1. S EXUAL R EPRODUCTION & M EIOSIS

2. S EXUAL R EPRODUCTION During Sexual Reproduction, two sex cells (sperm & egg) come together Sperm are formed in male reproductive organs Eggs are formed in female reproductive organs The joining of egg and sperm is called Fertilization The new cell that is formed is called a Zygote

3. D IPLOID C ELLS Your body forms two types of cells Body Cells & Sex Cells A human body cell has 46 chromosomes Each chromosome has a “mate” that is similar in size and has similar DNA The “mates” are paired into 23 pairs When cells have pairs of similar chromosomes they are called diploid.

5. H APLOID CELLS Sex cells do NOT have pairs of chromosomes Human sex cells have 23 chromosomes They are called haploid

7. M EIOSIS AND S EX C ELLS A process called meiosis produces haploid sex cells During meiosis, two divisions of the nucleus occur Meiosis I Meiosis II

8. M EIOSIS

9. M EIOSIS I Before the start of meiosis I, the chromosomes are duplicated just like mitosis Each set of duplicated chromosomes is called a chromatid

10. M EIOSIS I In Prophase I the chromatids are tightly coiled together, each duplicated chromosome comes near its mate. What are chromatids?

11. M EIOSIS I In Metaphase I, the pairs of duplicated chromosomes, or chromatids, line up at the equator. Each chromatid pair attaches to a _spindle at the centromere. What is a centromere?

14. Homologous Chromosomes

15. M EIOSIS I In Anaphase I the chromosome pairs are pulled to _opposite_sides of the cell. Each duplicated chromosome still has two _chromatids_.

17. M EIOSIS I In Telophase I the _cytoplasm divides and two new cells form. Each cell has one duplicated chromosome from each pair.

19. M EIOSIS II NO chromosome duplication occurs before meiosis II The two cells formed during meiosis I will begin meiosis II

20. M EIOSIS II In Prophase II the __chromosomes__ and __spindle reappear in each of the cells In Metaphase II the chromatid pairs move to the equator. Where is the equator? However, each centromere attaches to the spindleindividually.

23. M EIOSIS II In Anaphase II the __centromere divides and the _chromatids___ separate and move to opposite sides of the cell. Each _chromatid is now an individual chromosome.

26. M EIOSIS II In Telophase II the spindle disappears and the nuclear membrane forms. The cytoplasm divides and produces __4_ cells.

29. S UMMARY OF MEIOSIS During meiosis I, two cells formed During meiosis II, both of these cells formed two cells Four sex cells were formed Each sex cell has ½ the number of chromosomes that was in the original nucleus = 4 sex cells with 23 unpaired chromosomes

31. ZDONK Donkey (62 chromosomes) + Zebra(46 chromosomes) = Zonkey How many chromosomes does the Zdonk have? What happens when meiosis occurs in the Zdonk?

32. M ISTAKES IN M EIOSIS Mistakes in meiosis can produce sex cells with too many or too few chromosomes If the zygote survives, every cells in that organism will have the wrong number of chromosomes

33. Down Syndrome (Trisomy 21)