2. MEIOSIS

3. CHROMOSOME NUMBER The number of chromosomes varies from one kind of organism to another Humans have 46 chromosomes & Fruit Flies have 8

4. Karyotype – a picture of chromosomes that shows the number and the size of chromosomes that a cell has Amniocentesis and Chorionic Villi Sampling– tests done on pregnant women in order to look at the chromosomes of the developing baby

6. Chromosomes of animal cells are usually paired. Humans have 23 pairs, fruit flies have 4 pairs. Haploid - the # of pairs or n Diploid - the total number of chromosomes or 2n For humans: n=23 and 2n=46

7. In diploid cells, each chromosome pair has the same basic structure. Two members of a pair are homologous (similar) and are called homologs. Each homolog carries messages for the same trait, although not necessarily the same message.

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

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

11. Sex cells are also called gametes. –Female sex cell = egg or ovum –Male sex cell = sperm Gametes are haploid (half the chromosomes).

12. After fertilization, the developing organism is called a zygote and is diploid.

15. MEIOSIS Meiosis converts a diploid cell into 4 haploid gametes. It increases genetic diversity in the offspring.

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

17. –The nuclear membrane disappears. –Spindle fibers begin to appear.

19. METAPHASE I »The homologous pairs line up in the middle of the cell.

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

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

25. MEIOSIS II Meiosis II is similar to mitosis. However, there is no interphase and almost no prophase.

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

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

30. TELOPHASE II –Both new cells complete division making four new cells.

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