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Meiosis
NEREA LÓPEZ PEIRONA 4ºB
Significance of Meiosis
(1) Crossing over in bivalent
- Produces new combinations of
genes in both chromosomes
Chiasma and Crossing
Over
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
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.
e
Stages of Meiosis
 Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
• Meiosis II
Prophase II
Metaphase
II
Anaphase II
Telophase II
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.
Early prophase I
During early prophase it is the same as in mitosis
Chromosomes
Nuclear
envelope
Spindle pole
Prophase I
Now, during prophase, also
occurs the crossing over, where
the homologous chromosomes
exchange information.
Spindle fiber
Metaphase I
The chromosomes align in the
middle
Telophase I
Each chromosome
separates from the
other, making two
news.
Meiosis I
Flowchart
After telophase I
the second meiotic division occurs
(without DNA duplication before),
as a final result we have 4 haploid
cells)
Prophase II
 The nuclear membranes of the
daughter cells disintegrate
again. The spindle fibres re-
form in each daughter cell
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.
Metaphase II
 Each sister chromatid is attached
to the spindle fibres at the
centromere
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.
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.
Telophase II
 These haploid cells will develop
into gametes.
 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.
Human genetics = 46
chromosome (2n)
23 pairs of chromosome
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
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
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.
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.
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
Trisomy 21/ Down Syndrome
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.

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Meiosis Lesson

  • 2. Significance of Meiosis (1) Crossing over in bivalent - Produces new combinations of genes in both chromosomes
  • 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.
  • 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.
  • 21.
  • 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.
  • 23.
  • 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.
  • 29.
  • 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.