Meiosis and Sexual Reproduction Section 1 Section 2 Meiosis Sexual Reproduction

Formation of Haploid Cells Meiosis is a form of cell division that halves the number of chromosomes when forming gametes. Meiosis involves 2 divisions of the nucleus, (meiosis I and meiosis II) Before meiosis begins, DNA in the original cell is replicated. ( homologous chromosome)

Mitosis vs. Meiosis

Stages of Meiosis Prophase I - crossing over Metaphase I Anaphase I Telophase I (cytokinesis) Prophase II Metaphase II Anaphase II Telophase II See diagram on p

Fertilization and Meiosis

Genetic Variation Independent assortment random distribution of homologous chromos. Crossing over occurs when portions of a chromatid on one homolo. chromosome are broken and exchanged with other Random Fertilization - the random joining of two gametes

Importance of Genetic Variation 2 ways to achieve variation are mutation and sexual reproduction Variation is essential to evolution process The pace of evolution is sped up by genetic recombination Natural selection does not always favor genetic change - may favor existing combinations of genes, which slows evolution

Gamete Formation Spermatogenesis is the process in which sperm are produced in males animals Occurs in testes 4 haploid cells (sperm are formed) Oogenesis is the process in which gametes (ovum/egg) are formed in female animals Occurs in ovaries Only 1 haploid egg is formed (3 polar bodies)

Sexual & Asexual Reproduction Sexual reproduction - two parents each form reproductive cells that have one- half the # of chromosomes Offspring are genetically different from the parents Asexual reproduction- a single parent passes copies of all of its genes to its offspring (no fusion of gametes) Examples: fission/budding Clone - organism that is genetically identical to its parent

Haploid Life Cycle Examples: some fungi, algae, and other unicellular organisms Haploid cells give rise to haploid multicellular organisms that produce gametes by mitosis, the gametes fuse to produce a zygote, and then goes through meiosis ( page 152)

Diploid Life Cycle Examples: Humans, dogs, most animals Organism are diploid, have haploid gametes Gametes fuse during fertilization Diploid zygote is formed (more cells are made by mitosis)

Alteration of Generations Plants, algae, and some protists Life cycle that regularly alternates between haploid and diploid Diploid phase that produces spores (sporophyte) Spore - haploid reproductive cell that can develop into an adult without fusing w/ another cell (gametophyte) Gametophyte is the haploid phase that produces gametes by mitosis The 3 types of sexual life cycles differ from each other only in which phases become multicellular