2. MEIOSIS and Sexual Reproduction

3. Cell Division Simplified…

4. Types of Cells Body cells Body cells Sex cells Sex cells All cells contain chromosomes! All cells contain chromosomes!

5. Number of Chromosomes Any cell with the of chromosome pairs is called diploid Any cell with the total number of chromosome pairs is called diploid –2n –Humans  2n=46 chromosomes All BODY cells are diploid! All BODY cells are diploid!

6. The chromosomes of diploid cells come in pairs The chromosomes of diploid cells come in pairs Two members of a pair are considered homologous (similar) Two members of a pair are considered homologous (similar) –Are called homologs. Each homolog carries the message for the same trait, although not necessarily the same message. Each homolog carries the message for the same trait, although not necessarily the same message. Diploid Cells

7. Homologous Pair Example One homologous pair has a (gene) code for eye color. One homologous pair has a (gene) code for eye color. One homolog may code for brown and the other for blue. One homolog may code for brown and the other for blue. It is the combination of the two homologs that determines eye color. It is the combination of the two homologs that determines eye color.

8. Number of Chromosomes Any cell with of chromosomes pairs (or only 1 set) is called haploid Any cell with half the number of chromosomes pairs (or only 1 set) is called haploid –n –Humans  n=23 chromosomes Includes SEX cells Includes SEX cells –Also known as gametes

9. Making Haploid Cells We already learned mitosis is used to make diploid (body) cells We already learned mitosis is used to make diploid (body) cells Another process is used to make cells with only half of the genetic information (haploid cells)-- MEIOSIS!! Another process is used to make cells with only half of the genetic information (haploid cells)-- MEIOSIS!!

10. MEIOSIS Meiosis converts a diploid cell into 4 haploid gametes. Meiosis converts a diploid cell into 4 haploid gametes. Benefit of meiosis: Benefit of meiosis: –It increases genetic diversity in the offspring.

11. Stages of Meiosis Most stages are very similar to mitosis! Most stages are very similar to mitosis! –Also begins after the chromosomes have replicated (Interphase) Differs because it is broken down into 2 cells divisions: Differs because it is broken down into 2 cells divisions: –Meiosis I –Meiosis II

12. MEIOSIS I PROPHASE I PROPHASE I –The nuclear membrane disappears. –Spindle fibers begin to appear

13. MEIOSIS I PROPHASE I PROPHASE I –Homologous chromosomes pair up (a step unique to meiosis) –The two pairs of chromosomes (or four chromatids) are called a tetrad. Tetrad

14. METAPHASE I METAPHASE I  The homologous pairs line up in the middle of the cell. MEIOSIS I

15. ANAPHASE I ANAPHASE I –The pairs separate and the homologs move to separate poles. MEIOSIS I

16. TELOPHASE I TELOPHASE I –Each of the daughter cells is now haploid, but each chromosome still has two chromatids. MEIOSIS I

17. CYTOKINESIS I CYTOKINESIS I –Cytokinesis occurs and forms two new cells

18. MEIOSIS II Meiosis II is similar to mitosis. Meiosis II is similar to mitosis. However, there is no interphase. However, there is no interphase.

19. MEIOSIS II PROPHASE II PROPHASE II –A new spindle fiber forms

20. METAPHASE II METAPHASE II –The chromosomes line up in the middle of the two new cells. MEIOSIS II

21. ANAPHASE II ANAPHASE II –The chromatids separate and move to opposite ends of the two cells. MEIOSIS II

22. TELOPHASE II TELOPHASE II –Both new cells complete division making four new cells. –Cytokinesis occurs –The cells formed are gametes! 4 Haploid daughter cells are formed, each having only one chromosome of each homologous pair MEIOSIS II

23. Gametes Female: Egg or ovum Female: Egg or ovum –Occurs in the ovaries Male: Sperm Male: Sperm –Occurs in the testes

24. MEIOSIS END RESULT Meiosis Meiosis makes 4sex cells. Each Each cell has half half the number of chromosomes as the parent cell (haploid). Each Each cell has one chromatid from the original tetrad (one of each kind or size).

25. SEXUAL REPRODUCTION Fertilization- the combination of chromosomes (genetic information) from two sex cells Fertilization- the combination of chromosomes (genetic information) from two sex cells Each sex cell gives half of the total chromosomes. Each sex cell gives half of the total chromosomes.

26. After fertilization, the developing organism is called a zygote and is diploid. After fertilization, the developing organism is called a zygote and is diploid. –Haploid + Haploid= Diploid

28. Independent Assortment How do you get genetic diversity? How do you get genetic diversity? –By producing varied sex chromosomes –Notice your siblings are not identical even though the same two parents produced them! The arrangement of the chromosomes in Metaphase I is the key to this variation! The arrangement of the chromosomes in Metaphase I is the key to this variation!

29. How many different sex cells? Because there are many ways to arrange the chromosomes during Metaphase, the following math problem is used to calculate the number of DIFFERENT sex cells made: Because there are many ways to arrange the chromosomes during Metaphase, the following math problem is used to calculate the number of DIFFERENT sex cells made: –2 n –n=# of PAIRS (haploid #) – So….

30. Human Example Our haploid number is 23! Our haploid number is 23! –2 23 =8 million possibilities! What about the chances of a possible zygote during fertilization? What about the chances of a possible zygote during fertilization? –8 million x 8 million = 64 trillion

31. More Variation Other causes of varied offspring results from mom giving ½ the chromosomes and dad giving ½ Other causes of varied offspring results from mom giving ½ the chromosomes and dad giving ½ Though each parent gives the instructions for the same genes, the version is often DIFFERENT Though each parent gives the instructions for the same genes, the version is often DIFFERENT –Example: brown hair vs blonde hair