Meiosis & Mitosis

• Chromatin network: intertwined DNA strand only visible during interphase
• chromosome:

  • condensed piece of DNA
  • can be single stranded
  • can be double stranded

  
• chromatid:

  • formed after DNA replication
  • have two per chromosome

  
• Chromatids are formed after which process?

  DNA replication
• How many chromatids are there per chromosome?

  two
• What joins two chromatids?

  Centromere

  
• What is the term for two chromosomes carrying similar genes?

  Homologous pair
• Homologous pair:

  • two chromosomes carrying similar genes
  • one paternal and one maternal

  
• What is it called when homologous pairs move towards eachother?

  Synapsis
• What is the term for when homologous pairs are touching?

  Bivalent

  
• What is the point at which homologous pairs touch?

  Chiasmata
• What is the term for the gene location?

  locus
• What is the term for the thread that pulls chromatids/bivalent apart?

  spindle fibre
• What is a temporary organelle that houses centriole?

  centrosome
• Recombinant chromosome: when homologous chromosomes exchange genes and become different
• Meiosis:

  • Non-identical cells
  • 1 cell = 4 cells
  • Males: 4 sperm cells
  • Females: 1 ovum and 3 polar bodies
• Meiosis 1 stages:

  • chromosomes are visible (interphase)
  • double-stranded chromosomes
  • crossing over is taking place (chromosomes are touching)
  • homologous pairs line up at the equator
• Meiosis 2 stages:

  • recombinant chromosomes visible
  • no crossing over takes place
  • Single chromosomes line up at equator (no longer in homologous pairs)
• Where does meiosis occur?

  Ovaries or testes
• Why Meiosis occurs:

  
  

  To halve the chromosome number (haploid) in order to maintain the correct chromosome number and to introduce variation.
• Meiosis 1 purpose: To create 2 non-identical cells that are haploid with ONLY one of the homologous pairs present
• How many non-identical cells does Meiosis 1 create?

  2
• How many non-identical cells does Meiosis 2 create?

  4
• Meiosis 2 Purpose: to create 4 non-identical cells, with single stranded chromosomes.
• Is this image Meiosis 2 or Meiosis 1? Answer: Meiosis 1

  
• Is this image Meiosis 2 or Meiosis 1? Answer: Meiosis 2

  
• Cells in meiosis 2 will have:

  • no crossing over taking places
  • no homologous pair present
  • single-stranded chromosomes that may be recombinant
• Cells in meiosis 1 will have:

  • double-stranded chromosomes
  • homologous pairs crossing over
  • non-identical/recombinant chromosomes
• Where does meiosis take place in males?

  testes
• Where does meiosis take place in females?

  Ovaries
• Name TWO ways meiosis introduces variation.


  1. crossing over
  2. random arrangement
• You can only use the word chromatid when the centromere is still present.