1. Cell Division (Mitosis & Meiosis) Dr. Madhumita Bhattacharjee Assiatant Professor Botany deptt. P.G.G.C.G. -11,Chandigarh

2. Mitosis Mitosis is normal cell division, which goes on throughout life in all parts of the body. Meiosis is the special cell division that creates the sperm and eggs, the gametes. Mitosis and meiosis occur in eukaryotes. Humans have 46 chromosomes, 23 from each parent. Every cell has the same 46 chromosomes Each species has a characteristic number of chromosomes Meiosis

3. Chromosomes The essential part of a chromosome is a single very long strand of DNA. This DNA contains all the genetic information for creating and running the organism. Each chromosome has a central constricted region called a centromere that serves as an attachment point for the machinery of mitosis.

4. Chromosomes Chromosomes exist in 2 different states, before and after they replicate their DNA. Before replication, chromosomes have one chromatid. After replication, chromosomes have 2 sister chromatids, held together at the centromere. Each chromatid is one piece of DNA with its supporting proteins. In mitosis, the two chromatids of each chromosome separate, with each chromatid going into a daughter cell. Remember that diploid cells have two copies of each chromosome, one from each parent. These pairs of chromosomes are NOT attached together.

5. Interphase. During interphase the cell increases in size, but the chromosomes are invisible. The 3 stages of interphase are called G1, S, and G2. The S phase (“Synthesis”) is the time when the DNA is replicated, when the chromosome goes from having one chromatid to having 2 chromatids held together at the centromere. G1 (“Gap”) is the period between mitosis and S, when each chromosome has 1 chromatid. Cells spend mot of their time in G1: it is the time when the cell grows and performs its normal function. G2 is the period between S and mitosis. The chromosome have 2 chromatids, and the cell is getting ready to divide.

6. Machinery of Mitosis The chromosomes are pulled apart by the spindle, which is made of microtubules. The spindle fibers are attached to each centromere When the spindle fibers contract, the chromosomes are pulled to the opposing poles. The cell then divides to separate the two poles. Stages of mitosis: prophase, metaphase, anaphase, telophase.

7. Prophase In prophase, the cell begins the process of division. 1. The chromosomes condense. The proteins attached to the DNA cause the chromosomes to go from long thin structures to short fat one, which makes them easier to pull apart. 2. The nuclear envelope disappears. The double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division 3. The spindle starts to form.

8. Metaphase Metaphase is a short resting period where the chromosomes are lined up on the equator of the cell, and the spindle fibers attached to the centromeres.

9. Anaphase In anaphase, the centromeres divide. At this stage each individual chromosome has 2 chromatids Each centromere moves with its own chromatid & called daughter chromosome. Then the spindle fibers contract, and the chromosomes are pulled to opposite poles.

10. Telophase In telophase the cell actually divides. The chromosomes are at the poles of the spindle. The spindle disintegrates The nuclear envelope re-forms around the two sets of chromosomes.

11. Cytokinesis The organelles (other than the chromosomes) get divided up into the 2 daughter cells. Plant and animal cells divide the cytoplasm in different ways. In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei, about where the chromosomes lined up in metaphase. Cell membranes form along the surfaces of this wall. When the new wall joins with the existing side wall, the 2 cells have become separate. In animal cells, a ring of actin fibers (microfilaments are composed of actin) forms around the cell equator and contacts, pinching the cell in half.

12. Summary of Mitosis Prophase: Chromosomes condense Nuclear envelope disappears centrosomes move to opposite sides of the cell Spindle forms and attaches to centromeres on the chromosomes Metaphase Chromosomes lined up on equator of spindle centrosomes at opposite ends of cell Anaphase Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes Chromosomes pulled to opposite poles by the spindle Telophase Chromosomes de-condense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells

14. 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. Two divisions (meiosis I and meiosis II).

15. Meiosis Sex cells divide to produce gametes (sperm or egg). Gametes have half the No. of chromosomes. Occurs only in germ cells Meiosis is similar to mitosis with some chromosomal differences.

16. Spermatogenesis 2n=46 human sex cell diploid (2n) n=23 meiosis I n=23 sperm haploid (n) meiosis II

17. Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

18. Interphase I Nucleus and nucleolus visible. nuclear membrane nucleolus cell membrane chromatin

19. Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

20. Prophase I Longest and most complex phase (90%). It has 5 sub stages: Leptotene,Zygotene,Pachytene,Diplotene& diakinesis Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

21. Zygotene - Synapsis Homologous chromosomes sister chromatids Tetrad

22. Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. a.22 pairs of autosomes b.01 pair of sex chromosomes

23. Homologous Chromosomes PaternalMaternal eye color locus eye color locus hair color locus hair color locus

24. Pachytene - Crossing Over Crossing over (variation) may occur between nonsister chromatids at the chiasmata. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are the sites of crossing over.

25. Crossing Over - variation nonsister chromatids chiasmata: site of crossing over variation Tetrad

26. Prophase I centrioles spindle fiber aster fibers

27. Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example:2n = 4 then n = 2 thus 2 2 = 4 combinations

28. Metaphase I metaphase plate OR metaphase plate

29. Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.

30. Anaphase I

31. Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

32. Telophase I

33. Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

34. Prophase II same as prophase in mitosis

35. Metaphase II same as metaphase in mitosis metaphase plate

36. Anaphase II same as anaphase in mitosis sister chromatids separate

37. Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember:four haploid daughter cells produced. gametes = sperm or egg

38. Telophase II

39. Significant results of meiosis: 1.Haploid cells are produced because two rounds of division follow one round of chromosome replication. 2.Alignment of paternally and maternally inherited chromosomes is random in metaphase I, resulting in random combinations of chromosomes in each gamete. Number of possible chromosome arrangements = 2 n-1. 3.Crossing-over between maternal and paternal chromatids during meiosis I provides still more variation. Moreover, the crossing-over sites vary from one meiosis to another.

40. Meiosis 2n=4 sex cell diploid (2n) n=2 meiosis I n=2 sperm haploid (n) meiosis II