Physiology

Meiosis Part I

Anisha ValliLeave a comment

HUMAN CELLS

  1. Somatic cells: majority of the body’s cells
  • 46 chromosomes – diploid (2n)
  1. Reproductive cells: sperm and egg cells (gametes)
  • 23 chromosomes – haploid (1n)
  • Diploid germ line cells: precursors for reproductive cells; undergo meiosis

FERTILIZATION

  • 1 egg and 1 sperm fuse to form zygote (2n)
  • Followed by repeated cycles of mitosis to produce multicellular organism (2n)

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

PROPHASE I

  • >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

METAPHASE I

  • Tetrads align on metaphase plate
  • Sister chromatids face same pole
  • Homologous chromosomes face opposite poles

ANAPHASE I

  • Homologous chromosomes separate

TELOPHASE I AND CYTOKINESIS

  • Two haploid daughter cells: 1 tetrad in each

PROPHASE II

  • Each cell has one duplicated set of 23 chromosomes

METAPHASE II

  • Sister chromatids line up on metaphase plate and face opposite poles

ANAPHASE II

  • Sister chromatids separate

TELOPHASE II

  • Nuclear envelope reforms
  • Nucleolus reappears
  • Mitotic spindles depolymerize
  • Cleavage furrow

CYTOKINESIS

  • 4 haploid daughter cells
  • Daughter cells genetically distinct from each other and parent cells
  • Each develops into reproductive cell (egg or sperm cells)

CLINICAL CORRELATION

Down’s Syndrome (Trisomy 21): aneuploid 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

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