Meiosis

1. Meiosis

2. SEXUAL REPRODUCTION Asexual reproduction only needs one parent and produces identical offspring! Sexual Reproduction: • Requires 2 parents • Produces offspring that are genetically different from each other, from either parents, and from any other member of their species • These difference can be visible or invisible!

3. Genetic Diversity: variation, or inherited genetic differences, in a species • Occurs because sexual reproduction randomly sorts DNA • Mix of genes from parents may allow the offspring to be better equipped to cope with it’s environment

4. The Role of Gametes Review: • Genetic information is passed along through chromosomes • Every organism has a specific number of chromosomes Remember! Eukaryotic organisms are made of cells that contain a nucleus and organelles that are contained in the nuclear membrane)

5. Diploid Number (2n): the chromosome number in a eukaryotic organism • Means that a body cell has 2 sets of chromosomes • Human diploid number = 46 OR 2 sets of 23 chromosomes Mitosis makes sure that the diploid # always remains the same!

6. So how do humans become genetically diverse? • 1 set of chromosomes (23) come from the female parent and 1 set of chromosomes (23) from the male parent • Haploid Number (n): a set of inherited chromosomes (23) • Haploid chromosomes are carried in gametes • Gametes: specialized cells necessary for reproduction

7. In animals... • Male gametes are called sperm cells • Female gametes are called egg cells

8. Fertilization: an egg cell is penetrated by a sperm cell • Haploid genetic info from both gametes combine • Results in a diploid cell (46 chromosomes) called a zygote • Zygote receives half its chromosomes from the female parent and the other half from the male parent • Zygote undergoes mitosis and cell division, developing into an embryo

9. 23 chromosomes (sperm) + 23 chromosomes (egg) = 46 chromosomes (diploid zygote) The diploid zygote undergoes mitosis, eventually becoming...

11. The Process of Meiosis

12. Meiosis: Reducing Chromosome Number Meiosis: process that produces gametes with half the number of chromosomes as body cells Without meiosis, fertilization would result in offspring that have 2 times the original number of chromosomes as its parents!! During meiosis the DNA will only duplicate once even though cell division occurs twice!

14. Meiosis I In METAPHASE (mitosis) the 46 chromosomes line up along the middle of the cell. The sister chromatids are then split apart during ANAPHASE, and each half is pulled towards opposite ends of the cell.

15. In MEIOSIS I a pair of homologous chromosomes (1 from each parent) lines up at the equator. • Homologous chromosomes: pairs of matching chromosomes During MEIOSIS I the homologous chromosomes separate and move to opposite ends of the cell, resulting in 2 daughter cells.

17. MEIOSIS II • DNA will NOT be replicated again! • Like mitosis because the sister chromatids will be separated and pulled to opposite ends of the cell • Each daughter cell will inherit 1 chromatid from each chromosome, resulting in 4 haploid cells (each with ½ the number of chromosomes)

19. Events in Meiosis that Produce Genetic Variation There are 3 important events that cause genetic variation during Meiosis: • Crossing Over • Independent Assortment • Gamete Formation

20. 1. Crossing Over • Occurs between each chromosome pair in Meiosis I • Parts of non-sister chromatids “cross over” each other, exchanging DNA segments • Multiple cross-overs can occur • Results in an infinite numbers of genetic possibilities for just one gamete

22. 2. Independent Assortment • Homologous pairs of chromosomes separate at the equator and move towards opposite ends of the cell • For every pair there are 2 possibilities for how a chromosome will sort itself into daughter cells • More than 8 million combinations for these 23 pairs in the egg or sperm • 70 trillion different zygotes are possible once fertilization occurs

25. 3. Gamete Formation • Although meiosis is identical for males and females, gamete formation is NOT! In males... • Meiosis I occurs and produces 2 cells • Meiosis II then occurs (if enough nutrients are present), resulting in 4 cells with cytoplasm and organelles divided equally between them • All 4 cells can become mature sperm

27. In females... • Meiosis I occurs and produces 2 egg cells, but there is an unequal division of cytoplasm and organelles • Following Meiosis II, 3 of the cells will disintegrate • The last egg cell will hold most of the cytoplasm and is free for fertilization

30. Chromosome Mutations in Meiosis REVIEW: Small mutations in genes could have no effect on the organism OR they could cause disease. Big changes in genes/DNA could happen during meiosis when pieces of chromosomes are... • Lost (deleted) • Duplicated • Moved within a chromosome (inverted) • Moved to another chromosome

31. See Figure 6.8 on page 194 of your textbook for a clearer picture!

32. These mutations could change the proteins that are supposed to be made by those genes. Chromosome mutations also occur when exposed to mutagens! Example: Fruit flies • Legs growing where the antennae should be • Wrinkled heads • Mutated eye colour (pink, purple, or green instead of red)

33. Whole chromosome mutations can occur during Meiosis I (fail to separate) or Meiosis II (fail to separate) Result: 1 gamete will have 2 copies of 1 chromosomes while another will have no copy Many chromosome mutations are not passed to the next generation • Offspring fails to develop or does not live to a reproductive age

34. Diagnosing Genetic disorders • Karyotype: a picture that shows an individual’s chromosomes arranged in a particular order • Prepared by cutting and pasting chromosomes taken from body cells during mitosis • Homologous chromosomes identified and paired by size, centromere location, and banding patterns

36. Using karyotypes to understand which chromosome has been affected helps doctors diagnose and treat those with genetic disorders or syndromes. Syndrome: a particular disease or disorder with a specific group of symptoms that occur together Example: Down syndrome • Individual with Down syndrome have characteristic facial features and shorter statures • May be prone to developing heart defects and diseases like Alzheimer’s and leukemia

37. 95% of cases of Down syndrome are caused by an extra 21st chromosome