Week 3

• Steps of scientific inquiry


  • Observation
  • Hypothesis
  • Experiment
  • Experimental result
• Eukaryotic Cell Division


  Division of nucleus - Mitosis

  Asexual reproduction in single-celled eukaryotes and somatic cells.

  Division of cytoplasm - Cytokinesis

  Gamete formation - Meiosis

  Includes two rounds of nuclear division.

  Produces haploid gametes (sperm and eggs)

  
• Prokaryotes divide by binary fission
• There are similarities to mitosis (DNA replication, segregation of DNA and division of one cell into 2 cells) but eukaryotic cell division is more complicated

  
• How does mitosis fit into the eukaryotic cell cycle?


  Cell growth and division in eukaryotes proceeds through a number of steps that make up the cell cycle.

  Mitosis occurs during M phase after G1, S and G2 phase.

  
• Jargon (vocab)


  Two protein complexes called kinetochores are associated with the centromeres, one on each side of the centromere.

  Each kinetochore is associated with one of the two sister chromatids and forms the site of attachment for a single microtubule.

  This arrangement ensures that each sister chromatid is attached to a microtubule radiating from one of the poles of the cell.
• Eukaryotic Cell Division: Where does mitosis occur?


  Somatic Cells

  Non-reproductive cells

  Diploid (each chromosome is a pair) or in some organisms polyploid

  Many different types

  Divide by mitosis

  
• A human somatic cell has 23 pairs of homologous chromosomes. There are 46 chromosomes in total as shown in this karyotype.
• Phases of Mitosis - Prophase & Prometaphase

  
• Phases of Mitosis - Metaphase & Anaphase

  
• Cytokinesis differs when comparing animal and plant cells


  In animal cells, a contractile ring forms against the inner face of the cell membrane. This ring contracts and pinches the cytoplasm of the cell which divides it in two.

  Plant cells have a cell wall and divide via constructing a new cell wall (cell plate) in the middle of the dividing cell. Once the cell plate is large enough, it fuses with the original.

  
• Mitosis - Telophase & Cytokinesis
• What needs to occur for eukaryotic somatic cells to divide?


  • DNA needs to replicate. (S phase cell cycle)

  DNA needs to condense into chromosomes with sister chromatids.

  Sister chromatids need to attach to the spindle and align in the center of the cell.

  Sister chromatids need to separate and travel to opposite poles.

  A new nuclear membrane and a cell plate (plant cells) re-forms and chromosomes decondense.

  Product: Two genetically identical nuclei and daughter cells.

  
• This is mitosis summarized.

  
• This is the edge of the cell membrane where vesicles can fuse and the proteins inside them can be released or integrated into the membrane
• Human cells have 23 homologous chromosomes, 46 total and each chromosome has 2 sister chromatids
• Gametes


  • Reproductive cells
  • Products of meiosis in germ cells also called the germline
  • Haploid (one copy of each homologous chromosome). 23 chromosomes in human gametes
• In this karyotype are the chromosomes for a somatic human cell. In the resulting gametes, 1 gamete will have 1 chromosome from each pair as circled above
• All normal nucleated human somatic cells will have 46 chromosomes (diploid) in total. 22 pairs and 2 sex chromosomes
• Mitosis vs. Meiosis


  • DNA synthesis (S phase) required prior to both
  • Mitosis occurs in all eukaryotes and meiosis occurs in most eukaryotes
  • Pairing of homologous chromosomes is unique to meiosis I
  • Meiotic II daughter cells are haploid and genetically unique

  
• Meiosis I: Homologous pairs separate. Cells go from diploid to haploid


  Meiosis II: Sister chromatids separate, as in mitosis. Cells start and end haploid

  
• Prophase I (Meiosis I): Homologous Chromosomes Pair and Crossover


  1. The chromosomes (diploid) continue to shorten and thicken and the chiasmata (crossover) between non-sister chromatids become apparent

  2. The nuclear envelope begins to break down. Paternal and maternal genetic material has been exchanged

  
• • DNA replication has occurred and each chromosome has become two sister chromatids with one centromere
  • Chromosomes are condensed, shortening and thickening to allow for crossing over
  • Synapsis: Homologous chromosomes (same set of genes) pair and form a bivalent
• Crossing Over Increases Genetic Diversity


  • Homologous chromosomes
  • Recombinant chromatids
  • Bivalent
  • Chiasma like "Chimera" (Crossover)
  • Sister chromatids
  • Non-sister chromatids
  • Paternal homolog
  • Maternal homolog
• Metaphase I: Further increases genetic diversity


  1. Metaphase I: Homologous pairs line up in center of cell, with bivalents oriented randomly (paternal and maternal homologs on either side) with respect to each other

  2. Prometaphase I: Meiotic spindles attach to kinetochores (associated with the centromere) on the homologous chromosomes

  
• Anaphase I and Telophase I: Homologs separate and remain condensed for prophase II


  1. Anaphase I: Homologous chromosomes (homologs) separate, but sister chromatids do not separate

  2. Telophase I and cytokinesis: Daughter cells are ready to move into prophase II. Each human cell has 23 chromosomes (haploid) that remain mostly condensed

  
• Meiosis II
• • Mitosis and meiosis II are similar (prophase, metaphase, etc.. and chromatid separation)
  • Chromosomes from telophase I condense, spindles attach, align and chromatids separate
  • Cells at the end of meiosis II are haploid gametes (eggs or sperm)
  • Cells at the beginning of meiosis have the same number of chromosomes as cells at the end

  
• Nondisjunction results in gametes with an extra chromosome and some that are missing a chromosome
• First-division nondisjunction


  Homologs don't properly separate
• Second-division nondisjunction


  Chromatids/Chromosomes don't properly separate
• Nondisjunction for 1 homologous pair (1st division) and 1 pair of sister chromatids (2nd division)
• • 1st division and 2nd division nondisjunction are shown from left to right and indicated via orange arrows
  • The number of chromosomes per cell is indicated