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Meiosis
MEIOSIS
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
- \>90% of meiosis
- Chromosomes condense
- Tetrad forms via synapsis: each gene aligns with its homologue (4 chromatids)
- Synaptonemal 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 synaptonemal 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
- Tetrads align on metaphase plate
- Sister chromatids face same pole
- Homologous chromosomes face opposite poles
- Homologous chromosomes separate
- Two haploid daughter cells: 1 tetrad in each
- Each cell has one duplicated set of 23 chromosomes
- Sister chromatids line up on metaphase plate and face opposite poles
- Sister chromatids separate
- Nuclear envelope reforms
- Nucleolus reappears
- Mitotic spindles depolymerize
- Cleavage furrow
- 4 haploid daughter cells
- Daughter cells genetically distinct from each other and parent cells
- Each develops into reproductive cell (egg or sperm cells)
- 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