Abnormal Mitosis

• Nondisjunction:

  • Occurs when two homologous chromosomes move to the same pole during meiosis
  • One daughter cell has an extra chromosome, and the other is missing one chromosome
  • Can occur during anaphase in mitosis or meiosis, especially devastating in the later case
  • Produces gametes with 22 and 24 chromosomes, rather than 23
  • After fertilization with a normal (23-chromosome) gamete, the resulting zygote has 45 or 47 chromosomes
• Monosomy:

  • Single chromosome in place of a homologous pair
• Trisomy:

  • 3 homologous chromosomes in place of a pair
• Nondisjunction Disorders:

  • Down Syndrome:
  • -Trisomy
  • -Extra chromosome 21
  • -Risk increases with age of mother
  • Turners Syndrome:
  • -Monosomy
  • -Produces a sterile female with a single X chromosome (XO)
  • -Most are miscarried before the 20th week of pregnancy
  • Klinefelter Syndrome:
  • -Trisomy
  • -Produces a sterile male with two X and one Y chromosomes
  • -Some will have female characteristics
  • Patau Syndrome:
  • -Trisomy of chromosome 13
  • -Serious developmental problems
  • -Children rarely live more than a few months
  • Edwards Syndrome:
  • -Trisomy of chromosome 18
  • -Many organ system defects
• • -Most fetuses die before birth, or die within a month of being born
• Early Diagnosis:

  • Ultrasound
  • Amniocentesis: uses a needle to inject into the womb and extract amniotic fluid to test fetal DNA (risky procedure)
• Problems with Fertilization:

  • In Vitro Fertilization (IVF): Stimulation of ovaries using hormones, eggs are retrieved from ovaries, fertilized outside the body, and the embryo is transferred into the uterus
  • Artificial Insemination: Placement of sperm into the reproductive tract of a female
  • Assisted Reproductive Technologies (ARTs): Used to increase sperm and egg production (fertility drugs, selection)
• Abnormal Chromosomes Structure:

  • 4 main ways chromosomes can have abnormal structure during crossing-over process during meiosis 1
  • Deletion: Chromosome fragment fails to reattach to the homolog and is somehow lost
  • Duplication: Separated fragment reattaches to a complete homolog
  • Inversion: Segment reattaches to the correct homolog but in reverse order
  • Translocation: A segment of DNA attaches to a nonhomologous chromosome (a different chromosome, not part of the “pair”)