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# Meiosis Notes

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### Meiosis Notes

1. 1. 2n 2n egg (n) sperm (n) fertilization zygote Egg cell Sperm cell MEIOSIS MEIOSIS MITOSIS You MEIOSIS … making gametes….
2. 2. Each of your body cells has 2 sets of chromosomes – one from mum, one from dad. Any cell that has 2 complete sets of chromosomes is said to be DIPLOID OR 2n When 2 cells come together in fertilization to make a zygote, each can only have 1 set of chromosomes. These cells ( = egg/sperm, gametes) are said to be MONOPLOID or n.
3. 3. Why do meiosis? Meiosis is “ REDUCTION DIVISION”. Reduces chromosome number, parent cell is 2n while daughter cells are n. Why do ovary and testes cells go through meiosis?
4. 4. CHROMOSOME NUMBER In humans, 2n=46 This means we’re referring to a normal body cell that has a total of 46 chromosomes, but they are of 2 sets - 23 from mom, 23 from dad) In humans, n = 23 This means we’re talking about a cell that has only half the normal chromosome number (n) and that is only one set. This cell must therefore be a gamete.
5. 5. Chromosome number questions? <ul><li>If a horse retina cell has 36 chromosomes, how many does a horse sperm cell have? </li></ul><ul><li>If the n = 4 for fruit flies, how many chromosomes does a wing cell have? </li></ul><ul><li>3. If the 2n number of a pine tree is 86, how many chromosomes would be found in a pine ovum? </li></ul>
6. 6. Homologous pairs. In each body cell you have 23 homologous pairs. 2 chromosomes are homologous if: - one is from mum, one is from dad - they have the same types of genes on them (eg. Circled chromosomes may contain genes for blood type hair colour etc.)
7. 7. The chromosomes on the previous page are homologous because they have the same gene sequence. You can tell which chromosomes are homologous as they have: 1. the same type of genes. 2. The same banding pattern 3. The same position of the centromere.
8. 8. <ul><li>QUIZ TIME… </li></ul><ul><li>In streptomycin fungus n = 11. What is the diploid number for this species? </li></ul><ul><li>If a horse egg has 98 chromosomes how many does a horse egg cell have? </li></ul><ul><li>If n = 16 for goldfish how many chromosome in a fin cell? </li></ul><ul><li>If 2n = 108 for black spruce trees then what is the haploid number? </li></ul>2n = 14 for this species. Fill in the chromosome numbers for each cell. A B C What is process A, B, and C?
9. 9. <ul><li>KARYOTYPE </li></ul><ul><li>Photograph a cell in metaphase. </li></ul><ul><li>Cut out the chromosomes from the picture. </li></ul><ul><li>Paste homologous pairs together according to size, banding pattern and centromere. </li></ul><ul><li>Lay pairs out largest to smallest, sex chromosomes at end. </li></ul>
10. 10. <ul><li>Karyotype is used to determine </li></ul><ul><li>If there is an abnormality in number or structure of the chromosomes </li></ul><ul><li>(eg. Down’s syndrome) </li></ul><ul><li>2. The gender </li></ul>
11. 11. <ul><li>Karyotypes of embryos are obtained from: </li></ul><ul><li>CVS (chorionic villus sampling) and amniocentesis. </li></ul><ul><li>Karyotypes of adults can be obtained from any growing cells. </li></ul><ul><li>Karyotypes can not tell if there are mutations of a gene. </li></ul>
12. 12. Products of MITOSIS: 2 x 2n cells. 2 diploid cells Products of MEIOSIS: 4 x n cells (4 monoploid cells)   2n 2n 2n 2n n n n n
13. 13. MEIOSIS is two mitotic divisions in a row, (meiosis I and II) except you will see the homologous chromosomes coming together . -    -- in prophase I the double homologous chromosomes come together to form a tetrad - Crossing-over (synapsis ) occurs in the tetrad in prophase to increase the variability in gametes. - There is no dna replication in the interphase between the first and second divisions
14. 15. Interphase I <ul><li>Similar to mitosis interphase. </li></ul><ul><li>Chromosomes replicate (S phase). </li></ul><ul><li>Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres . </li></ul><ul><li>Centriole pairs also replicate (G2). </li></ul>
15. 16. Interphase I <ul><li>Nucleus and nucleolus visible. </li></ul>nuclear membrane cell membrane Sister chromatin
16. 17. Meiosis I (four phases) <ul><li>Cell division that reduces the chromosome number by one-half. </li></ul><ul><li>four phases : </li></ul><ul><li>a. prophase I </li></ul><ul><li>b. metaphase I </li></ul><ul><li>c. anaphase I </li></ul><ul><li>d. telophase I </li></ul>
17. 18. Prophase I <ul><li>Longest and most complex phase (90%). </li></ul><ul><li>Chromosomes condense. </li></ul><ul><li>Synapsis occurs: homologous chromosomes come together to form a tetrad . </li></ul><ul><li>Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) . </li></ul>
18. 19. Prophase I - Synapsis From Mum From Dad Homologous chromosomes sister chromatids sister chromatids Tetrad
19. 20. Homologous Chromosomes <ul><li>Pair of chromosomes ( maternal and paternal ) that are similar in shape and size. </li></ul><ul><li>Homologous pairs (tetrads) carry genes controlling the same inherited traits. </li></ul><ul><li>Each locus (position of a gene) is in the same position on homologues. </li></ul><ul><li>Humans have 23 pairs of homologous chromosomes. </li></ul><ul><li>a. 22 pairs of autosomes </li></ul><ul><li>b. 1 pair of sex chromosomes </li></ul>
20. 21. Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal
21. 22. Crossing Over (SYNAPSIS) <ul><li>Crossing over (variation) may occur between nonsister chromatids at the chiasmata . </li></ul><ul><li>Crossing over : segments of nonsister chromatids break and reattach to the other chromatid . </li></ul><ul><li>Chiasmata (chiasma) are the sites of crossing over . </li></ul>Synapsis increases genetic variability – and that’s a good thing…
22. 23. Crossing Over - variation variation nonsister chromatids chiasmata: site of crossing over Tetrad
23. 24. Sex Chromosomes XX chromosome - female XY chromosome - male
24. 25. Prophase I centrioles spindle fiber aster fibers
25. 26. Metaphase I <ul><li>Shortest phase </li></ul><ul><li>Tetrads align on the metaphase plate . </li></ul><ul><li>INDEPENDENT ASSORTMENT OCCURS: </li></ul><ul><li>1. Orientation of homologous pair to poles is random. </li></ul><ul><li>2. Variation </li></ul><ul><li>3. Formula: 2 n </li></ul><ul><li>Example: 2n = 4 </li></ul><ul><li>then n = 2 </li></ul><ul><li> thus 2 2 = 4 combinations </li></ul>
26. 27. Metaphase I metaphase plate OR metaphase plate
27. 28. Anaphase I <ul><li>Homologous chromosomes separate and move towards the poles. </li></ul><ul><li>Sister chromatids remain attached at their centromeres . </li></ul>
28. 29. Anaphase I
29. 30. Telophase I <ul><li>Each pole now has haploid set of chromosomes . </li></ul><ul><li>Cytokinesis occurs and two haploid daughter cells are formed. </li></ul>
30. 31. Telophase I
31. 32. Meiosis II <ul><li>No interphase II </li></ul><ul><li>(or very short - no more DNA replication ) </li></ul><ul><li>Remember: Meiosis II is similar to mitosis </li></ul>Reduction Division occurs during Meiosis II because it is defined as the point at which mum’s genetic information is separated from dad’s genetic information whereas a normal body cell has both mom’s and dad’s chromosomes.
32. 33. MEIOSIS II
33. 34. Prophase II <ul><li>same as prophase in mitosis </li></ul>
34. 35. Metaphase II <ul><li>same as metaphase in mitosis </li></ul>metaphase plate metaphase plate
35. 36. Anaphase II <ul><li>same as anaphase in mitosis </li></ul><ul><li>sister chromatids separate </li></ul>
36. 37. Telophase II <ul><li>Same as telophase in mitosis . </li></ul><ul><li>Nuclei form. </li></ul><ul><li>Cytokinesis occurs. </li></ul><ul><li>Remember: four haploid daughter cells produced. </li></ul><ul><li>gametes = sperm or egg </li></ul>
37. 38. Telophase II
38. 39. Cytokinesis: cytoplasm divides. Total product is 4 cells, each with only a mom or Dad’s chromosome , in other words, half the normal chromosome number , or 4 monoploid (n) cells, or 4 gametes. Uneven cytokinesis in females: In oogenesis, get one big egg and 3 non-functional polar bodies: ( who die off) and 1 big functional gamete
39. 40. First division: results in one big primary oocyte and a little polar body. Second division: One secondary oocyte and 3 polar bodies. ( These polar bodies die.) OOGENESIS Oocyte Polar Bodies Oogonium Primary Oocyte Mitosis Polar Body Secondary Oocyte Meiosis I Meiosis II
40. 42. Spermatogenesis There are equal divisions producing four equal sized sperm .
41. 43. COMPARISONS: Include similarities and differences. <ul><li>Compare Mitosis and Meiosis </li></ul><ul><li>Compare spermatogenesis and oogenesis </li></ul><ul><li>Compare Meiosis I and Meiosis II </li></ul><ul><li>Compare haploid and diploid cells. </li></ul><ul><li>Compare a somatic cell to a gamete. </li></ul>
42. 44. Compared to the number of chromosomes contained in a body cell of a parent, how many chromosomes would normally be contained in a gamete? F. the same number G. twice as many H. one-fourth as many J. half as many During mitosis, the chromosomes A. are located at the cell equator during prophase B. are located at the cell equator during telophase C. move toward the poles of the cell during anaphase D. move toward the poles of the cell during metaphase Some questions on mitosis and meiosis!
43. 45. 1. Meiosis occurs in this life cycle when A. an egg and a sperm fuse to form a zygote B. a mature polyp produces a medusa C. a planula produces a young polyp D. a medusa produces gametes . Which structures normally have a haploid number of chromosomes? A. Seed plant spores and animal zygotes B. Seed plant spores and animal gametes C. Seed plant zygotes and animal zygotes D. Seed plant gametes and animal zygotes
44. 46. Processes II and IV illustrate budding. The information in the diagram indicates that budding is a type of A. asexual reproduction that produces haploid or diploid cells B. asexual reproduction that produces diploid cells C. sexual reproduction that produces haploid or diploid cells D. sexual reproduction that produces haploid cells
45. 47. = Non-separation of chromosomes resulting in diploid or empty gametes. Non-disjunction
46. 48. Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a total of 47 chromosomes. = 2n + 1(chance of occurring in oogenesis increases with maternal age) Turners syndrome = Monosomy X - has only one X chromosomes totalling only 45 chromosomes in her body cells. (monosomy X) = 2n-1 Klinefelter’s syndrome = XXY - male which has an extra X chromosome = 2n + 1 Conditions caused by non-disjunction
47. 49. A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome. Down’s syndrome – trisomy 21
48. 50. Normal boy who develops some female secondary sex character-istics at puberty. -slightly lower IQ, infertile, delayed motor, speech, maturation -treated with testosterone. Klinefelter’s syndrome XXY