Meiosis and Sexual Reproduction

1. Meiosis and Sexual Reproduction CHAPTER 7

2. MEIOSIS: • Gametes are formed during Meiosis • Gametes are haploid • Meiosis is the process of nuclear division that reduces the number of chromosomes by half. • In meiosis, two nuclear division take place instead of one producing 4 cells. • They are referred to as, Meiosis I and Meiosis II

3. STEPS OF MEIOSIS I Prophase I: • Same as in mitosis, but chromosomes line up next to a partner during a process called SYNAPSIS. • The chromosomes twist around and line up with each other to form a • TETRAD.

4. Genetic information within segments of DNA may be exchanged:CROSSING OVER

5. Metaphase I: The tetrads are moved by spindle fibers to the equator of the cell. The homologous pairs of chromosomes remain together

7. Anaphase I: • The homologous pairs of chromosomes are separated. • Each chromosome is still composed of two chromatids joined by a centromere

8. Telephase I: • The cytoplasm divides and forms two daughter cells. • Each has ½ the original # of chromosomes

9. STEPS OF MEIOSIS II Prophase II: If the chromosomes uncoiled, they are reformed from the chromatin New spindles form

10. Metaphase II: The chromosomes are moved to the cell equator Each is attached to spindle fibers, at the centromere

11. Anaphase II: The centromeres joining the chromatids divide, freeing the sister chromatids from each other They move to opposite poles within the cell.

12. Telophase II: • Spindles dissolve and nucleus reappears • Cytokinesis occurs after Telophase II. Division of cytoplasm

15. MITOSIS VS. MEIOSIS • Mitosis: • produces two exact copies of the original cell • daughter cells have same # of chromosomes as parent cell • process done in body cells • Meiosis • produces four cells • daughter cells have half the # of original chromosomes • genetic information can be exchanged - crossing over • process done in sex organs

16. Meiosis and Genetic Variation 3 occurrences account for genetic variation also called variability: • Independent Assortment • Crossing-Over • Random Fertilization Importance: “Essential for Evolution”

17. INDEPENDENT ASSORTMENT: The random distribution of homologous chromosomes • Each of the 23 pairs segregate independently • 2^23 Or 8 million gametes with different gene combinations

19. Crossing-Over and Random Fertilization • Adds to the different possible combinations from independent assortment • Unlimited number of genetic variations are possible

20. Meiosis and Gamete Formation: -Develop when the individual reaches PUBERTY Making of Egg Cells (oogenesis) : Form in the female OVARY • Creates two cells after first division - one VIABLE and one POLAR BODY(nonfunctioning). • Second division creates a total of 3 polar bodies and one viable ovum or more commonly known as egg.

22. Sperm: • Formed in the TESTES • One cell is able to create 4 new viable (usable) cells

25. CHANGES IN THE RATES OF MITOSIS: • -Mitosis rate can speed up or slow down… • -Two effects caused by a change in the rate of mitosis include…. AGING and CANCER • Aging: • -Process of becoming older • -Slows down mitosis ( hair, nails, etc )

26. The union of sperm and egg is called fertilization. • Producing offspring is called reproduction and can be asexual or sexual.

27. Plants go through the sexual life cycle described as Alternation of Generations

30. Liverwort has a gametophyte dominant life cycle

31. A spore is a 1n structure formed during the sporophyte stage of alteration of generations in plants. Budding is a type of asexual reproduction in which a new organism grows on another one. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical to the parent organism. Ex. Hydra and Corals

32. Cancer: • Disease in which the cells of the body reproduce at an unusually high rate. • Agents that cause cancer include; chemicals, radiation, and viruses. • Things that cause cancer are often referred to as CARCINOGENS