BIOLOGY 111 1st Edition Lecture 15 Outline of Last Lecture I Cell Division prokaryotes vs eukaryotes II DNA and Chromosomes III Cell Cycle IV Regulations V Checkpoints VI Cancer Cells Outline of Current Lecture I Meiosis II Sexual Reproduction III Phases of Meiosis IV Genetic Variation Current Lecture meiosis sexual reproduction o requires fertilization two haploid gametes one from mother one from father come together to create a diploid cell zygote o reduces the number of chromosomes in the gametes to one half the number of diploid cells Diploid cells two copies of each cell o each homologue is nearly identical in size and genetic composition it is very important that two divisions occur so that each cell results in half the number of chromosomes These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o mitosis and meiosis 1 starts with 46 replicates to make 92 cell divides in two with 46 each o meiosis 2 divides the 46 again so that there are 23 in each homologous chromosomes carry the same set of genes non homologous chromosomes are not the same or identical containing different genes replicated pair of the homologous pairs Karyotype map of chromosomes o chromosome 1 one from mother one from father o chromosome 2 one from mother one from father o and so on o in sex cells sexual reproduction o requires meiosis and fertilization o gametes sperm and egg are the only haploid cells in mammals different forms of sexual life cycles are different in how prevalent the haploid stage is o in plants when producing spores they are haploid they are diploid when sporophytes produce spores first because plants don t want to make gametes when the environment is not suitable spores are able to be transported various ways to environments that are suitable if not suitable plants keep them as gametophyte until it is suitable in which they are released as spores o Fungi and protists do not make diploid multicellular organism just haploid unicellular some multicellular haploid organisms Meiosis reduces chromosomes to the haploid number by performing two rounds of chromosome division without an intervening round of DNA synthesis and chromosome doubling o interphase start with a homologous pair of chromosomes in diploid parent cell these chromosomes replicate creating homologous pairs of replicated chromosomes each with 2 sister chromatids a diploid cell with replicated chromosomes o meiosis 1 homologous chromosomes separate producing haploid cells with replicated chromosomes o meiosis 2 the sister chromatids separate producing 4 haploid cells with unreplicated chromosomes Mitosis o prophase o metaphase align so that no recombination can occur no genetic variation o anaphase o telophase and cytokinesis Meiosis 1 o prophase non sister chromatids form a bivalent by a synaptonemal complex that forms between them crossing over chiasma may happen at this point in the bivalent when they begin to separate in division o all the other steps are the same as Mitosis o Metaphase chromosomes assort randomly when they align on the metaphase line leads to genetic variation o Anaphase pulls the non sister chromatids apart connections between bivalents break but not the connections between sister chromatids o Telophase and cytokinesis the original diploid cell had its chromosomes in homologous pairs but the two cells produced at the end of meiosis are haploid cells Meiosis 2 o there is no interphase 2 because we don t want synthesis again o prophase 2 same as above o metaphase 2 same as above o anaphase 2 sister chromatids are separated o telophase 2 and cytokinesis end up with 4 daughter cells each containing a random mix of 3 chromosomes Genetic Variation Recombination o occurs in the metaphase of meiosis o combinations in humans more than 8 million due to our 23 pairs o recombination between Random fertilization o family resemblance