(46 Pairs)

46 Single Chromosomes

(23 Pairs)

46 Single Chromosomes

(23 Pairs)

46Single

Chromosomes(23 Pairs)

92Single

Chromosomes(46 Pairs)

46Single

Chromosomes

(23 Pairs)

46Single

Chromosomes

(23 Pairs)

23 singles

23 singles

23 singles

23 singles

46 Single Chromosomes

(23 Pairs)

WhatWhat’’s the difference between Mitosis & s the difference between Mitosis & Meiosis ?Meiosis ?