Cell Replication

• Mitosis

  • Cell division that results in two genetically identical daughter cells
  • Each genetically identical daughter cell has the same number and kind of chromosomes as the parent nucleus (diploid)
  • Required for growth, repair and asexual reproduction
• Diploid

  Cell containing two complete sets of chromosomes, one from each parent
• Haploid

  Cell containing a single set of unpaired chromosomes (eg. gametes)
• Homologous Chromosomes

  Sets of one paternal and maternal chromosomes that pair up with each other inside a cell containing identical genes
• Stages of Mitosis

  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase and Cytokinesis
• Interphase (Mitosis)


  • Period of cell growth and normal activity
  • DNA of the cell undergoes duplication during interphase
  • Creates two copies of each chromosomes (sister chromatids)
• Prophase (Mitosis)


  • First and longest stage
  • Chromosomes become visible under a strong light microscope
  • Centrioles separate and move to opposite poles of the cell
  • Earliest appearance of mitotic spindle fibres
  • Sister chromatids are joined to the centromere
• Metaphase (Mitosis)


  • Second stage
  • Chromosomes line up across the centre (plate) of the cell
  • Become connected to the spindle fibre at their centromere
  • Each of the sister chromatids are pulled to opposite ends of the cell
• Anaphase (Mitosis)


  • Third stage
  • Sister chromatids are pulled apart from each other
  • Separated into individual chromosomes
• Telophase (Mitosis)


  • Chromosomes gather at opposite ends of the cell
  • Lose their distinct rod-like shapes
  • Two nuclear membranes form around each set of chromosomes, which leads to the formation of the nucleolus
  • The spindle fibres disappear from the cell
• Cytokinesis (Mitosis)


  • Cell membrane pinches and divides the cytoplasm in half (cleavage furrow)
  • Results in two identical daughter cells, each containing a complete copy of the original DNA and all organelles
• Summary of Mitosis

  • The two original homologous chromosomes from the diploid parent cell make copies of themselves to form sister chromatids
  • Sister chromatids are joined in the centre by the centromere
  • Once duplication has occurred, spindle fibres draw one of each set of the sister chromatids to either end of the cell
  • Chromatids separate, spindle fibres break off and the cell separates
  • Results in two genetically identical (diploid) daughter cells
• Meiosis

  • Cell division that results in four genetically unique daughter cells
  • Each genetically unique daughter cell contains half the number of chromosomes (haploid) of the parent cell (diploid)
• Prophase I (Meiosis)


  • Homologous chromosomes make copies of themselves (duplicate)
  • Chromosomes come together and join the chiasmata
  • Alleles are exchanged from the crossing over of homologous chromosomes
  • Crossing over introduces variation
• Metaphase I (Meiosis)


  • Pairs of homologous chromosomes move to the middle of the cell
  • Independent assortment occurs (ensures variation)
  • Homologous chromosomes randomly end up on either side
• Anaphase I (Meiosis)


  • Random segregation occurs (ensures variation)
  • Chromosomes are segregated to either end of the poles of the cell
• Telophase & Cytokinesis I (Meiosis)


  • First cell division of meiosis
  • Results in two genetically unique haploid cells
  • Due to crossing over, independent assortment and segregation
• Prophase II (Meiosis)


  • Formation of new spindle fibres
  • Chromosomes will start to move towards the middle of the cell
• Metaphase II (Meiosis)


  • Sister chromatids have lined up along the middle of the cell
  • Random segregation occurs (ensures variation)
  • Chromatids are segregated at random towards each side of the middle
• Anaphase II (Meiosis)


  • Sister chromatids are randomly assorted along the middle
  • Segregated one to each side of the cell, being drawn by the spindle fibres
• Telophase & Cytokinesis II (Meiosis)


  • New nuclear envelope forms around each of the sets of chromosomes
  • Cleavage furrow forms along each of the sets of daugther cells
  • Results in the formation of four genetically unique (haploid) daughter cells
• DNA Replication

  • Double stranded DNA molecule, is 'unzipped' by enzymes
  • Free nucleotides bind to each of the exposed backbones
  • Creates two identical DNA molecules
• Helicase (DNA)

  Enzyme that separates strands of DNA
• Polymerase (DNA)

  Enzyme that creates the new DNA molecules by assembling nucleotides
• Scientific Model

  Simplified representation of a complex system or concept that is usually too small or too large to be seen with the naked eye
• Advantages of Scientific Models

  • Simplify complex concepts
  • Have predictive power
  • Lead to increased understanding and future research
• Disadvantages of Scientific Models

  • May not be accurate
  • Over simplification can be misleading
  • Lack detail
• Phoebus Levene (1919)


  • Discovered that nucleic acids (DNA and RNA) are composed of nucleotides
  • Each nucleotide is composed of a sugar molecule, phosphate group, and 1 of 4 nitrogen containing bases (Adenine, Thymine, Guanine, Cytosine)
• Erwin Chargaff (1950)


  • Amount of Adenine (A) = amount of Thymine (T)
  • Amount of Guanine (G) = amount of Cytosine (C)
• Watson and Crick DNA Model

  • Discovered in 1953 by Francis Crick and James Watson
  • Found that DNA is a double stranded, helical molecule
  • Consisted of two sugar-phosphate backbones on the outside, which were held together by hydrogen bonds between pairs of nitrogenous bases (ATCG) on the inside
  • Each strand of the DNA molecule was a template for the other
  • The two strands separate during cell division
  • Each strand a new "other half" is built, just like the one before
  • DNA can reproduce itself without changing its structure
• Properties of Mitosis

  • Stage that DNA replication occurs = Interphase (before mitosis)
  • Number of divisions = 1
  • Number of daughter cells = 2
  • Composition of daughter cells = Diploid, genetically identical
  • Role = Growth and repair
• Properties of Meiosis

  • Stage that DNA replication occurs = Interphase (before meiosis)
  • Number of divisions = 2
  • Number of daughter cells = 4
  • Composition of daughter cells = Haploid, genetically different
  • Role = Production of gametes, genetic diversity
• Importance of Cell Replication (Pt.1)

  • Genome of an individual is set from the point of fertilisation
  • Essential that the daughter cells are genetically identical in mitosis to ensure functional proteins are proteins are produced
  • Meiosis ensures genetic diversity which is essential for the survival of a species
• Importance of Cell Replication (Pt.2)

  • Mitosis and meiosis are essential for the continuity of sexually reproducing organisms
  • Meiosis ensures genetic variation within a species is maintained
  • Genetic variation is central to a species surviving changes to the environment
  • Mitosis ensures every single cell contains the same genetic information which is necessary for growth, repair and protein production