Meiosis

1. Meiosis

2. Reproduction • Mitosis produces somatic cells (body cells) • Somatic cells have 46 chromosomes (23 pairs) • What if somatic cells were involved in reproduction? How do we solve this problem? Oocyte 46 Zygote 92 46 Spermatocte I’ve got too many chromosomes!!!

3. Reproduction • Mitosis produces somatic cells (body cells) • Somatic cells have 46 chromosomes (23 pairs) • What if somatic cells were involved in reproduction? Much better 23 46 23 Haploid Diploid

4. Gametes • Haploid gametes keeps the chromosome number from doubling in each succeeding generation • Haploid gametes are produced by a special sort of cell division called meiosis • Meiosis • occurs only in reproductive organs, ovaries and testes • Purpose of meiosis is to produce sperm and egg • Spermatogenesis – Sperm production • Oogenesis – Egg cell production

5. Summary of Meiosis • Meiosis involves 2 cell divisions • Meiosis produces 4 cells from 1 parental cell • Each of the 4 daughter cells has 23 individual chromosomes rather than 23 pairs of chromosomes • Meiosis reduces the chromosome number from diploid to haploid • Meiosis, like mitosis, is preceded by chromosome duplication • However, in meiosis the cell divides twice to form four daughter cells

6. Meiosis I

7. Meiosis II

8. MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes(withcentriolepairs) Microtubules attached tokinetochore Metaphaseplate Sister chromatidsremain attached Sites of crossing over Spindle Nuclearenvelope Sisterchromatids Tetrad Centromere(with kinetochore) Homologouschromosomes separate Chromatin

9. MEIOSIS II: Sister chromatids separate TELOPHASE IAND CYTOKINESIS TELOPHASE IIAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavagefurrow Sister chromatidsseparate Haploiddaughter cellsforming

10. Genetic Variation in Meiosis • Meiosis provides for a vast amount of genetic variation • Crossing over – in prophase I, chromosomes exchange DNA • Independent Assortment – in anaphase I homologs independently separate, in anaphase II sister chromatids independently separate • Additional genetic variation is achieved through mating of non-related individuals

11. Crossing Over

12. Independent Assortment POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4

13. Independent Assortment • The number of genetically distinct gametes that can be produced from a diploid cell is 2n • n = the number of chromosome pairs I wonder how many distinct gametes can be produced from independent assortment in humans? 223 = 8,388,608 combinations

14. MITOSIS MEIOSIS PARENT CELL(before chromosome replication) Site ofcrossing over MEIOSIS I PROPHASE I Crossing over occurs PROPHASE Chromosomereplication Chromosomereplication Duplicatedchromosome(two sister chromatids) 2n = 4 Chromosomes align at the metaphase plate Homologous pairs line up at the equator METAPHASE I METAPHASE ANAPHASE I TELOPHASE I ANAPHASETELOPHASE Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together Sister chromatidsseparate duringanaphase Haploidn = 2 Daughtercells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cellsof mitosis n n n n Daughter cells of meiosis II

15. Karyotype What does this karyotype tell you about the person? • A karyotype is a display of a persons chromosomes

18. Errors in Meiosis • Nondisjunction occurs when either: • homologues fail to separate during anaphase I of meiosis, or  • sister chromatids fail to separate during anaphase II. • Trisomy- one extra chromosome (2n +1) • Monosomy - one missing chromosome (2n - 1)

19. Down syndrome(trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are 47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely: - characteristic facial features, short stature; heart defects - susceptibility to respiratory disease, shorter lifespan - prone to developing early Alzheimer's and leukemia - often sexually underdeveloped and sterile, usually some degree of mental retardation. - Down Syndrome is correlated with age of mother but can also be the result of nondisjunction of the father's chromosome 21.

20. Patau syndrome (trisomy 13): serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months.

21. Edward's syndrome (trisomy 18): almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months.

22. Klinefelter syndrome: 47, XXY males. Male sex organs; unusually small testes, sterile. Breast enlargement and other feminine body characteristics. Normal intelligence.

23. 47, XYY males: Individuals are somewhat taller than average and often have below normal intelligence. At one time (~1970s), it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time.

24. Trisomy X: 47, XXX females. 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female except by karyotype

25. Monosomy X (Turner's syndrome): 1:5000 live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)