Meiosis reduces the number of chromosomes in cells from diploid to haploid, forming four haploid gametes from one diploid cell. This occurs through the processes of independent assortment, crossing over, and random fertilization, which contribute to genetic variation and allow for evolution. Meiosis produces either sperm cells in males through spermatogenesis or egg cells in females through oogenesis, where the cytoplasm unequally divides. Sexual reproduction then involves the union of haploid gametes from two parents to form a diploid zygote, resulting in offspring that are genetically different from the parents.

1. “ Meiosis and Sexual Reproduction” Chapter 7 Notes

2. I. MEIOSIS

3. A. Formation of Haploid Cells 1. Meiosis reduces the number of chromosomes by half to form gametes . 2. During fertilization the gametes unite to form a zygote and restore the original diploid number. 3. Result of meiosis is 4 haploid gametes from 1 diploid cell.

4. MEIOSIS

5. Picture of a sperm cell fertilizing an egg cell

6. B. Meiosis and Genetic Variation 1. Genetic variation is necessary for evolution to occur. 2. Three events during meiosis contribute to genetic variation: a. independent assortment – random distribution of homologous chromosomes during meiosis. b. crossing-over – exchange of genetic material between homologous chromosomes during meiosis c. random fertilization – random fertilization of an egg by a sperm

7. a. independent assortment

8. b. crossing-over

9. c. random fertilization

10. SEGREGATION & CROSSING OVER together make even more combinations See ananimation

11. http://www.tokyo-med.ac.jp/genet/anm/mimov.gi

12. C. Meiosis and Gamete Formation 1. Spermatogenesis – process by which sperm are produced in males. A diploid cell increases in size to become a germ cell , goes through meiosis , forms 4 haploid sperm cells.

13. C. Meiosis and Gamete Formation 2. Oogenesis – process by which an ovum (egg cell) is produced in females. A diploid cell increases in size to become a germ cell . Unequal division of the cytoplasm during meiosis results in a larger cell that becomes the egg and 3 smaller cells that become polar bodies and die.

15. II. SEXUAL REPRODUCTION

16. A. Sexual and Asexual Reproduction 1 . Asexual – formation of offspring from 1 parent. Produces clones that are genetically identical to parents. a. fission b. fragmentation c. budding Disadvantage of asexual reproduction is lack of genetic variation . (always produces a clone)

17. A. Sexual and Asexual Reproduction 2. Sexual – formation of offspring through union of gametes from 2 parents. Offspring are genetically different than the parents. 23 chromosomes 23 chromosomes 46 chromosomes

18. B. Sexual Life Cycles in Eukaryotes 1. Haploid life cycle – simplest of all life cycles. Haploid cell major portion of life cycle. Ex: algae and fungi 2. Diploid life cycle – adults are diploid and are major portion of life cycle. Only haploid cells are gametes . Ex: animals including humans 3. Alternation of Generations – some organisms such as plants have a life cycle that alternates between haploid and diploid phases .