Meiosis is a type of cell division that produces gametes, such as sperm and egg cells, with half the normal number of chromosomes. It involves two rounds of division called Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and crossover can occur, generating genetic diversity. The paired chromosomes then separate, reducing the number of chromosomes by half. In Meiosis II, the sister chromatids separate, resulting in four haploid cells each containing a single set of chromosomes. This ensures fertilization restores the diploid number. Errors in meiosis can result in conditions like Down syndrome due to an extra chromosome 21.

1. Meiosis
NEREA LÓPEZ PEIRONA 4ºB

2. Significance of Meiosis
(1) Crossing over in bivalent
- Produces new combinations of
genes in both chromosomes

3. Chiasma and Crossing
Over

4. Significance of Meiosis
(2) Reduction and fusion of gametes
- Meiosis produces haploid gametes
- In sexual reproduction, a male
gamete fertilizes a haploid female
gamete to produce a normal
diploid zygote

5. Significance in Mitosis
(3) Independent (random assortment)
- During metaphase I, homologous
pairs of chromosomes align at the
equator
- It is by chance which “way round”
each pair lies, before these
homologous pairs of chromosomes
separate into two different
daughter cells.

6. e

7. Stages of Meiosis
 Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
• Meiosis II
Prophase II
Metaphase
II
Anaphase II
Telophase II

8. Interphase
 Interphase is an important stage preceding
meiosis. Without this stage meiosis would
not occur.
 During this stage, each individual
chromatid replicates, similar to mitosis.
 B replicates B and b replicates b
 At this stage, the chromosomes are long
and stringy and are not visible.
 **Remember: All somatic cells are diploid in
number (2n), therefore for each chromatid
there also exists its homolog, which also
replicates during interphase.

9. Early prophase I
During early prophase it is the same as in mitosis
Chromosomes
Nuclear
envelope
Spindle pole

10. Prophase I
Now, during prophase, also
occurs the crossing over, where
the homologous chromosomes
exchange information.
Spindle fiber

11. Metaphase I
The chromosomes align in the
middle

12. Telophase I
Each chromosome
separates from the
other, making two
news.

13. Meiosis I
Flowchart

14. After telophase I
the second meiotic division occurs
(without DNA duplication before),
as a final result we have 4 haploid
cells)

15. Prophase II
 The nuclear membranes of the
daughter cells disintegrate
again. The spindle fibres re-
form in each daughter cell

16. Metaphase II
 The chromosomes, each still
made up of sister chromatids, are
positioned randomly on the
metaphase plate with the sister
chromatids of each chromosome
pointing towards the opposite
poles.

17. Metaphase II
 Each sister chromatid is attached
to the spindle fibres at the
centromere

18. Anaphase II
 The centromeres of the sister
chromatids finally separate, and
the sister chromatids of each
chromosome are now individual
chromosomes.
 The chromosomes move towards
the opposite poles of the cell.

19. Telophase II
 Finally, the nucleoli and
nuclear membranes re-form.
The spindle fibres break
down.
 Cytokinesis follows and four
haploid daughter cells are
formed, each containing half
the number of chromosomes
and is genetically different
from the parent diploid cell.

20. Telophase II
 These haploid cells will develop
into gametes.

22.  Males: all 4 hapoid cells become
sperm
Females: in oogenesis, only 1 of the
haploid cells becomes an egg, and
the other 3 are reabsorbed by the
body.

24. Human genetics = 46
chromosome (2n)
23 pairs of chromosome

25. Mutation
 Mutation is a change in structure,
arrangement or quantity of the DNA in
the chromosome
 May be caused by:
 Mistakes in the replication of DNA
 Damage to the DNA by radioactive
and carcinogenic substance
 Disruption to the orderly movement of
chromosomes during cell division

26. In Mitosis
 If the functions of these genes are
disrupted due to mutation, cancers
may form.
 Somatic mutations are not
transmitted to the offspring, but
may cause body cells to
malfunction
 Cancers are caused by somatic
mutation

27. In Meiosis
 Meiosis involves an orderly movement
and reduction (in meiosis I) of a diploid
cell to two haploid cells that subsequently
divide (in meiosis II) to form four haploid
gametes
 Since these are gametes, so any mistakes
– caused by disorderly movement of
chromosomes during meiosis --- are
inherited by the offspring.

28. Example: non-disjunction or
improper segregation
(separation) of chromosome
 During anaphase I, certain
homologous chromosomes fail to
segregate, resulting in the
production of gametes with either
an extra chromosome (n+1) or a
missing chromosome (n-1)
 If this abnormal gametes unites with
a normal gamete, an abnormal
zygote will be produced.

30. Down’s syndrome
 3 copies of chromosomes
number 21, instead of the
normal 2 chromosomes
 This means a down
syndrome patient has (2n+1
= 47) 47 chromosomes
instead of the normal
(2n=46) chromosomes

31. Trisomy 21/ Down Syndrome

32. References
 TRabajo de clase para Biología MEC-BC. IES Pedro de Luna- 2011-
Zaragoza SPAIN
 by imkaelah on Jul 24, 2013
 http://www.slideshare.net/imkaelah/meiosis-lesson?qid=95dae3f0-
5bd5-4b9a-9ffa-9f69c1d878a2&v=default&b=&from_search=1
 Rebecca Choong
 http://www.slideshare.net/rchoong/mitosis-meiosis-lesson-
3?qid=5905ba13-12a1-4b46-ae5f-
969f78f84434&v=default&b=&from_search=3
 by Anesh Jeyakumar on Oct 31, 2011
 Slides abducte.d from slideshare.