Lecture 7 - Comparing Mitosis & Meiosis

• Gametogenesis

  • The formation of sex cells during meiosis

  1. Oogenesis
  2. Spermatogenesis
• Oogenesis

  • It begins with a diploid cell called oogonium or oocyte cell 
  • One daughter cell (ootid) will receive the majority of cytoplasm during Meiosis to produce ONE viable egg cell. 
  • Other cells produced (polar bodies) will die and nutrients will be reabsorbed into the body.
• Oogenesis

  • The egg cell does not have to move around and thus carry all nutrients and organelles for future cell divisions if the egg becomes fertilized. 
  • Before birth the oogonium is produced in a  female by mitosis and will begin meiosis but stop at prophase 1 “Follicle cells”
  • Meiosis stages will continue for one follicle cell each month after puberty
• Spermatogenesis

  • The spermatocyte cell or spermatogonium will have equal divisions of cytoplasm and have much less cytoplasm.
  • Sperm are designed for movement, thus are streamlined and cannot carry excess weight
  • After meiosis 4 sperm cells will be produced
  • Males are able to do MANY Meiotic divisions to make spermatocytes (sperm) and then undergo meiosis to produce 1 billion sperm per day!
• • Sex Chromosomes ~ are the chromosomes that determine the sex of an individual. In humans they are on location 23. 
  • Females have XX chromosome
  • Males have XY chromosome
  • Autosomes ~ any chromosomes that are not sex chromosomes.
• Karyotypes

  • A picture of the chromosomes of an individual have been sorted and arranged according to size and type. 
• • Sometimes the Meiosis process can make mistakes.
  • For Example: Nondisjunction occurs when two homologous chromosomes move to the same poles. Thus one of the daughter cells is missing one chromosome while the other will retain an extra chromosome.
• • Cells that lack genetic information or have too much information will not function properly.
• • If a daughter cell of 24 joins another normal gamete cell (sex cell) the zygote will have 3 chromosomes in a location instead of a normal pair. ~ Trisomy 
• • If a daughter cell of 22 joins another normal gamete cell (sex cell) the zygote will be missing a chromosome in a location ~ Monosomy
• • Nondisjunction can occur during mitosis too, but the effects are more devastating in meiosis
• • We can determine if an individual is missing or has extra chromosomes by creating a Karyotype chart ~ picture of chromosomes arranged in homologous pairs
• Nondisjunction Disorders

  1. Down Syndrome

  • Trisomic disorder which a zygote receives three homologous chromosomes for chromosome pair number 21.
  • Common Traits: full face, enlarged and creased tongue, short height, large forehead, & mental lateness.
• Nondisjunction Disorders

  1. Turner Syndrome

  •  A monosomic disorder in which a female has a single X Chromosome 
  • Common Traits: Appear female but do not usually develop sexually and tend to be short and have thick widened necks and are sterile.
• Nondisjunction Disorders

  1. Klinefelter Syndrome

  • A trisomic disorder in which a male carries an XXY condition 
  • Common Traits: Sterile male, often long arms and legs and below normal intelligence