1. MEIOSIS Process of reductional division in which the number of chromosomes per cell is cut in half.

2. Meiosis This Cell division occurs in the gametes, giving rise to the sperm and egg. This is the Characteristic of eukaryotes only: not in prokaryotes. Normal cells are diploid: 2n copies of every gene. Diploidy is useful because there are 2 copies of every gene, that means there is a backup copy, if one gets mutated. Mutations are very frequent in the cells of large organisms. Gametes are haploid: 1 copy of every gene. Meiosis produces 4 haploid cells. ***Mitosis produces 2 diploid cells. Why do we have sexual reproduction? Shuffling of alleles between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

3. Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis I reduces 2n to n, while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I.

4. Meiosis is a series of two nuclear divisions --- meiosis I and meiosis II. These two divisions are each divided into further phases: - Prophase –Metaphase – Anaphase –Telophase

5. Meiosis I Meiosis I encompasses four stages: –Prophase I –Metaphase I –Anaphase I –Telophase I

6. The stages are similar to the stages in mitosis but the largest differences occur in prophase I. In most cases, at the end of meiosis I, two daughter cells are produced. Each of these cells are haploid, having one homologous chromosome with two chromatids.

7. Meiosis II Meiosis II also encompasses four stages: –Prophase II –Metaphase II –Anaphase II –Telophase II

8. At the end of meiosis II, four daughter cells are produced. Each of these resulting daughter cells is haploid(n) having one chromatid making up the chromosome.

10. Start with a diploid cell, with 2 copies of each chromosome, one from each parent. The two copies are called homologues. Chromosomes each has 2 chromatids attached at the centromere.  Use 2 cell divisions: Meiosis 1. First separate the homologous chromosomes Meiosis 2. Then separate the 2 chromatids. The stages of meiosis have the same names as in mitosis: prophase 1, metaphase 1, anaphase 1, telophase 1. Each of the 2 cell divisions has all of these stages.

11. Prophase 1 Its one of the most important stages of meiosis. During this stage, many crucial events occur. The chromatid threads begin to twist and condense, creating chromosomal structures which are visible to the microscope. Each chromosome then actively seeks out its homologous chromosome. After the homologous chromosomes pair, the structure is referred to as a tetrad (four chromatids). The point at which two non-sister chromatids intertwine is known as a chiasma. Sometimes a process known as “crossing over "occurs at this point. This is where two non-sister chromatids exchange genetic material. This exchange does not become evident, however, until the two homologous pairs separate.

14. Types of Prophase I Prophase I has so many processes happening that it is usually separated into five stages. Leptonema ( leptotene). Zygonema (Zygotene). Pachynema (Pachytene). Diplonema ( Diplotene). Diakinesis.

15. Leptonema During this stage, the chromosomes begin to condense and become visible. Its also believed that homologous pair searching begins also at this stage.

16. Zygonema The chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as 'rough pairing'. Lateral elements also form between the two homologous pairs, forming a synaptonemal complex.

17. Pachynema Coiling and shortening continues as the chromosomes become more condense. A synapsis forms between the pairs, forming a tetrad.

18. Diplonema The sister chromatids begin to separate slightly, revealing points of the chiasma. This is where genetic exchange occurs between two non-sister chromatids, a process known as “crossing over”.

19. Two important events occur: – crossing over in prophase, and –the pairing of homologues in metaphase. Crossing over: Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.

20. A chiasma (chiasmata), in genetics, is thought to be the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis (sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells). The chiasmata becomes visible during the diplotene stage of prophase I of meiosis, but the actual "crossing- over" of genetic material is thought to occur during the previous pachytene stage. When each bivalent, which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata.

22. Diakinesis The chromosomes continue to pull apart, but non-sister chromatids are still loosely associated via the chiasma. The chiasma begin to move toward the ends of the tetrad as separation continues. This process is known as terminalization. Also during diakinesis, the nuclear envelope breaks down and the spindle fibers begin to interact with the tetrad.

25. Metaphase 1 The chromosomes begin their migration to the equator. The main event in this is the pairing and separation of the homologues ‘C’. At metaphase, the pairs of homologous chromosomes line up side by side. This does not happen in meiosis 2, but only in meiosis 1.

26. Metaphase I

27. Anaphase I Its very similar to anaphase in mitosis. The following changes occur: At anaphase1, the pairs of homologues ‘C’ are pulled to opposite poles by the spindle. Note: the centromeres do NOT divide; the chromosomes remain in the 2-chromatid X- shaped state, or sister chromatids remain together.

28. Anaphase I

29. Telophase I The following changes occur: The spindles continue to move the homologous chromosomes to the poles. Once movement is complete, each pole has a haploid number of chromosomes. In most cases, cytokinesis occurs at the same time as telophase I. At the end of telophase I and cytokinesis, two daughter cells are produced, each with one half the number of chromosomes of the original parent cell.

30. Telophase I

31. Meiosis 2 Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I. The following changes occur: Prophase II If needed, the nuclear membrane and nuclei break up while the spindle "network" appears and the chromosomes begin migrating to the metaphase II plate (at the cell's equator).

32. Metaphase II The chromosomes line up at the equatorial plate at the cell's center. The kinetochores ( centromere) of the sister chromatids point toward opposite poles. Anaphase II The sister chromatids separate and move toward the opposite cell poles.There is longitudinal splitting of the centromere. Telophase II Distinct nuclei form at the opposite poles and cytokinesis occurs.

34. Result of Meiosis 1 Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each. For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes. As a result of crossing over, each chromosome is the mixture of the original homologues.

35. Meiosis 2 Meiosis 2 is just like mitosis No replication of DNA between meiosis 1 and meiosis 2. Chromosomes line up individually on the equator of the spindle at metaphase. At anaphase the centromeres divide, splitting the 2 chromatids. The one-chromatid chromosomes are pulled to opposite poles.

37. Summary of Meiosis 2 cell divisions. Start with 2 copies of each chromosome (homologues), each with 2 chromatids. In meiosis 1, crossing over in prophase mixes alleles between the homologues. In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells. Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

38. Chromosomal Abnormalities

39. Deletion A deletion can happen in every chromosome and exhibit every size. The consequences of a deletion depends on the size of the missing segment and which genes are found on it. A partial deletion on the short arm (p) of chromosome 5, for example, is responsible for the "cri du chat" syndrome.

41. Duplication A chromosome duplication is the doubling of a chromosome piece. A duplication is sometimes termed a "partial trisomy". If a duplication is present, the person is equipped with 3 copies of the genes in the associated chromosome segment. This means that extra directions (genes) are present, leading to congenital abnormalities or developmental problems.

42. Inversion If chromosome pieces that have been broken out become inserted again, but reversed, an inversion has occurred. The phenotype of this disorder is usually unobtrusive, since the entire chromosomal information is still present. When the interchanged region includes the centromere, one refers to it as a pericentric inversion, otherwise to a paracentric inversion.

43. Normal chromosome Chromosome with paracentric inversion Normal chromosome Chromosome with pericentric inversion

44. Insertion If chromosome pieces are reinserted somewhere else, this is referred to as an insertion. Carriers of such insertions can be phenotypically inconspicuous because no information has been lost.

45. 2 normal chromosome pairs Insertion of a broken off piece on another chromosome

46. Reciprocal Translocation In a reciprocal translocation two broken off chromosome pieces of non-homologous chromosomes are exchanged. This is a relatively frequent anomaly. One finds it with an incidence of 1:500 newborns. Reciprocal translocations are frequently balanced because the entire genetic material is present. Problems occur, though, in gamete formation.

47. Two non- homologous chromosome pairs Reciprocal translocation