Chapter 12 Meiosis

1. Chapter 12Meiosis

2. Meiosis During sexual reproduction: a sperm and an egg unite form a new individual process called fertilization

3. Meiosis: nuclear division that precedes formation of gametes results in the halving of the chromosome number

4. Gene: section of DNA that influences one or more hereditary traits Different versions of a specific gene are called alleles

5. Chromosomes of the same type, with the same genes in the same locations, are homologous chromosomes (homologs).

6. Sex chromosomes: determine the sex of the individual all other chromosomes are = autosomes.

7. Karyotype: number and types of chromosomes present in an organism

8. haploid number: letter n stands for the number of distinct types of chromosomes in a given cell

9. During meiosis: the number of chromosomes is reduced by half

10. An Overview of Meiosis Meiosis consists of two cell divisions: meiosis I and meiosis II.

11. Prior to meiosis begins: each chromosome in diploid (2n) cell is replicated Once completed: each chromosome consists of two identical sister chromatids attached to centromere

13. In meiosis I: homologous chromosomes of each chromosome pair separate go to different daughter cells daughter cells are haploid each chromosome has two identical sister chromatids attached to centromere

15. meiosis II: sister chromatids of each chromosome separate go to each of two daughter cells. end of meiosis II: four haploid gametes each containing one copy of each chromosome.

16. Phases of Meiosis I Meiosis I is a continuous process with five distinct phases

18. Early Prophase I: homologous chromosome pairs come together (synapsis)

19. Late Prophase I: non-sister chromatids begin to separate Crossing over between homologous non-sister chromatids occurs

20. Metaphase I: pairs of homologous chromosomes line up at the metaphase plate

21. Anaphase I: paired homologs separate and migrate to opposite ends of cell Telophase I: homologs finish migrating to the poles of cell.

22. At end of meiosis I: cell divides to form two haploid daughter cells

23. Phases of Meiosis II At start of meiosis II: each daughter cell is haploid but each replicated chromosome is composed of two identical sister chromatids

25. Meiosis II also consists of five phases

26. Prophase II: spindle apparatus forms If nuclear envelope formed at the end of meiosis I� �it breaks apart

27. Metaphase II: Replicated chromosomes�consisting of two sister chromatids line up at the metaphase plate

28. Anaphase II: sister chromatids separate unreplicated chromosomes move to opposite sides of the cell

29. Telophase II: Chromosomes finish moving to opposite sides of the cell nuclear envelope forms around each haploid set of chromosomes

30. At end of meiosis II: each cell divides four haploid daughter cells are produced

32. A Closer Look at Key Events in Prophase of Meiosis I During synapsis in prophase I: the two pairs of sister chromatids are held tightly together both sets of non-sister chromatids may undergo crossing over and exchange segments

35. The Consequencesof Meiosis Chromosome shuffling and crossing over in meiosis I: four gametes whose chromosome composition is different from parent cell. Offspring have a chromosome makeup different from that of either parent

36. Chromosomes and Heredity Chromosomes contain the cell's hereditary material and are composed of genes.

37. How Does the Separation and Distribution of Homologous Chromosomes ProduceGenetic Variation?

38. The random separation (segregation) of homologous chromosomes: generates a great deal of genetic diversity in the subsequent gametes

39. The Role of Crossing Over Crossing over = genetic recombination produces new combinations of alleles on the same chromosome greatly increases the genetic variability of gametes

40. Why Does Meiosis Exist? Why Sex? Sexual reproduction common among multicellular organisms �but organisms in most lineages of the tree of life undergo asexual reproduction

42. The Changing-Environment Hypothesis assumes that "all other things are equal" when comparing asexual and sexual reproduction. However, if the environment changes from one generation to the next, this assumption is incorrect.

43. Offspring that are genetically different from their parents� � may be more likely to survive and produce offspring if the environment changes� �than offspring that are genetically identical to their parents (as in asexual reproduction).

44. Changing-Environment Hypothesis Studies support the changing-environment hypothesis: thus, sexual reproduction an adaptation that increases fitness of individuals in certain environments

45. In the life cycle of humans and most animals haploid cells are found only in production of gametes. Mitotic division of the diploid zygote generates all cells of the adult

46. Mistakes in Meiosis If a mistake occurs during meiosis I and the chromosomes from the parent cells are not properly distributed to each daughter cell� resulting gametes will contain an abnormal set of chromosomes. Down syndrome = an extra copy of chromosome 21.

47. How Do Mistakes Occur? The failure of homologous chromosomes to separate is called nondisjunction

49. If nondisjunction occurs in meiosis I� two gametes will have an extra copy of a chromosome (trisomy) two gametes will lack that chromosome completely (monosomy)

50. Nondisjunction may occur in as many as 10% of meiotic divisions� aneuploid zygotes (those with too few or too many chromosomes) typically do not survive to produce viable offspring

51. Why Do Mistakes Occur? Meiotic errors appear to be accidental, with no genetic predisposition