Meiosis reduces the chromosome number by half to produce gametes for sexual reproduction. It involves two cell divisions. In the first division, homologous chromosome pairs separate, reducing the number by half. Crossing over and random assortment during meiosis increases genetic variation. Fusion of male and female gametes through fertilization combines the genetic material of the two parents, maximizing genetic diversity in offspring. Errors in meiosis can result in chromosomal abnormalities like Down syndrome. Methods to obtain fetal cells for analysis include amniocentesis and chorionic villus sampling.

1. 3.3 Meiosis
Essential Question: Alleles segregate during meiosis allowing
new combinations to be formed by the fusion of gametes.
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Grandmother Granddaughter

2. Understandings
Statement Guidance
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
3.3U2
The halving of the chromosome number allows a sexual life cycle with
fusion of gametes.
3.3.U3
DNA is replicated before meiosis so that all chromosomes consist of two
sister chromatids.
3.3.U4
The early stages of meiosis involve pairing of homologous chromosomes
and crossing over followed by condensation. [The process of chiasmata
formation need not be explained.]
3.3.U5
Orientation of pairs of homologous chromosomes prior to separation is
random.
3.3 U6
Separation of pairs of homologous chromosomes in the first division of
meiosis halves the chromosome number.
3.3 U7 Crossing over and random orientation promotes genetic variation.
3.3 U8 Fusion of gametes from different parents promotes genetic variation.

3. Applications and Skills
Statement Guidance
3.3 A1
Non-disjunction can cause Down syndrome and other chromosome
abnormalities.
3.3 A2 Studies showing age of parents influences chances of non-disjunction.
3.3 A3
Description of methods used to obtain cells for karyotype analysis e.g.
chorionic villus sampling and amniocentesis and the associated risks.
3.3 S1
Drawing diagrams to show the stages of meiosis resulting in the formation
of four haploid cells. [Drawings of the stages of meiosis do not need to
include chiasmata. Preparation of microscope slides showing meiosis is
challenging and permanent slides should be available in case no cells in
meiosis are visible in temporary mounts.]

4. 3.3 U1 One diploid nucleus divides by meiosis to
produce four haploid nuclei.
http://cis.payap.ac.th/wp-content/uploads/2011/11/142.png
• Cells divide twice
• Result: 4 daughter cells, each
with half as many chromosomes
as parent cell

5. • What if a complex multicellular organism (like
us) wants to reproduce?
– joining of egg + sperm
• Do we make egg & sperm by mitosis?
46 46+ 92
egg sperm zygote
What if we did, then….
Doesn’t work!
No!
3.3 U.2 The halving of the chromosome number allows a
sexual life cycle with fusion of gametes.

6. 3.3 U.2 The halving of the chromosome number allows a
sexual life cycle with fusion of gametes.
http://lh6.ggpht.com/n2T0xB_8BXE/UY4bhlV52uI/AAAAAAAABxg/7ZjWF
QMBfx4/s1600-h/Humanlifecycle9.jpg
Sexual life cycle
• Meiosis: reduces #
of chromosomes
creating gametes (n)
• Fertilization:
combine gametes
(sperm + egg)
creating a fertilized
egg or zygote (2n)

7. Duplication of Chromosomes During S stage of Interphase
Single chromosome
(called chromatin
during Interphase)
Two identical sister
chromatids joined by
a centromere
DNA replication during
S stage of Interphase
3.3 U.3 DNA is replicated before meiosis so that all chromosomes
consist of two sister chromatids.
.

8. Crossover
• Red and Blue chromosomes are a homologous pair.
• They have replicated during the interphase and each copy is
held together by the centromere (black dot)
3.3 U.4 The early stages of meiosis involve pairing of homologous chromosomes and
crossing over followed by condensation. [The process of chiasmata formation need not
be explained.]

9. •The chromosomes overlap and exchange lengths of DNA.
•The chiasmata are the points at which the red and blue chromosomes
overlap.
•At a molecular level the exchange has to be exact avoiding loss of
bases. Such an error would have profound effect on the genes.
3.3 U.4 The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation. [The process
of chiasmata formation need not be explained.]

10. • At anaphase I the homologous pairs are separated.
• Notice that sections of DNA have been exchanged.
• These will mean that the alleles on these section have also been
exchanged
• Crossing over, Independent Assortment, combined with Fertilization:
A couple can produce over 64 trillion (8.3 million x 8.3 million)
different combinations in a zygotes during fertilization
3.3 U.4 The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation. [The process
of chiasmata formation need not be explained.]

11. 3.3 U.5 Orientation of pairs of homologous chromosomes
prior to separation is random.
http://cnx.org/resources/9d7cd91abd65e9c7fa857cf6c6d133ec/Figure_08_03_06.jpg
Independent assortment of chromosomes
• meiosis introduces genetic variation in several ways
• gametes of offspring do not have same combination
of genes as gametes from parents

12. 3.3 U.6 Separation of pairs of homologous chromosomes in the first
division of meiosis halves the chromosome number.
• Each individual has a pair
chromosomes both
chromosomes of a pair carry
“matching” genes control same
inherited characters
homologous = same
information
• To create sex cells the number
of chromosomes must be
reduce for Humans from 46
chromosomes  to 23.
Reduced by half.
http://41.media.tumblr.com/4c08ea694bcf30c5a9d8423
187fba2de/tumblr_mx5zd49kWN1qjofuoo1_1280.jpg

13. 3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.
http://upload.wikimedia.org/wikipedia/commons/5/54/Meiosis_diagram.jpg

14. Meiosis 1
• interphase
• prophase 1
• metaphase 1
• anaphase 1
• telophase 1
Meiosis 2
• prophase 2
• metaphase 2
• anaphase 2
• telophase 2
2nd division of meiosis
separates sister
chromatids
(1n  1n)
* just like mitosis *
1st division of
meiosis separates
homologous pairs
(2n  1n)
“reduction division”
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

15. Meiosis: Generates haploid gametes
• Reduces the number of chromosomes by half.
• Also produces genetic variability, each gamete is different,
ensuring that two offspring from the same parents are
never identical.
• Two divisions: Meiosis I and meiosis II. Chromosomes are
duplicated in interphase prior to Meiosis I.
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

16. Meiosis I: Separation of Homologous Chromosomes
1. Prophase I:
• Chromatin condenses into chromosomes.
• Nuclear membrane disappear.
• Centrosomes move to opposite poles of cell and
microtubules attach to chromatids.
• Synapsis: Homologous chromosomes pair up and
form a tetrad of 4 sister chromatids.
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

17. Prophase I: Crossing Over Between Homologous Chromosomes
Crossing over combined with Fertilization: A couple can produce over 64
trillion (8.3 million x 8.3 million) different combinations in a zygotes during
fertilization.
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

18. Meiosis I:
2. Metaphase I:
3. Anaphase I:
4. Telophase I and Cytokinesis:
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

19. Random Assortment of Homologous Chromosomes
During Meiosis I Generates Many Possible Gametes

20. Meiosis II: Separation of Sister Chromatids
During interphase that follows meiosis I, no DNA replication
occurs.
Interphase may be very brief or absent.
Meiosis II is very similar to mitosis.
1. Prophase II:
• Very brief, chromosomes reform.
• No crossing over or synapsis.
• Spindle forms and starts to move chromosomes towards
center of the cell.
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

21. Meiosis II: Separation of Sister Chromatids
2. Metaphase II:
• Very brief, individual chromosomes line up in the middle
of the cell.
3. Anaphase II:
• Chromatids separate and move towards opposite ends of
the cell.
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

22. Meiosis II: Separation of Sister Chromatids
4. Telophase II:
• Nuclei form at opposite ends of the
cell.
• Cytokinesis occurs.
Product of meiosis:
Four (4) haploid gametes, each
genetically different from the other.
3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.

23. • Independent assortment of chromosomes
– meiosis introduces genetic variation
– gametes of offspring do not have same combination of
genes as gametes from parents
• random assortment in humans produces
223 (8,388,608) different combinations in gametes
from Dadfrom Mom offspring
new gametes
made by offspring
3.3 U.7 Crossing over and random orientation promotes
genetic variation.

24. Fertilization Sources of Genetic Variation
– Human Sperm + Human Egg come together to produce
8 million X 8 million = 64 trillion combinations!
3.3 U8 Fusion of gametes from different parents promotes
genetic variation.
http://news.bbcimg.co.uk/media/images/63005000/jpg/_63005
026_p6480029-sperm_fertilizing_egg-spl.jpg
http://worms.zoology.wisc.edu/dd2/echino/fert/files/page19_2.gif

25. • Sperm + Egg = ?
– any 2 parents will produce a zygote with over 70
trillion (223 x 223) possible diploid combinations
3.3 U8 Fusion of gametes from different parents promotes
genetic variation.
http://worms.zoology.wisc.edu/dd2/e
chino/fert/files/page19_2.gif

26. Sexual reproduction allows us to maintain both genetic
similarity & differences.
Baldwin brothers
Jonas
Brothers
3.3 U8 Fusion of gametes from different parents promotes
genetic variation.
http://upload.wikimedia.org/wikipedia/commons/f/ff/Jonas_Brothers_2009.jpg
Martin & Charlie Sheen, Emilio Estevez

27. 3.3 A.1 Non-disjunction can cause Down syndrome and
other chromosome abnormalities.
Accidents During Meiosis Can Cause Chromosomal Abnormalities
– Nondisjunction: Chromosomes fail to separate.
– Gametes (and zygotes) will have an extra chromosome, others will be
missing a chromosome.
• Trisomy: Individuals with one extra chromosome, three instead of
pair. Have 47 chromosomes in cells.
• Monosomy: Missing a chromosome, one instead of pair. Have 45
chromosomes in cells.
http://upload.wikimedia.org/wikipedia/commons/3/31/Mitotic_nondisjunction.png

28. 3.3 A.2 Studies showing age of parents influences chances of
non-disjunction.
http://knowgenetics.org/wp-content/uploads/2012/12/DownSyndromeRisk.png
• Weakening of
cohesive ties holding
together
chromosomes in sex
cells may contribute
to maternal age-
related errors.
• The loss of cohesion
may contribute to
incorrect microtubule
attachment during
meiotic divisions

29. 3.3 A.3 Description of methods used to obtain cells for karyotype
analysis e.g. chorionic villus sampling and amniocentesis and the
associated risks.
http://www.hopkinsmedicine.org/healthlibrary/GetImage.aspx?ImageId=161385
• Amniocentesis medical
procedure used in prenatal
diagnosis of chromosomal
abnormalities and also used
for sex determination.
• A small amount of amniotic
fluid, which contains fetal
tissues, is sampled from
the amniotic
sac surrounding a
developing fetus, and the
fetal DNA is examined for
genetic abnormalities.