Chapter 7 General Biology

1. Chapter 7General Biology Meiosis and Sexual Reproduction

2. Meiosis • Meiosis: A form of cell division that halves the number of chromosomes. • Meiosis: A process that guarantees that he number of chromosomes in gametes is half the number of chromosomes in body cells • The cells resulting from meiosis in either males or females are called gametes • Meiosis – process by which a haploid reproductive cell is produced (eggs, sperms, spores) • Before meiosis begins, the DNA in the original cell is replicated. • Meiosis starts with homologous chromosomes.

3. The process by which sperm are produced in males animals is called Spermatogenesis • The process by which egg cells are produced in female animals is called Oogenesis • Spermatogenesis will produce four haploid (sperm) cells for every one parent cell • Oogenesis will only produce one haploid (ovum) cell for every one parent cell • Therefore meiosis in female animals does NOT result in the same number of ova as sperm that were produced by meiosis

4. Spermatogenesis occurs in the male reproductive organs (testes) • Oogenesis occurs in the female reproductive organs (ovary) • In most animals, including humans, meiosis produces sperm and egg cells. • The more common name for the ovum is EGG

5. Crossing-over is the exchange of corresponding portions of chromatids between homologous chromosomes • Crossing-over: A reciprocal exchange of corresponding segments of DNA • At the conclusion of crossing over, genetic recombination has occurred. • When crossing-over takes place, chromosomes exchange corresponding segments of DNA • The exchange of segments of DNA between members of a pair of chromosomes – acts as a source of variation within a species • Crossing-over occurs during prophase 1

6. Crossing-over, independent assortment, and random fertilization all provide NEW genetic recombination.

7. First Stages of Meiosis • Prophase 1 (crossing over occurs) • Metaphase 1 (Homologues line up along the equator of the cell) • Anaphase 1 (first separation of homologous chromosomes) • Telophase 1 ( results in two cells) NOTE: No replication of chromosomes between Meiosis 1 and Meiosis II

8. 2nd stage of Meiosis • Prophase II • Metaphase II • Anaphase II ( 2nd separation of homologous chromosomes) • Telophase II (result is four (4) haploid cells • ( results in 4 cells in males (sperm) • (Results in 4 cells in females but only ONE matures the other three are absorbed by the body)

9. The difference between anaphase of mitosis and anaphase I of meiosis is that the homologous chromosomes separate but the chromatids do not separate at the centromere in anaphase I

10. After a new nuclear membrane forms during telophase of meiosis, the cytoplasm divides. • During cytokinesis in the female, the cytoplasm divides unequally.

11. Asexual and Sexual Reproduction • The process of producing offspring is either a form of sexual or asexual reproduction • Three types of asexual reproduction • Budding ex. hydra • Fragmentation ex. Flat worm • Fission ex. Amoeba • Budding – • Fragmentation- • Fission- the separation of a parent into two or more individuals of about equal size (binary fission – bacteria/amoeba) – simplest and most primitive form of reproduction • Asexual – a single parent passes copies of all its genes to each of its offspring • An individual produced by asexual reproduction that is genetically identical to its parent is called a CLONE

12. The process by which sperm and egg cells join is called fertilization • fertilization of the haploid sperm cell and egg results in the restoration of the diploid number of chromosomes in the zygote • Fertilization – form of sexual reproduction • Haploid life cycle – simplest of sexual life cycles • Adult individuals are diploid • The gametes are the only haploid cells in the diploid life cycle; all of the other cells (body cells) are diploid

13. Alternation of GenerationsSporophyte and Gametophyte • Plants and some Protists have a life cycle that alternates between a haploid phase and a diploid phase. • Spore-forming cells in the sporophyte phase undergo meiosis to produce spores • This sporophyte phase is the diploid phase in the life cycle of plants. • During alternation of generations, cells reproduce by both mitosis and meiosis • A diploid spore forming cell will give rise to four haploid spores

14. Gametophyte Phase • In the life cycle of plants, the gametophyte is the haploid phase that produces gametes by meiosis.