Meiosis + Nondisjunction

Cell Division

Meiosis is the formation of gametes (eggs and sperm).

• One parent cell → 4 genetically different daughter cells, each with half the number of chromosomes.
  
  

• Includes two cycles:
  
  

  • Interphase I
    
    

  • Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I)
    
    

  • Interphase II
    
    

  • Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II)
    
    

• Fertilization:
  
  

  • Sperm (23 chromosomes) + Egg (23 chromosomes) → Zygote (46 chromosomes) → Embryo → Adult
    
    

• Diploid (2n): 2 sets of homologous chromosomes = 46 total chromosomes
  
  

  • Found in most body cells
    
    

• Haploid (n): 1 set of chromosomes = 23 chromosomes
  
  

  • Found in gametes (sperm and egg)
    
    

Meiosis I - Prophase I

• Duplicated chromosomes condense.

• Nuclear membrane begins to fragment.

• Nucleolus disappears.

• Centrosomes form microtubules and move to opposite sides.
  
  

Prophase I & Crossing Over

• Homologous chromosomes pair up via synapsis.
  
  

• Crossing over occurs – segments are exchanged between homologous chromosomes.
  
  

• This creates genetic diversity.
  
  

Meiosis I - Metaphase I

• Nuclear envelope disappears.
  
  

• Centrosomes are at the poles.
  
  

• Microtubules attach to chromosome pairs.
  
  

• Homologous pairs align at the metaphase plate.

 Meiosis I - Anaphase I

• Cell elongates as microtubules stretch.
  
  

• Homologous chromosomes are pulled apart to opposite sides.
  
  

• Sister chromatids stay together, unlike mitosis.
  
  

Meiosis I - Telophase I and Cytokinesis

• Nuclear membrane reforms.
  
  

• Nucleolus reappears.
  
  

• Mitotic spindle disappears.
  
  

• Cytokinesis begins: cell splits into two haploid cells.
  
  

Interphase II (Interkinesis)

• Cells grow and prepare for meiosis II.
  
  

• Chromosomes remain condensed.
  
  

• No DNA replication occurs.
  
  

Meiosis II Overview

• Like mitosis, but in haploid cells.
  
  

• Prophase II: Chromosomes condense; spindle forms.
  
  

• Metaphase II: Chromosomes line up at the equator.
  
  

• Anaphase II: Sister chromatids separate.
  
  

• Telophase II & Cytokinesis: Four unique haploid cells form.
  
  

Mitosis vs. Meiosis 

Mitosis:

• Chromosomes duplicate → line up → sister chromatids separate
  
  

• One 2n parent → Two identical 2n cells

Meiosis:

• Meiosis I: Homologous chromosomes pair, cross over, and separate
  
  

• Meiosis II: Sister chromatids separate
  
  

• One 2n parent → Four different n cells
  
  

Genetic Variation

Genetic variation results from:

1. Crossing over (Prophase I)
   
   

2. Independent orientation (Metaphase I)
   
   

3. Random fertilization
   
   

Nondisjunction

• Chromosomes fail to separate properly during meiosis.
  
  

• Can occur in:
  
  

  • Anaphase I (homologous pairs don't separate)
    
    

  • Anaphase II (sister chromatids don't separate)
    
    

• Results:
  
  

  • Normal: 2 copies (46 total)
    
    

  • Trisomy: 3 copies (47 total)
    
    

  • Monosomy: 1 copy (45 total)
    
    

Nondisjunction Outcomes 

Normal Meiosis II: Produces normal gametes (n) Abnormal Meiosis:

• Gametes can be:
  
  

  • n + 1
    
    

  • n – 1

• Fertilization of these leads to trisomy or monosomy in zygotes
  
  

Karyotyping

• A karyotype shows magnified images of chromosomes arranged in pairs.
  
  

• Made from cells in metaphase.
  
  

• Useful to detect:
  
  

  • Homologous pairs
    
    

  • Chromosome count
    
    

  • Structural abnormalities
    
    

Chromosome Anatomy

Labels:

• Centromere
  
  

• Sister chromatids
  
  

• Homologous pairs
  
  

• Sex chromosomes
  
  

Down Syndrome (Trisomy 21)

• 3 copies of chromosome 21

• Symptoms:
  
  

  • Distinct facial features

  • Short stature

  • Heart defects

  • Higher risk of infections, leukemia, Alzheimer’s

  • Developmental disabilities

• Maternal age increases risk
  
  

Down Syndrome Data Graph: Risk of Down syndrome increases with mother’s age
(Notable increases after age 35)

Other Trisomies

• Trisomy 18 (Edwards syndrome):
  
  

  • Mental retardation
    
    

  • Clenched hands, rocker-bottom feet
    
    

  • Small head and jaw
    
    

• Trisomy 13 (Patau syndrome):
  
  

  • Severe defects
    
    

  • Polydactyly, fused eyes, small head
    
    

  • Babies rarely survive beyond a year
    
    

• Sex Chromosome Abnormalities

• XYY: Jacob’s syndrome (tall, possible autism/ADHD)
  
  

• XXX: Triple-X syndrome (tall, few symptoms)
  
  

• XXY: Klinefelter syndrome (low testosterone, infertility)
  
  

• XO: Turner syndrome (short, webbed neck, heart defects, sterile)