Meiosis and its different stages

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

2. 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.

3. 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.

4. 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.

5. PHASES OF MEIOSIS I Interphase I Prophase I Metaphase I Anaphase I Telophase I

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

7. 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).

8. 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.

9. 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

10. 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.

11. 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).

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

13. Stages of Meiosis II Prophase II Metaphase II Anaphase II Telophase II

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

15. 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.

16. Anaphase II Sister chromatrids are separate from each other.

17. 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.

18. 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.

19. 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.

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

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