Nondisjuncion

• Gamete formation:

  • At the end of meiosis, spermatids are formed which differentiate into sperm cells
• Sperm formation (spermatogenesis):

  • Diploid SPERMATOGONIA → primarySPERMATOCYTES → 4 SPERMATIDS
• Egg formation (oogenesis):

  • Diploid OOGONIA → primary OOCYTES → 1 EGG + polar bodies
  • The cells stay in the undifferentiated stage until puberty starts - then they differentiate to the appropriate sex cell
• Mistakes in Meiosis:

  • Can occur when chromosomes do not separate properly during meiosis
  • Results in abnormal pairing during meiosis
• Zygote will have abnormal number of chromosomes: ANEUPLOIDY (45 OR 47)
• Nondisjunction:

  • HOMOLOGOUS CHROMOSOMES fail to move apart properly during meiosis I
  • OR
  • SISTER CHROMATIDS don’t separate during meiosis II
• Monosomy: FERTILIZED CELL IS MISSING A CHROMOSOME
• Trisomy: FERTILIZED CELL HAS AN EXTRA COPY OF A CHROMOSOME
• Nondisjunction Disorders:

  • Results in cells not being able to function properly
  • Responsible for a lot of human genetic disorders
  • Eg. DOWN’S SYNDROME (trisomy 21): result of nondisjunction of chromosome 21. The offspring receives 3 copies instead of 2. THE KARYOTYPE IMAGE SHOWS THE TRISOMY ON CHROMOSOME 21
  • An excess of genetic information causing people with this disorder to have mental and physical differences/challenges
• Diagnosing Nondisjunction Disorders:

  • Karyotyping can be used to diagnose chromosomal abnormalities prenatally
  • Cells for testing can be collected from the amniotic fluid (amniocentesis) or from villi along the lining of the uterus (chorionic villus)
• Abnormal Chromosome Structure:

  • These occur during the crossing over process
  • Deletion: A Chromosome fragment fails to reattach properly to the chromosomes involved
  • Duplication: If a segment reattaches to a complete homologue
  • Inversion: If segment reattaches to correct homologue but in reverse order
  • Translocation: Segment attaches to a nonhomologous chromosome