1. 1 CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES Section B: Meiosis الانقسام الميوزي

2. 2  Many steps of meiosis resemble steps in mitosis.  In meiosis, chromosomes replicate once followed by two consecutive متعاقب cell divisions, meiosis I and meiosis II, which results in four daughter cells.  Each final daughter cell has only half chromosomes number (haploid (فردى.  Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice.  The first division (meiosis I ) separates homologous chromosomes.  The second (meiosis II ) separates sister chromatids. Meiosis ( Reduction Division ) الإنقسام الإختزالى R educes chromosome number from diploid to haploid :

3. 3 2- Meiosis Division Reduction Division) 2- Meiosis Division (Reduction Division) A)- Meiosis I B)- Meiosis II - Separate homologous chromosomes. - Results in 2 haploid cells with replicated chromosomes. - Separate homologous chromosomes. - Results in 2 haploid cells with replicated chromosomes. - No further replication of chromosomes. -Occurs in the newly resulting cells from Meiosis I. (4 haploid cells) - No further replication of chromosomes. -Occurs in the newly resulting cells from Meiosis I. (4 haploid cells) Gametes It occurs mainly in sex gonads to form Gametes (sperms and ova) Reduction Division Each of the resulting cells has half number of chromosomes of the original cell (23 in human). Thus, it called Reduction Division Occurs in two steps

4. 4 1)- interphase the chromosomes are replicated تتضاعف to form sister chromatids. Meiosis A)- Meiosis I : is very similar to mitosis. 2)- Prophase I, the chromosomes condense and homologous chromosomes pair up تزدوج to form tetrads رباعية مجموعات.

5. 5 3)- Metaphase I, the tetrads are all arranged at the metaphase plate. 4)- Anaphase I, the homologous chromosomes separate and are pulled يتم دفعها toward opposite poles. Fig. 13.7

6. 6 5)- Telophase I, Cytokinesis by the same mechanisms as mitosisCytokinesis by the same mechanisms as mitosis Fig. 13.7

7. 7 B)- Meiosis II 1)- Prophase II a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them around. Fig. 13.7 2)- Metaphase II, the sister chromatids are arranged at the metaphase plate. 3)- Anaphase II, the centromeres of sister chromatids separate and travel toward opposite poles.

8. 8 4)- Telophase II, separated sister chromatids arrive at opposite poles. –Nuclei form around the chromatids. Cytokinesis separates the cytoplasm.Cytokinesis separates the cytoplasm. At the end of meiosis, there are four haploid daughter cells.At the end of meiosis, there are four haploid daughter cells. Fig. 13.7

9. 9 Fig. 13.8, Page 242 Comparison between Mitosis and meiosis