A short presentation on the Meiosis.

1. Introduction to Biology
Meiosis
Muhammad Hamza (Bs-phy-15-064)
Sultan Ahmed (Bs-phy-15-068)
Aqdas Butt (Bs-phy-15-036)
Komal Shahbaz (Bs-phy-15-019)

2. Cell Cycle
• Series of events from the time a cell is produced till it is
divided into two/four.
• It has two phases:
-Interphase (Preparatory phase)
-M Phase ( Mitosis or Meiosis)

3. Mitosis and Meiosis (M Phase)
Mitosis: Type of cell division in which one parent
cell(Somatic Cell) is divided into two cells.
Meiosis: Type of cell division in which one diploid cell
is divided into four haploid cells(Germ Cell).

4. Meiosis
• Gametes with half number of chromosomes, are
produced.
i.e. Diploid (2n)  haploid (n)
• Reduction division.
• Occurs only in sexual reproduction.
• Two divisions (meiosis I and meiosis II).

5. Meiosis
• Meiosis in males is called spermatogenesis and
produces sperm.
• Meiosis in females is called oogenesis and produces
egg cell or ovum.

6. Interphase I
• Similar to interphase of mitosis.
-Chromosomes replicate (S phase).
• Each duplicated chromosome has two identical
sister chromatids attached at
centromeres.
• Centriole also replicate.

7. Meiosis I (Phases)
• Karyokinesis: Division of nucleus.
-Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
• Cytokinesis: Division of Cell.

8. Prophase I
• Longest and most complex phase (90% of total time).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come together
to form a tetrad.
(Homologous are chromosomes with similar shape and size)
• Tetrad is two chromosomes or four chromatids (sister
and non-sister chromatids).

9. Prophase I
Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

10. Crossing Over
• Crossing over may occur between non-sister
chromatids of Homologous chromosomes
• Crossing over: Exchange of segments between non-
sister chromatids at chiasmata.
-Chiasmata are the sites of crossing over.

11. Crossing Over
Non-sister chromatids
chiasmata: site
of crossing over
Exchange of
segments
Tetrad

12. Prophase I
• Nuclear membrane disappears.
• Centrioles go to opposite poles.
• Spindle fibers attach to centromeres
CentriolesSpindle fiber

13. Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
metaphase plate
OR
metaphase plate

14. Anaphase I
• Homologous chromosomes separate and move
towards the poles.
• Sister chromatids remain attached at their
centromeres.

15. Telophase I
• Each pole now has haploid (n) set of
chromosomes.
• Cytokinesis occurs and two haploid daughter cells
are formed.
(Cytokinesis: Division of cell)

16. Telophase I
Cleavage Furrow

17. Meiosis II
• Interphase II
(very short - no more DNA replication i.e. No S phase)
• It is similar to mitosis

18. Prophase II
• Same as prophase in mitosis.
• Nuclear envelop disappears
• Spindle fibers attach to centromeres

19. Metaphase II
• Same as metaphase in mitosis.
• Chromosomes align on Equator/ metaphase
plate
metaphase platemetaphase plate

20. Anaphase II
• Same as anaphase in mitosis
• Sister chromatids separate and move towards
opposite poles.

21. Telophase II
• Same as telophase in mitosis.
• Nuclear envelop and nuclei are formed.
• Cytokinesis occurs by cleavage furrow.
• 4 Haploid daughter cells are formed.

22. Telophase II
Cleavage furrow

23. Meiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II

24. Significance
• To maintain the no. of chromosomes in offspring.
• Bringing the genetic variations.

25. Animation of Meiosis

26. Questions are welcomed
Thanks For your Attention