Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Making Haploid Gametes Both chromosomal problems (monosomy/trisomy & breakage) are result of improper haploid cell formation/division in process known as meiosis Two phases: meiosis I & meiosis II Goal: 2n (diploid) n (haploid) Body or somatic cell division is called mitosis, which generates the EXACT same cell Mitosis: cell division which produces cells containing SAME number of chromosomes as parent’s cell ○ Start: One 2n skin cell ○ End: Two 2n skin cells Gamete or sex cell n division is called meiosis, which generates cell with HALF number of chromosomes as original Start: One 2n cell End: Four n sperm (spermatogenesis) or egg (oogenesis) cells Video of Mitosis vs. Meiosis n n oogenesis 2n spermatogenesis n MITOSIS MEIOSIS Growth & repair In body/somatic cells Making gametes In sex cells/gametes One 2n cell Two 2n cells One 2n cell Four n cells One diploid two diploids One diploid four haploids Exact copy of parent Half copy of parent 2n 2n 2n 2n n n n n Steps of meiosis Two repeating stages with precursor stage: Interphase Meiosis I Meiosis II MEIOSIS II – 2n n MEIOSIS I = 4n 2n 1. 2. 3. 4. Interphase – DNA replication (2n 4n) Prophase I – homologous chromosomes visible; crossing over occurs to add genetic variety Metaphase I – homologs move to equator Anaphase I – homologs move to opposite poles Telophase I – 2n sets move to poles; cytokinesis Prophase II – new spindle forms around chromosomes 6. Metaphase II – chromosomes move to equator 7. Anaphase II – centromeres divide; n chromatids to poles 8. Telophase II – n sets move to poles; cytokinesis 5. Interphase Doubling of chromosomes (2n 4n) Cell looks same Nucleus is darker because of chromosomal duplication AA aa B B b b “tetrad” CC cc A Aa a BB b b CC c c Prophase I centrioles spindle fibers Nuclear envelope disappears Spindle fibers form from centrioles Chromosomes in homologous pairs Crossing-over occurs (genetic diversity!) A A a a Same gene, A aA a BB b b CC c c BB b b CC c c different variety Crossing over Homologous pair Nuclear envelope Metaphase I Homologs move to equator How they line up varies (genetic diversity!) = law of independent assortment Spindle fibers attach to centromeres Anaphase I Spindle fibers contract Homologs pulled to poles Anaphase Mistakes What happens when anaphase goes wrong? If tetrads are not pulled apart evenly, chromosomal abnormalities result Trisomy (extra chromosome Monosomy (missing) Telophase I Chromosomes go to poles Cytokinesis occurs; cells split in two 4n 2n (halfway to goal!) Prophase II New spindle fibers form 2n (2 sets of 23 = 46) Metaphase II Same as metaphase I Homologs move to equator Spindle fibers attach to centromeres Anaphase II Sister chromatids pulled apart at centromere Chromatids move to opposite poles Anaphase Mistakes – A Second Chance What happens when anaphase goes wrong? If chromatids are not pulled apart evenly, chromosomal abnormalities result Trisomy (extra chromosome Deletion/Monosomy (missing) Telophase II Cytokinesis takes place, splitting two cells into four (sperm or eggs) 2n n Meiosis Overview Watch Meiosis Video Overview Precursor cell 2n diploid INTERPHASE 4n polyploid MEIOSIS I 2n 2n diploid MEIOSIS II n n n n haploid