Meiosis

1. Meiosis

2. Meiosis • Meiosis is cell division that produces gametes. • Gametes are sex cells (sperm and eggs). • During meiosis, a cell divides twice after copying its DNA. • This reduces the number of chromosomes in each ‘daughter’ cell to half the original.

3. Meiosis is a part of Sexual Reproduction • Meiosis produces gametes. • In males, 4 sperm cells are produced. • They are equally small in size. • In females, 1 egg cell & 3 ‘polar bodies’ are produced. • The egg is larger in size. • Human gametes have 23 chromosomes.

4. Oogenesis: the making of an egg egg cell polar bodies

5. Spermatogenesis: the making of sperm cells

6. The phases of meiosis • Meiosis is a “reduction-division” process. • 1 ‘Diploid’ ends up as 4 ‘Haploid’ • The number of chromosomes per cell is cut in half. • Homologous chromosomes separate. • There are two parts to meiosis: • Meiosis I (similar to Mitosis) • Meiosis II

7. Remember… • Cell division is a very small part of a cell’s life cycle Copy the text part of this diagram onto the drawing found on the back of your worksheet.

8. Remember… Why cells are small • Diffusion limits cell size • Essential substances (nutrients, water, enzymes, wastes, etc.) must diffuse into and out of the cell. • Surface area to volume ratio • The volume increases faster than the surface area • If size of the cell doubles, the cell would require 8 times more nutrients • A small cell size insures that materials can get in and out quickly.

9. Remember… DNA is first copied during Interphase before either Mitosis or Meiosis occurs.

10. Remember… Complete the drawing on your worksheet with the terms below spindle fibers centrioles Prophase centromere nuclear envelope chromatin centrioles sister chromatids Mitosis spindle fibers Metaphase Daughter cells centrioles Telophase sister chromatids Anaphase nuclear envelope

11. Meiosis I • Meiosis I has four phases. They are: • Prophase I • Metaphase I • Anaphase I • Telophase I and cytokinesis

12. Prophase I • Each chromosome pairs with its corresponding homologous chromosome to form a tetrad (4 chromosomes). • This is the phase when crossing over occurs. • Crossing over is when tetrads exchange portions of their chromatids. • This creates new, unique chromosome combinations

13. Remember… • Chromosomes are made up of sequences of genes • Homologous chromosomes have the ‘same’ gene sequences from ‘mom’ & ‘dad’

14. Remember… • These ‘same’ genes are called alleles • Some may be chemically stronger and are called ‘dominant’. • Those not as strong are called ‘recessive’

15. Crossing over during Prophase I: Complete the drawing on your worksheet using the labels shown below. homologous chromosomes in a tetrad crossing over (gene jumping) later separation into gamete cells 2 sets of sister chromosomes from DNA replication a little bit of ‘mom’ & ‘dad’ end up in all cells from ‘mom’ from ‘dad’

16. Metaphase I • During metaphase I, the spindle fibers attach to the chromosomes. Prophase I Metaphase I

17. Anaphase I • During anaphase I, the fibers pull the homologous chromosomes toward opposite ends of the cell.

18. Telophase I and Cytokinesis • During this phase, the nuclear membranes reform and the cell divides. • After cytokinesis, meiosis II occurs.

19. Result of Meiosis I • Two daughter cells with ‘half’ the number of chromosomes (‘N’) as the original cell (‘2N’). • Homologous chromosomes, with their ‘crossed over’ genes in each cell • Each homologous chromosome still has the ‘sister’ formed when DNA was replicated (also possibly ‘crossed over’).

20. Meiosis II The four phases in Meiosis II are: • Prophase II • Metaphase II • Anaphase II • Telophase II

21. Prophase II • After a brief resting period, the nuclear envelope breaks down • chromatin again condenses into chromosomes • Spindle fibers reform daughter cells from meiosis I

22. Metaphase II • The chromosomes line up along the middle (equator) of the cell • Spindles attach to sister chromatids daughter cells from meiosis I

23. Anaphase II • The sister chromatids separate and are pulled toward opposite ends of the cell daughter cells from meiosis I

24. Telophase II and cytokinesis • During this phase, the nuclear membranes reform and the cell divides. • After cytokinesis, there are 4 haploid sex cells (gametes)

25. The difference between mitosis and meiosis • Mitosis results in two genetically identical diploid cells (2N) • Meiosis produces four genetically different haploid (N) cells.

26. The difference between mitosis and meiosis • Mitosis creates ‘clones’ • necessary for growth and repair of tissues in multicellular creatures • Prokaryotes (e.g. bacteria) reproduce this way • Meiosis produces ‘gametes’ • leads to greater diversity and chance for favorable mutations

27. Complete the drawing on your worksheet by copying the circled terms shown here Diploid cells (2N) Haploid cells (N)

28. Fertilization • Fertilization is the union of 2 gametes (sperm cell + egg cell). • ‘Haploid’ + ‘Haploid’ = ‘Diploid’ • The zygote is the fertilized egg. • It is the first cell of a new organism. • In humans, 23 + 23 = 46 chromosomes • homologous chromosomes come from each parent

29. Fertilization • Sperm (N) + Egg (N)  Zygote (2N)

30. Finire! (Finished!) • Double-check all blanks are filled in on your worksheet • Double-check all diagrams are labeled and color-coded • Double-check you answered the Essential Question (2-word answer from notes) • Place worksheet in notebook with “Genetics & Heredity” notes. Your next test will be over these concepts.