CH. 13.2 Sexual Reproduction and meiosis

1. CH. 13.2Sexual Reproduction and meiosis Mr. Perez

2. Important Vocabulary • Organism • Sexual reproduction • Sperm • Egg • Fertilization • Zygote • Diploid • Haploid • Meiosis

3. Sexual Reproduction • Sexual reproduction: two sex cells, usually called an egg and sperm, come together • Sperm: sex cells formed in the male reproductive organs • Eggs: sex cells formed in the female reproductive organs • When these two sex cells meet, it is called fertilization • Fertilization creates a new cell, called a zygote • After fertilization, cell division begins and a new organism with a unique identity begins to grow and develop

4. Diploid & Haploid cells • There are two types of cells in your body • Body cells- brain, skin, bones, and other tissues and organs • Contain 23 pairs of chromosomes (46 chromosomes) • Each chromosome has a mate that is similar in size, shape and DNA • Sex cells • Body cells are said to be diploid cells because they have a pair of similar chromosomes (2n) • Sex cells do not have pairs of chromosomes, so they are considered to be haploid cells (n)

5. Diploid & Haploid cells

6. Meiosis and Sex cells • Meiosis is the process which results in haploid sex cells • Meiosis ensures that the offspring will have the same diploid number as the parent • Two haploid cells unite and form a diploid zygote, which develops and grows into a diploid organism • During meiosis, two divisions of the nucleus occur • Meiosis I • Meiosis II • Steps of each division have names like those in mitosis

7. Meiosis

8. Meiosis I (Draw in your notes)

9. Meiosis I (Write Descriptions underneath drawing) • Before meiosis I, each chromosome is duplicated (like in mitosis) • Prophase I: each duplicated chromosome comes near its similar duplicated mate (not in mitosis) • Metaphase I: pairs of duplicated chromosomes (tetrads) lines up at the center of the cell • Centromere becomes attached to a spindle fiber • Anaphase I: chromatids DO NOT split apart, instead they move with their pair to the opposite end of the cell • Telophase I: cytoplasm divides and two new cells are formed • Each new cell has one duplicated chromosome of each similar pair

10. Meiosis II (draw in your notes)

11. Meiosis II (Write descriptions underneath drawing) • The two cells formed in meiosis I now begin meiosis II • Prophase II: duplicated chromosome and spindle fibers reappear in each new cell • Metaphase II: duplicated chromosomes move to the center of the cell • Each centromere attaches to TWO spindle fibers instead of one (unlike metaphase I) • Anaphase II: chromatids separate and move to opposite ends of the cell • Each chromatid is now an individual chromosome • Telophase II: spindle fibers disappear, nuclear membrane forms around chromosomes at each end of cell • Cytoplasm divides

12. Summary of meiosis • Meiosis I • Two diploid cells form (2n) • In humans, two cells with 23 pairs of chromosomes • Meiosis II • Four haploid cells form (n) • Each has one-half the number of chromosomes in its nucleus • In humans, four cells with 23 unpaired chromosomes

13. Meiosis animation • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html • Crossing over occurs in meiosis…this means that DNA is exchanged between homologous chromosomes ( prophase I) • This results in mixing of parental characteristics in offspring

14. Mistakes in meiosis • Mistakes are common in plants and less common in animals • Mistakes can produce sex cells with too many or too few chromosomes • Sometimes, the zygotes die • If the zygote lives, the organism with the wrong number of chromosome may not grow normally • Down syndrome is an example of a mistake in meiosis • Trisomy 21- there is an extra chromosome on the 21st chromosome pair

15. Karyotype (normal human)

16. Karyotype (down syndrome)

17. Mitosis vs. Meiosis • http://www.pbs.org/wgbh/nova/miracle/divi_flash.html

18. Resources • Florida Science Grade 8 Glencoe Science & McGraw Hill Publishing • Google Images