Meiosis is a type of cell division that produces gametes, such as sperm and egg cells, with half the number of chromosomes as the original parent cell. It involves two rounds of division called Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and crossover can occur, followed by the separation of homologous chromosomes into daughter cells. Meiosis II separates the sister chromatids, resulting in four haploid daughter cells with half the chromosome number of the original diploid parent cell. Meiosis ensures genetic variation in offspring and maintains the chromosome number from one generation to the next.

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