CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES

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

2. Meiosis (Reduction Division) الإنقسام الإختزالىReduces chromosome number from diploid to haploid : • Many steps of meiosis resemble steps in mitosis. • In meiosis, chromosomes replicate once followed by two consecutive متعاقب cell divisions, meiosis Iand 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.

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

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

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

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. B)- Meiosis II1)- Prophase II a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them around. Fig. 13.7

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

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