Meiosis is the process by which germ cells are produced with half the normal number of chromosomes. It occurs in two stages, Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and may exchange genetic material through crossing over. The homologous chromosomes then separate, reducing the chromosome number by half. Meiosis II then divides the remaining chromatids, resulting in four haploid daughter cells that are genetically unique from each other and the original cell. Meiosis ensures genetic variation in offspring and prevents organisms from accumulating extra chromosomes over successive generations.

1. MEIOSIS
Formation of gametes, division of the
sex cell (egg and sperm)
(Readapted from Slideshare):
1. Jay Swan
2. Karl Pointer
3. Mbrown

3. Chromosome Matching
 In humans, somatic cells (body cells) have:
• 23 pairs of homologous chromosomes
and
• one member of each pair from each
parent.
 The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic
composition.
 The other 22 pairs of chromosomes are
autosomes with the same size and genetic
composition.

5. • Homologous chromosomes are matched in:
• Similar length,
• Centromere (attaches sister chromatids together)
position
• gene locations (locus).
• A locus (plural, loci) is the position of a gene.
• Different versions or variations (alleles) of a
gene may be found at the same locus on
maternal and paternal chromosomes.

7. MEIOSIS
 The process to make cells with half the number of
chromosomes for sexual reproduction
 Usually humans and most animals and some plants have
diploid (2n) body cells. Meaning that they have two sets of
chromosomes (one from each parent)
 Meiosis occurs in our germ cells that produce gametes
(Sperm & egg)
• Meiosis results in four cells which are genetically different
from parent cell and from each other.
• The end products of Meiosis are 4 Haploid (n) cells

9.  Meiosis is a process that converts diploid
nuclei to haploid nuclei.
• Diploid cells have 2 sets of chromosomes.
• Haploid cells have 1 set of chromosomes.
• Meiosis occurs in the sex organs,
producing gametes—sperm and eggs.
 Fertilization is the fusion of a sperm and egg
cell.
 The zygote has a diploid chromosome
number, one set from each parent.

11. Why do we need
Meiosis?
It is the fundamental basis of sexual reproduction
Two haploid (n) gametes are brought together
through fertilization to form a diploid (2n) zygote
If egg and sperm had the same number of
chromosomes as other body cells then the
offspring would have too many chromosomes.

13. Meiosis must reduce the chromosome number by half
(n)
Fertilization then restores the 2n number

14. Summary of the Meiotic
process

15. MEIOSIS has two distinct
stages
MEIOSIS I consisting of 5
phases:
 Interphase I, Prophase I,
Metaphase I, Anaphase I,
Telophase I.
 MEIOSIS II consisting of 4 phases
 Prophase II, Metaphase II,
Anaphase II, Telophase II.

16. MEIOSIS 1:
Interphase
 Cell build up
energy
 DNA Replication
(to make
duplicated
chromosomes
 Cell doesn’t
change
structurally.

17. MEIOSIS 1:
Prophase 1

18. Prophase 1
in detail
 Events occurring in the nucleus:
• Chromosomes coil and become individual
chromosomes, nucleolus and nuclear envelope
disappear.
• Homologous chromosomes come together as pairs
by synapsis forming a tetrad (Each pair, with four
chromatids)
• Non-sister chromatids exchange genetic material
through the process of crossing over to ensure
genetic variation.
• Centrioli move to opposite poles with spindle fibers
between them.

19. PROPHASE 1
• Early prophase 1
 Homologous pair.
 Crossing over occurs.
Late Prophase 1
 Chromosomes condense.
 Spindle forms.
 Nuclear envelope
fragments.

20. Prophase 1: Crossing over
• Synapsis – the pairing of homologous
chromosomes
• Group of 4 chromatids
Homologous chromosomes
(each with sister chromatids)
Join to form a
TETRAD

21. Prophase 1: Crossing over
Homologous
chromosomes in
a tetrad cross
over each other
- Genes are
exchanged

22. Crossing over

23. Metaphase I
Spindle fibre
attached to a
kinetochore
Metaphase
plate
 Homologous pairs of
chromosomes align along
the equator of the cell
 The two chromosomes
attach to one spindle
fiber by means of the
kinetochore of the
centromere.

24. Anaphase 1
Spindle fibers contact.
Homologous chromosomes separate and
move to opposite poles.
Sister chromatids remain
attached at their centromeres.

25. Telophase 1
Nuclear envelopes reappear
Spindle fibres disappear.
Cytokinesis (when the cytoplasm divides)
divides cell into two.

26. Telophase 1 and Cytokinesis
• Duplicated chromosomes have
reached the poles.
• A nuclear envelope and
nucleolus re-forms around
chromosomes.
• Each nucleus now has the
haploid number of
chromosomes.
• Cell invaginates forming a
cleavage furrow, which extends
to for 2 separate haploid cells.

27. Meiosis ii: prophase II
• Chromosomes coil and
become compact (if
uncoiled after telophase
I).
• Nuclear envelope and
nucleolus, if re-formed,
dissappears again.
• Centrioli move to
opposite poles, forming
spindle fibers between
them.

28. Meiosis ii: metaphase II
• Individual duplicated
chromosomes align on the
equator.
• One chromosome per
spindle fiber attached by
means of kinetochore of
centromere.
• Centrioli has reached the
poles.

29. Anaphase 2
- Spindle fibers
contract.
- Duplicated
chromosomes
split in half
(centromere
dividing in 2)
Sister chromatids separate and
move to opposite poles.

30. Meiosis 2: Telophase 2
• Daughter chromosomes
has reached the poles.
• Two cells invaginate and
form 4 daughter haploid
cells (gametes)
• They uncoil and form
chromatin.
• Nuclear envelope and
nucleolus for around
chromatin again.
• Centrioli for centrosome.

31. MEOITIC DIVISION 2:
SUMMARY
Prophase II
Metaphase II
Anaphase II
Telophase II
and Cytokinesis

32. Results of Meiosis
 Gametes (egg & sperm) form
 Four haploid cells (n) with one
copy of each chromosome
 One allele of each gene
 Different combinations of alleles
for different genes along the
chromosome

34. When Chromosome number is
altered
 An extra copy of chromosome 21
causes Down syndrome or also known
as TRISOMY 21.
 A. Trisomy 21
• involves the inheritance of three
copies of chromosome 21 and
• is the most common human
chromosome abnormality.

36.  Trisomy 21 (Down Syndrome) produces a
characteristic set of symptoms, which include:
1. mental retardation,
characteristic facial features,
2. short stature,
3. heart defects,
4. susceptibility to respiratory infections,
leukemia, and Alzheimer’s disease, and
5. shortened life span.
 The incidence increases with the age of the
mother.

37. Bibliography
• Jackson, R., & Jackson, J. M. (2011). Henderson's
Dictionary of BIOLOGY (15th ed.). (E. Lawrence, Ed.)
(pp. 181-183). London: Pearson Education Limited.
• Livingstone, C. D., & Nobbe, M. E. (1998). The
Molecules of Life. Oxford: Biochemistry Department,
University of Oxford.
• Reece, J., Urry, L. A., Cane, M. L., Wasserman, S. A.,
Minrsky, P. V., & Jackson, R. B. (2011). Campbell Biology
(9th ed.). San Francisco: Pearson Benjamin Cummings.
• Urry, L. A., & Cane, M. L. (2011). The Molecular Basis of
Inheritance. In J. B. Reece, Campbell Biology (pp. 351370). San Francisco: Pearson Benjamin Cummings.