<p> Notes for week of February 29th</p><p>HUMAN CELLS 2. Somatic cells: majority of the body’s cells • 46 chromosomes – diploid (2n) 2. Reproductive cells: sperm and egg cells (gametes) • 23 chromosomes – haploid (1n) • Diploid germ line cells: precursors for reproductive cells; undergo meiosis</p><p>FERTILIZATION • 1 egg and 1 sperm fuse to form zygote (2n) • Followed by repeated cycles of mitosis to produce multicellular organism (2n)</p><p>INTERPHASE • Parent cell (2n): two sets of 23 chromosomes • Homologous chromosomes: contain the same genes in the same order, each from a different parent (contain different alleles) • S-phase: each set of 23 chromosomes duplicates (92 chromosomes total), sister chromatids pair at the centromere</p><p>PROPHASE I • >90% of meiosis • Chromosomes condense • Tetrad forms via synapsis: each gene aligns with its homologue (4 chromatids) • Synaptomenal complex: zipper-like structure holds chromosomes together until crossing over occurs • Crossing over: paternal chromosome crosses over to maternal and vice versa • Chiasma (site of crossing over) holds tetrad together after synaptomenal complex disassembles Other features of this phase: • Nuclear envelope fragments • Nucleolus disperses • Centrosomes move to opposite poles • Microtubules form spindle & attach kinetochores of homologous chromosomes</p><p>METAPHASE I • Tetrads align on metaphase plate • Sister chromatids face same pole • Homologous chromosomes face opposite poles</p><p>ANAPHASE I • Homologous chromosomes separate</p><p>TELOPHASE I AND CYTOKINESIS • Two haploid daughter cells: 1 tetrad in each</p><p>PROPHASE II • Each cell has one duplicated set of 23 chromosomes</p><p>METAPHASE II • Sister chromatids line up on metaphase plate and face opposite poles</p><p>ANAPHASE II • Sister chromatids separate</p><p>TELOPHASE II • Nuclear envelope reforms • Nucleolus reappears • Mitotic spindles depolymerize • Cleavage furrow</p><p>CYTOKINESIS • 4 haploid daughter cells • Daughter cells genetically distinct from each other and parent cells • Each develops into reproductive cell (egg or sperm cells)</p><p>CLINICAL CORRELATION Down’s Syndrome (Trisomy 21): neuploidy
gametes • Nondisjunction: chromosome 21 fails to separate properly during meiosis I • 2 daughter cells with extra chromosome 21 copy • 2 daughter cells missing chromosome 21 Trisomy 21: gamete with extra chromosome fuses with normal gamete during fertilization = zygote with 3 copies of chromosome 21 Fertilization Meiosis I and II: Drawing </p>