Post on 13-Jan-2016
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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 ?