Meiosis is the special type of cell division in which the number of chromosomes in daughter cells reduces to half, as compared to the parent cell. It takes place in diploid cells only, in animals at the time of gamete formation, while in plants when spores are produced.

1. MEIOSIS
BUSHRA HAFEEZ

2. MEIOSIS
• Meiosis is the special type of cell
division in which the number of
chromosomes in daughter cells reduces
to half, as compared to the parent cell.
• It takes place in diploid cells only, in
animals at the time of gamete
formation, while in plants when spores
are produced.

3. DIVISIONS OF MEIOSIS
There are two divisions of meiosis:
• Meiosis I
• Meiosis II
Meiosis I:
It is reduction division which is further be
divided into sub stages like Prophase I, Metaphase I,
Anaphase I and Telophase I.

4. Meiosis II:
It is just like the mitosis. It is
further divided into sub stages like
Prophase II, Metaphase II, Anaphase II
and Telophase II.

6. MEIOSIS I
Prophase I:
• This is very prolonged phase, in this phase chromosomes
behave as homologous pairs.
It consists of following sub stages:
I. Leptotene
II. Zygotene
III. Pachytene
IV. Diplotene
V. Diakinesis

7. LEPTOTENE:
• The chromosomes become visible, shorten and
thick.
• The size of nucleus increases.
• The homologous chromosomes star getting closer to
each other.

8. ZYGOTENE:
• Pairing of homologous chromosomes called
Synapsis starts.
• This pairing is highly specific and exactly pointed.
• Paired but not fused, complex structures, Bivalent
or Tetrad are formed.

9. PACHYTENE:
• The pairing of homologous chromosomes is
completed.
• Non-sister chromatids exchange their segments due
to chiasmeta, during crossing over.
• Reshuffling of genetic material occurs which
produces recombinations.

10. DIPLOTENE:
• The paired chromosomes repel each other and begin
to separate.
• Separation is not completed because homologous
chromosomes remain united by their point of
interchange(chiasmata).

11. DIAKINESIS:
• The condensation of chromosome reaches to its
maximum.
• Separation of homologous chromosomes is
completed
• Nucleoli disappear.

13. Prophase I

14. METAPHASE I:
• Nuclear membrane disorganize at the beginning.
• Spindle fibres originate and kinetochore fibres
attach to the kinetochore of homologous
chromosomes from each pole and arrange them at
equator.

15. Metaphase I

16. ANAPHASE I:
• The kinetochore fibres contract.
• The spindle or pole fibres elongate, which pull the
individual chromosome towards their respective
poles.
• This is reduction phase because each pole receives
half of the total number of chromosomes.

17. Anaphase I

18. TELOPHASE I:
• Nuclear membrane reorganizes.
• Nucleoli reappear thus two nuclei are formed.
• Cytoplasm divides thus terminating the first meiotic
division.

19. Telophase I

20. MEIOSIS II
• After Telophase I two daughter cells experience
small interphase but there is no replication of
chromosomes.
• The stages of meiosis II are just like respective
phases of mitosis.

21. PROPHASE II:
• The centrioles duplicate. This occurs by
separation of the two members of the pair, and
then the formation of a daughter centriole
perpendicular to each original centriole.
• The two pairs of centrioles separate into two
centrosomes.
• The nuclear envelope breaks down, and the
spindle apparatus forms.

22. Prophase II

23. METAPHASE II:
• Each of the daughter cells completes the formation of a spindle
apparatus.
• Single chromosomes align on the metaphase plate, much as
chromosomes do in mitosis. This is in contrast to metaphase I, in
which homologous pairs of chromosomes align on the metaphase
plate.
• For each chromosome, the kinetochores of the sister chromatids
face the opposite poles, and each is attached to a kinetochore
microtubule coming from that pole.

24. Metaphase
II

25. ANAPHASE II:
• The centromeres separate, and the two chromatids
of each chromosome move to opposite poles on the
spindle. The separated chromatids are now called
chromosomes in their own right.

26. Anaphase
II

27. TELOPHASE II:
• A nuclear envelope forms around
each set of chromosomes.
• Cytokines is takes place, producing
four daughter cells (gametes, in
animals), each with a haploid set of
chromosomes.
• Because of crossing-over, some
chromosomes are seen to have

28. Telophase
II

29. SIGNIFICANCE OF
MEIOSIS
• It maintains the same chromosome number in the
sexually reproducing organisms. From a diploid cell,
haploid gametes are produced which in turn fuse to
form a diploid cell.
• It restricts the multiplication of chromosome
number and maintains the stability of the species.

30. SIGNIFICANCE OF
MEIOSIS
• Maternal and paternal genes get exchanged during
crossing over. It results in variations among the
offspring.
• All the four chromatids of a homologous pair of
chromosomes segregate and go over separately to
four different daughter cells. This leads to variation
in the daughter cells genetically.