3.2.2

• Not all cells retain the ability to divide
• Retaining the ability to divide shows a cell cycle
• Cell cycle has 4 stages

  • G1: Cell growth; new organelles and proteins are made
  • S phase: DNA replicates
  • G2: growth and proof reading
  • Mitosis: Cell division
• G1 + S + G2 = Interphase
• Many eukaryotic cells lose the ability to divide as they become specialised for specific functions through differenciation
• Interphase

  Cell copies its chromosomes by DNA replication

  • Cell grows and prepares to divide
• Prophase

  Chromosomes are visible though an optical microscope

  • Chromosomes coil; becomes shorter and fatter
  • Nuclear envelope disappears
  • Protein fibres form spindles in cell.
• Anaphase

  Centromere holds each pair of sister chromatids

  • Centromere divides
  • Spindle fibres contract; pulls sister chromatids to opposite poles of the cell
  • Chromatids now can be referred to as chromosomes
• Telophase

  Two sets of separated chromosomes collect at opposite ends of the cell

  • New nuclear envelope forms around each set
  • Chromosomes become long and thin
  • Cytoplasm divides to form two new cells
  • No longer visible through an optical microscope
• Mitosis

  Part of cell cycle which a eukaryotic cell divides to produce two daughter cells

  • Each with identical copies of DNA produced by parent during DNA replication
• Mitosis is a controlled process

  • Uncontrolled cell division = cancer/tumors
• Mitosis is essential for growth, repair and asexual reproduction.
• Mitosis produces identical daughter cells for growth and asexual reproduction
• All cells produced by mitosis are genetically identical

  • Does not give rise to genetic variation
• Cytokinesis

  Parent and replicated organelles move to opposite sides of the cell

  • Cytoplasm divides; produces two daughter cells
• Binary Fission

  Only occurs in prokaryotic cells

  1. Circular DNA in cells replicates
  2. Both copies attach to the cell membrane.
  3. Plasmids also replicate
  4. Cell membrane begins to grow between the two DNA molecules and pinches inwards; divides cytoplasm in two
  5. New cell wall forms between the two DNA molecules. Identical daughter cells have single copy of circular DNA and a variable number of plasmid copies.
• Viruses are non-living, do not undergo cell division


  Injects their nucleic acids into another cell

  • Infected host cell replicates virus particles
• Haploid

  Cells with a set of unpaired chromasomes
• When chromosomes are replicated; the replicated pairs are called sister chromatids.
• Sister chromatids are held together by the centromere
• First mitotic division

  Diploid cell divides to form two haploid cells with double the amount of DNA
• Second mitotic division

  Each cell from first mitotic division divides to form two other haploid cells wth the correct amount of DNA.
• Crossing over

  The homologous pairs line up

  • The chromatids synapse (cross over)
  • Forms a chiasmata; holds the chromosomes together
  • Allows the crossing over and exchange of alleles
• Crossing over in Meiosis 1 creates new genetically different chromosomes.

  • Means of genetic variation.
• Non-disjunction is the failure of homologous chromosomes to separate during meiosis
• Metaphase

  The chromosomes align Along the equator of the cell.

  • Spindle fibres released from the poles now attach to the centromere and chromatid.
• Binary Fission

  Circular DNA molecules replicates

  • Both copies attach to cell membrane

  Plasmids replicate

  Cell membrane begins to grow between the two DNA molecules and begin to pinch inwards

  • Divides cytoplasm into two

  New cell wall forms between the two DNA molecules

  • Divides original cell into two daughter cells
  • Each with a single copy of circular DNA
  • Variable number of copies of plasmids.