3.CELL CYCLE AND CELL DIVISION-MIOSIS - Copy.pptx

Cell cycle

1. MEIOSIS Meiosis is Two cell divisions (called meiosis I and meiosis II ) with only one duplication of chromosomes . MERCY-RAKER [email protected] 0703891895/0779864326 “ And if any one saved a life, it would be as if he saved the life of the whole mankind”. 1

2. MEOSIS Meiosis is a is a specialized form of cell division that occurs in organisms that reproduce sexually/ or type of cell division that occurs in gamete cells. As mentioned above, it produces reproductive cells, such as sperm cells, egg cells, and spores in plants and fungi. In humans, special cells called germ cells undergo meiosis and ultimately give rise to sperm or eggs. Germ cells contain a complete set of 46 chromosomes (23 maternal chromosomes and 23 paternal chromosomes). 2 CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326

3. By the end of meiosis, the resulting reproductive cells, or gametes each have 23 genetically unique chromosomes ( During meiosis the genetic material of a diploid germ cell undergoes two nuclear divisions resulting into 4 haploid daughter cells ) There are two successful nuclear division in meiosis as compared to the one division found in mitosis. Meiosis in males is called spermatogenesis and produces sperms while in females is called oogenesis and produces ova. CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 3

4. Terms related to meiosis Homologous chromosome Chromosome with similar size, shape and the position of their centromeres . Synapsis Condition of chromosomes pairing Bivalent Homologous chromosome pair , which consists of two chromosomes . One of the two chromosomes has a maternal origin and the other has a paternal origin. Tetrad Homologous chromosomes that have 4 chromatids 4

5. Sister chromatid Chromatid from same homologous chromososmes Chiasma Point at which crossing over occurs CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 5

6. In this form of division the number of chromosomes is halved (Reduction division), so that each new sex cell has only 23 (haploid) chromosomes When fertilization takes place, the full complement of 46 (diploid) chromosomes is restored. The 2 stages of meiosis are i.e ; Meiosis 1 also called reduction division and meiosis 11 also referred to as equational division. Before a dividing cell enters meiosis, it undergoes a period of growth called interphase. Interphase Similar to mitosis interphase. CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 6

7. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate. MEIOSIS 1( reduction division) Cell division that reduces the chromosome number by one-half. It consists of 4 phases namely; Prophase 1 Metaphase 1 Anaphase 1 CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 7

8. d) Telophase 1 Prophase 1 Longest and most complex phase in which 90% of meiotic process is spent in. Stages of prophase 1 Leptotene ( condesation ); T he DNA of the chromosomes begin to twist and condense, making the DNA visible under the microscope. Zygotene (pairing); homologous chromosomes align next to each other and pair up in a synapsis complex forming tetrads. CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 8

9. Pachytene (recombination); exchange of genetic segments occurs between non-sister chromatids.(crossing over which occurs at chiasmata/chiasma) Diplotene (coiling); homologous chromosomes begin to separate from each other but remain attached at chiasmata. Diakinesis (termination); The chiasmata and nucleolus disappear, the nuclear membrane degenerates, and the spindle fibers form. CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 9

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12. Metaphase 1 The tetrads(homologous chromosomes) line up along the equator/metaphase plate. 12 metaphase plate OR metaphase plate

13. Anaphase 1 Homologous chromosomes separate and move towards the poles but sister chromatids remain attached at their centromeres. 13

14. Telophase 1 Sister chromosomes arrive at their poles, spindle fibers disappear, a new nuclear envelop forms around the haploid sets of chromosomes Cytokinesis occurs and two haploid daughter cells are formed . Note ; sister chromatids are still Attached together. 14

15. MEIOSIS 11 No interphase II very short and no more DNA replication Involves 4 stages Note; Meiosis II is similar to mitosis Prophase 11 Nucleus & nucleolus disappear Chromosomes condense Spindle forms 15

16. Metaphase 11 The chromosomes line up the middle of the cell with the help of spindle fibers attached to the centromere of each replicated chromosome. Nucleus completely disappears. Chromosomes condenses to their fullest . Each Chromatid of a chromosome faces the opposite pole. 16 metaphase plate metaphase plate

17. Anaphase 11 The chromosomes split in the middle and the sister chromatids are pulled by the spindle fibers to opposite poles of the cell. 17

18. Telophase 11 Sister chromosomes arrive at the opposite poles Chromosomes uncoil and lengthen Spindles disappear and nuclear envelop forms around haploid sets Cytokinesis Occurs and four haploid cells are produced which are c alled GAMETES (eggs and sperm) 18

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20. SIMILARITIES BETWEEN MITOSIS AND MEIOSIS They are both types of cell division in which one cell gives rise to more than one cell. Both involve duplication of chromosomes. Both include breakdown of the nuclear membrane. Both require spindle fibers to move chromosomes to opposite poles of the cell. Both require chromosomes to line up in the middle of the cell Both end with cytokinesis. CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 20

21. CYTOLOGY LECTURE NOTES BY MERCY-RAKER-0703891895/0779864326 21 DIFFERENCES BETWEEN MITOSIS AND MEIOSIS . Mitosis Meiosis Involves one cell division Involves two successive cell divisions Results in two daughter cells Results in four daughter cells Results in diploid daughter cells (chromosome number remains the same as parent cell) Results in haploid daughter cells (chromosome number is halved from the parent cell) Daughter cells are genetically identical Daughter cells are genetically different

22. 22 Occurs in all organisms except viruses Occurs only in animals, plants and fungi Creates all body cells (somatic) apart from the germ cells (eggs and sperm) Creates germ cells (eggs and sperm) only Prophase is much shorter Prophase I takes much longer In metaphase individual chromosomes (pairs of chromatids) line up along the equator. In metaphase I pairs of chromosomes line up along the equator.

23. Reference Molecular biology of the cell by Alberts . Human Anatomy and Physiology 7 th edition by Elaine N. Marieb Essentials of Medical Physiology 6 th edition by K Sembulingam Wheater's Functional Histology-A text and Color Atlas 5 th edition by Barbara Young at.el Text book of pathology 6 th edition by Harsh Mohan “And if any one saved a life, it would be as if he saved the life of the whole mankind”. Qn 5:32 23

24. References Essentials of Medical Physiology 6 th edition by K Sembulingam Guyton and Hall text book of medical physiology 13 th edition by John E.Hall Review of medical physiology 22 nd edition by william F.Ganong Molecular Biology of the Cell, 4th ed. Garland Science, 2002 by Albert B . et al “And if any one saved a life, it would be as if he saved the life of the whole mankind ”. 24