1. Journal Why are you not identical to your siblings?

2. Meiosis

3. Chromosome Number –Chromosomes—those strands of DNA and protein inside the cell nucleus—are the carriers of genes. –The genes are located in specific positions on chromosomes.

4. Diploid Cells A body cell in an adult fruit fly has eight chromosomes, as shown in the figure. Four of the chromosomes come from its male parent, and four come from its female parent. These two sets of chromosomes are homologous, meaning that each of the four chromosomes from the male parent has a corresponding chromosome from the female parent.

5. A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.” The diploid number of chromosomes is sometimes represented by the symbol 2N. For the fruit fly, the diploid number is 8, which can be written as 2N = 8, where N represents twice the number of chromosomes in a sperm or egg cell. For a human the diploid number is 46 or 2n=46.

6. Haploid Cells –Some cells contain only a single set of chromosomes, and therefore a single set of genes. Such cells are haploid, meaning “one set.” –The gametes of sexually reproducing organisms are haploid. –For fruit fly gametes, the haploid number is 4, which can be written as N = 4. The haploid number in humans is 23 or n=23.

7. How many sets of genes do multicellular organisms inherit? The diploid cells of most adult organisms contain two complete sets of inherited chromosomes and two complete sets of genes. The number of chromosomes is reduced from 46 (23 pairs) to 23 during the process of meiosis. In a human egg or sperm, there are 23 chromosomes. A human egg is haploid (has 23 chromosomes) and a sperm is haploid (has 23 chromosomes). Upon fertilization, the new baby now has the correct human number of 46 chromosomes in each of its somatic cells. Fertilization of the egg by the sperm restores the diploid number of 46 chromosomes.

8. Understanding the process of meiosis is fundamental to understanding human health and development

9. Mitosis A normal cell with, with 2 copies of each chromosome Daughter cells are exact copies of the mother cell with 2 copies of each chromosome This is the usual process of division by which cells in out bodies renew themselves

10. What is Meiosis? Meiosis is the production of sperm and egg cells. These cells are "Gametes" or "Sex" cells. Each cell has to go through the division process twice in order for the cell to end up with half the number of chromosomes. Meiosis usually involves two distinct divisions, called meiosis I and meiosis II. By the end of meiosis II, the diploid cell becomes four haploid cells.

11. Meiosis The purpose of meiosis is to reduce the normal diploid cells to haploid cells called gametes

12. In humans, special haploid cells resulting from meiosis are eggs (female) or sperm (male) In yeast, the haploid cells are called spores

13. To make a diploid again— haploid gametes produced by each parent will fuse to form a zygote (during fertilization) Offspring receive one copy of each chromosome from each parent

14. Depending on the recombination events that occurred to produce a gamete, genetic information may be rearranged

15. This explains why siblings get different combinations of genes from their parents, which is why they look related but not identical

16. Meiosis vs. Mitosis 1.In Meiosis, two cell divisions take place (4 daughter cells produced) 2.During meiosis, chromatids may exchange similar parts, so they are NOT identical when distributed 3.In meiosis, chromosomes duplicate only ONCE, even though they divide twice 4.During meiosis, chromosomes are distributed into daughter cells randomly, not equally as it is in mitosis

18. Types of Reproduction Sexual reproduction involves the fusion of two gametes (parent cells) to make a genetically different offspring. Two gametes fusing creates a ZYGOTE (new individual, embryo) through the process of fertilization. Asexual Reproduction occurs when one parent donates all chromosomes to an offspring and there is no union of gametes

19. Meiosis explains Mendel's Results Meiosis explains the physical basis of Mendel's results. In Anaphase I, when the segregation of chromosomes occurs, Mendel's observation that each parent gives one allele for each trait at random to each offspring, regardless of whether the allele is expressed, is explained. It explains Mendel's observation that factors, or genes, for different traits are inherited independently for each other.

20. Steps of Meiosis

21. To carry out meiosis cells duplicate their chromosomes but follow this by 2 rounds of cell division, instead of one like mitosis

22. 1 st Division of Meiosis Separates duplicated chromosomes from each other Reduces the number of chromosomes in each cell

23. 2 nd Step of Meiosis Allows for chromosome copies to separate

24. Offspring from meiosis have ½ the number of chromosomes as their parent cell, because they receive just one copy of each chromosome rather than 2

25. Phases of Meiosis

26. Remember Meiosis is the cell division that produces gametes containing half the number of chromosomes as a parent’s body cell.

27. Meiosis contains 2 separate divisions Meiosis I Meiosis II

28. Meiosis I Interphase I –The cell first carries out its usual metabolic activities & replicates its chromosomes

29. Prophase I Each chromosome consists of 2 identical sister chromatids, held together by a centromere Chromosomes coil up & a spindle forms

30. Each pair of chromosomes comes together to from a 4-part structure called a tetrad

31. Tetrad Made up of 2 homologous chromosomes, each of which is made up of 2 sister chromatids

32. Crossing Over Chromatids of homologous chromosomes cross over each other Crossed sections—containing alleles—are exchanged Produces new combinations of alleles in the cell

33. Metaphase I Tetrads line up on the middle of the spindle

34. Anaphase I Homologous chromosomes separate and move to opposite ends of the cell because the centrometers do not split

35. Telophase I Spindle breaks down Chromosomes uncoil Cytoplasm divides so there are 2 new cells The chromatids are Still attached.

36. Meiosis I Summary Meiosis I results in two cells, called daughter cells, each of which has four chromatids, as it would after mitosis.Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell. The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

37. Meiosis II Newly formed cells undergo a short interphase in which the chromosomes do not replicate Sometimes the cells go from Meiosis I right into Meiosis II

38. Prophase II Each cell contains one member of each homologous pair. A spindle forms in each of the two new cells

39. Metaphase II Chromosomes, still made up of sister chromatids, line up at the middle of the cell.

40. Anaphase II The centromere of each chromosome splits, allowing sister chromatids to separate and move to opposite poles of the cell

41. Telophase II Nuclei reform Spindles break down Cytoplasm divides

42. Meiosis results in 4 haploid cells Each cell containing half of the chromosomes of the parent cell

43. Gametes to Zygotes The haploid cells produced by meiosis II are gametes. –In male animals, these gametes are called sperm. In some plants, pollen grains contain haploid sperm cells. –In female animals, generally only one of the cells produced by meiosis is involved in reproduction. The female gamete is called an egg in animals and an egg cell in some plants.

44. Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote. The zygote undergoes cell division by mitosis and eventually forms a new organism.

45. Spermatogenesis –Meiosis in a male organism, producing 4 sperm cells Oogenesis –Meiosis in a female organism, producing 1 egg and 3 inactive polar bodies

47. Identical Twins A single egg is fertilized to form one zygote The zygote divides to form two separate embryos

48. Meiosis Stages Unique Features of Meiosis