MEIOSIS

2. Meiosis – A Source of DistinctionMeiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the
answers lie in
meiosis.

3. Meiosis does two things -Meiosis does two things -
1) Meiosis takes a cell with1) Meiosis takes a cell with two copiestwo copies of everyof every
chromosome (diploid) and makes cells with achromosome (diploid) and makes cells with a
single copysingle copy of every chromosome (haploid).of every chromosome (haploid).
This is a good idea if you’re going to combineThis is a good idea if you’re going to combine
two cells to make a new organism. This tricktwo cells to make a new organism. This trick
is accomplished byis accomplished by halvinghalving chromosomechromosome
number.number.
In meiosis,In meiosis, one diploid cells produces fourone diploid cells produces four
haploid cells.haploid cells.

4. Why do we need meiosis?Why do we need meiosis?
Meiosis is necessary to halve the numberMeiosis is necessary to halve the number
of chromosomes going into the sex cellsof chromosomes going into the sex cells
Why halve the chromosomes in gametes?Why halve the chromosomes in gametes?
At fertilization the male and female sexAt fertilization the male and female sex
cells will providecells will provide ½ of the chromosomes½ of the chromosomes
each – so the offspring has genes fromeach – so the offspring has genes from
both parentsboth parents

5. 2) Meiosis2) Meiosis scramblesscrambles the specific forms ofthe specific forms of
each gene that each sex cell (egg or sperm)each gene that each sex cell (egg or sperm)
receives.receives.
This makes for a lot ofThis makes for a lot of genetic diversitygenetic diversity. This. This
trick is accomplished throughtrick is accomplished through independentindependent
assortmentassortment andand crossing-overcrossing-over..
Genetic diversity is important for theGenetic diversity is important for the evolutionevolution
of populations and speciesof populations and species..

6. MeiosisMeiosis
Parent cell –
chromosome pair
Chromosomes
copied
1st
division - pairs split
2nd
division – produces
4 gamete cells with ½
the original no. of
chromosomes

7. Meiosis – mouse testesMeiosis – mouse testes
Parent cell
4 gametes
1st
division
2nd
division

8. The Stages of Meiosis:
aka: Reduction Division

9. Meiosis I : SeparatesMeiosis I : Separates
Homologous ChromosomesHomologous Chromosomes
InterphaseInterphase
Each of the chromosomes replicateEach of the chromosomes replicate
The result is two genetically identicalThe result is two genetically identical
sister chromatids which remainsister chromatids which remain
attached at their centromeresattached at their centromeres

10. Prophase IProphase I
This is a crucial phase for mitosis.This is a crucial phase for mitosis.
During this phase each pair of chromatidsDuring this phase each pair of chromatids
don’t move to the equator alone, theydon’t move to the equator alone, they
match up with their homologous pair andmatch up with their homologous pair and
fasten together (synapsis) in a group offasten together (synapsis) in a group of
four called a tetrad.four called a tetrad.
Extremely IMPORTANT!!! It is during thisExtremely IMPORTANT!!! It is during this
phase that crossing over can occur.phase that crossing over can occur.
Crossing Over is the exchange ofCrossing Over is the exchange of
segments during synapsis.segments during synapsis.

11. Metaphase IMetaphase I
The chromosomes line up at theThe chromosomes line up at the
equator attached by theirequator attached by their
centromeres to spindle fibers fromcentromeres to spindle fibers from
centrioles.centrioles.
Still in homologous pairsStill in homologous pairs

12. Anaphase IAnaphase I
The spindle guides the movementThe spindle guides the movement
of the chromosomes toward theof the chromosomes toward the
polespoles
Sister chromatids remain attachedSister chromatids remain attached
Move as a unit towards the same poleMove as a unit towards the same pole
The homologous chromosomeThe homologous chromosome
moves toward the opposite polemoves toward the opposite pole
Contrasts mitosis – chromosomesContrasts mitosis – chromosomes
appear as individuals instead of pairsappear as individuals instead of pairs
(meiosis)(meiosis)

13. Telophase ITelophase I
This is the end of the first meiotic cellThis is the end of the first meiotic cell
division.division.
The cytoplasm divides, forming twoThe cytoplasm divides, forming two
new daughter cells.new daughter cells.
Each of the newly formed cells hasEach of the newly formed cells has
half the number of the parent cell’shalf the number of the parent cell’s
chromosomes, but each chromosomechromosomes, but each chromosome
is already replicated ready for theis already replicated ready for the
second meiotic cell divisionsecond meiotic cell division

14. CytokinesisCytokinesis
Occurs simultaneously withOccurs simultaneously with
telophase Itelophase I
Forms 2 daughter cellsForms 2 daughter cells
Plant cells – cell platePlant cells – cell plate
Animal cells – cleavage furrowsAnimal cells – cleavage furrows
NO FURTHER REPLICATION OFNO FURTHER REPLICATION OF
GENETIC MATERIAL PRIOR TOGENETIC MATERIAL PRIOR TO
THE SECOND DIVISION OFTHE SECOND DIVISION OF
MEIOSISMEIOSIS

15. Figure 13.7 The stages of meiotic cell division: Meiosis I

16. Meiosis II :Meiosis II :
Separates sister chromatidsSeparates sister chromatids
Proceeds similar to mitosisProceeds similar to mitosis
THERE IS NO INTERPHASE II !THERE IS NO INTERPHASE II !

17. Prophase IIProphase II
Each of the daughter cells forms aEach of the daughter cells forms a
spindle, and the double strandedspindle, and the double stranded
chromosomes move toward thechromosomes move toward the
equatorequator

18. Metaphase IIMetaphase II
The chromosomes are positioned onThe chromosomes are positioned on
the metaphase plate in a mitosis-likethe metaphase plate in a mitosis-like
fashionfashion

19. Anaphase IIAnaphase II
The centromeres of sister chromatidsThe centromeres of sister chromatids
finally separatefinally separate
The sister chromatids of each pairThe sister chromatids of each pair
move toward opposite polesmove toward opposite poles
Now individual chromosomesNow individual chromosomes

20. Telophase II and CytokinesisTelophase II and Cytokinesis
Nuclei form at opposite poles of theNuclei form at opposite poles of the
cell and cytokinesis occurscell and cytokinesis occurs
After completion of cytokinesis thereAfter completion of cytokinesis there
are four daughter cellsare four daughter cells
All are haploid (n)All are haploid (n)

21. Figure 13.7 The stages of meiotic cell division: Meiosis II

22. One Way Meiosis Makes Lots ofOne Way Meiosis Makes Lots of
Different Sex Cells (Gametes) –Different Sex Cells (Gametes) –
Independent AssortmentIndependent Assortment
Independent assortment produces 2Independent assortment produces 2nn
distinct gametes, where n = the numberdistinct gametes, where n = the number
of unique chromosomes.of unique chromosomes.
That’s a lot of diversity by thisThat’s a lot of diversity by this
mechanism alone.mechanism alone.
In humans, n = 23 and 2In humans, n = 23 and 22323
= 6,000,0000.= 6,000,0000.

24. Another Way Meiosis Makes Lots of DifferentAnother Way Meiosis Makes Lots of Different
Sex Cells – Crossing-OverSex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gameteCrossing-over multiplies the already huge number of different gamete
types produced by independent assortment.types produced by independent assortment.

26. The Key Difference Between Mitosis and Meiosis isThe Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align inthe Way Chromosomes Uniquely Pair and Align in
MeiosisMeiosis
MitosisMitosis The first (andThe first (and
distinguishing)distinguishing)
division of meiosisdivision of meiosis

28. Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”
X chromosomeX chromosome
Y chromosomeY chromosome

29. Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

30. Meiosis – division errorMeiosis – division error
Chromosome pair

31. Meiosis error - fertilizationMeiosis error - fertilization
Should the gamete with theShould the gamete with the
chromosome pair be fertilizedchromosome pair be fertilized
then the offspring will not bethen the offspring will not be
‘normal’.‘normal’.
In humans this often occursIn humans this often occurs
with the 21with the 21stst
pair – producingpair – producing
a child with Downs Syndromea child with Downs Syndrome

32. 21 trisomy – Downs Syndrome21 trisomy – Downs Syndrome
Can you see the
extra 21st
chromosome?
Is this person male
or female?

33. REFERENCES
 Mauri , S. (2010). Meiosis.
http://www.slideshare.net/SergioMauri98/presentacin-mitosis-y-meiosis-31090980?qid=4359ea49-4b39-4171-b4b4-65faee1
Accessed: on 2014 March 07
 Swan, J.(2011). Meiosis and sexual life cycle.
http://www.slideshare.net/jayswan/chapter-13-presentation-6504581
Accessed: on 2014 March 07