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3. Prepared By SAFI ULLAH
B-11
B.S Botany
3rd Semester

4. Meiosis
 The form of cell division by which Gametes, with half
the number of chromosomes, are produced.
 Diploid (2n) Haploid (n).
 Meiosis is sexual reproduction.
 The term Meiosis is coined by J.B Farmer (1905).
 The cells in which meiosis occur are called meiocytes.

5. THE IMPORTANCE OF MEIOSIS
 Meiosis halves the number of chromosomes going to
sex cells so that the offspring will have both the
mother and father’s genetic material.
 Meiosis reduces the chromosome number so that the
offspring will be normal.
 It brings about variation of off springs or individuals.

6. STAGES OF MEIOSIS
 MEIOSIS I is divided into five phases. It begins with
one diploid parent cell and ends with two haploid
daughter cells, halving the number of chromosomes in
each cell.
 MEIOSIS II is divided into four phases. Meiosis II
starts with two haploid parent cells and ends with four
haploid daughter cells, maintaining the number of
chromosomes in each cell.

9. PHASES OF MEIOSIS I
 Interphase I
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I

10. Interphase I
 Cell build up energy .
 DNA Replication (to make duplicated chromosomes).
 Cell doesn’t change structurally.

11. Prophase I
 Longest and most complex phase.
 90% of the meiotic process is spent in Prophase I.
 Chromosomes condense.
 Synapsis occurs: homologous chromosomes come
togather to form a Tetrad.
 Tetrad is two chromosomes or four chromatids
(sister and non-sister chromatids).

13. Crossing Over
 Genetic recombination is the production of new
combinations of genes due to crossing over.
 Crossing over is an exchange of genes between
separate (non-sister) chromatids on homologous
chromosomes.
 Non-sister chromatids join at a chiasma (plural,
chiasmata), the site ofattachment.
 Genetic material are exchanged between maternal and
paternal (no sister) Chromatids.

15. by the end of prophase I
 the spindle has formed
 the nuclear membrane has vesicularized
 nucleoli have disintegrated
 homologous chromosomes are attached by their
kinetochores to spindle fibers from opposite poles
 homologous chromosomes are held together only at
chiasmata, the sites where crossing-over occurred

16. Metaphase I
 Centrioli has reached the poles.
 Homologous pairs align at the cell equator.
 The two chromosomes attach to one spindle fiber by
means of the
kinetochore of the centromere.

17. Anaphase I
 homologous chromosomes separate and are moved toward
opposite poles.
 each pole gets one set of homologous chromosomes.
 the initial “maternal” or “paternal” chromosome sets are mixed
and distributed randomly (crossing-over largely blurs such
identity anyway).

18. Telophase I
 the spindle fibers disintegrate
 the chromosomes partially de-condense
 nuclear membranes may form around the genetic
material
 cytokinesis occurs

19. Stages of Meiosis II
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II

20. Prophase II
 During prophase II the nuclear envelopes dissolve and
new spindle fibers forms.

21. Metaphase II
 Individual duplicated chromosomes align on the
equator.
 One chromosome per spindle fiber attached by means
of kinetochore of centromere.
 Centrioli has reached the poles.

22. Anaphase II
 Sister chromatrids are separate from each other.

23. Telophase II
 Daughter chromosomes has reached the poles.
 Two cells invaginate and form 4 daughter haploid cells
(gametes).
 They uncoil and form chromatin.
 Nuclear envelope and nucleolus for around chromatin
again.
 Centrioli for centrosome.

24. Final result
 Four cells containing haploid sets of chromosomes.
 In animals, develop directly into gametes--- Sperms
and Eggs.
 In plants, fungi, and many protists, divide mitotically
to produce greater of gametes.

26. Errors in meiosis
 Non- disjunction: failure of chromosomes to
separate from one another as they move to opposite
poles during either meiotic division.
 Aneuploid Gametes: gametes with missing or extra
chromosomes.

28. Reference
 www.slideshare.net
 www.google-scholer.com
 www.slideplayer.com
 www.google.com
 Principles of Botany
By: Tanveer Ahmad

29. .
THE END
Prepared By: Safi Ullah
Student of B.S Botany 3rd Semester.