Genetics BIOL 2120

Prophase I:
    1. Leptonema
         a. lepto - thin
          b. Appear as thin threads - can’t distinguish between sister chromatids
         c. tips of chromosomes attached to nuclear membrane
    2. Zygonema
         a. zygo - yoke shaped: pairing of homologous chromosomes
         b. synapsis - proteins zip up chromosomes
              i. synaptonemal complex made of proteins
              ii. point to point pairing of sister chromatids
              iii. as the chromosomes zip up, there are Y shaped junctions - hence yoke shaped = zygo
              iv. each chromosome consists of a ‘bivalent’ - two sister chromatids zipped together
         c. Once all chromosomes are zipped up, zygonema is over
    3. Pachynema
         a. pachy - thick
         b. Chromosomes are shortening and thickening
         c. crossing over occurs here
         d. increases genetic variability
    4. Diplonema
         a. diplo - double
         b. visualize sister chromatids because finally short and thick enough
         c. called tetrads at this stage (can see 4 strands per homologous pair)
         d. chiasmata - X shaped places where crossovers occur
         e. rest of synaptonemal complex disappears
         f. chiasmata help hold tetrads together
         g. can decondense and express at this point
         h. point at which human female reproduction stalls from fetus to puberty
    5. Diakinesis
         a. Dia = across [diagonal]
         b. maternal and paternal chromosomes intertwine
         c. So tightly condensed, crossover sometimes move down to ends of chromosomes

For questions, comments and additional information, contact mfhicks@pstcc.edu
Last Updated: August 31, 2001
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