AQA Biology Unit 2: 11 The Cell Cycle

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Replication of DNA

  • Cell division occurs in two main stages:
    • Nuclear Division: nucleus divides either in mitosis or meiosis
    • Cell Division: follows nuclear division, it is where the whole cell divides
  • Before a nucleus divides its DNA must be replicated
  • This makes sure daughter cells have genetic information to produce enzymes and other needed proteins

Semi-Conservative Replication

  • Has four requirements:
    • Four types of nucleotide must be present with their bases adenine, guanine, cytosine and thymine
    • Both DNA molecule strands must act as a template for the attachment of nucleotides
    • The enzyme DNA polymerase is needed to catalyse the reaction
    • A source of energy is required
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Replication of DNA Cont

Semi-Conservaive Replication Process

  • DNA helicase breaks hydrogen bonds linking base pairs
  • The double helix separates into two strands
  • Each exposed polynucleotide strand acts as a template to which complementary nucleotides are attracted
  • Energy used to activate the nucleotides
  • Activated nucleotides are joined together by DNA polymerase to form missing polynucleotide strand
  • Each new DNA molecule has one original DNA strand and one new DNA strand
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  • Division of the nucleus of a cell that results in each daughter cell having an exact copy of the DNA of the parent cell
  • Genetic make up of the two daughter nuclei is identical to the parent unless mutation
  • The period when the cell is not dividing is called interphase
  • Interphase: cell is actively synthesising proteins, chromosomes are visible and DNA replicates
  • Mitosis is divided into four stages:
    • Prophase: Chromosomes are visible, nuclear envelope disappears
    • Metaphase: Chromosomes arrange themselves at the equator of the cell, spindles form
    • Anaphase: The chromatids are pulled towards poles as the spindles contract
    • Telophase: Nuclear envelope reforms, spindles disappear and cell division commences
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Mitosis Cont

The Importance of Mitosis

  • It makes daughter cells identical to the parent cells
  • Growth: When two haploid cells fuse to form a diploid cell the diploid has all genetic information needed to from the new organism. All the cells must have same set of genetic information so it resembles its parents
  • Differentiation: Cells change to give specialised cells, the cells divide by mitosis to give tissues made of identical cells which perform particular functions. Essential for efficient functioning of cells
  • Repair: If cell is damaged or dies in needs to be replaced with a cell identical in structure and function. Without mitosis an identical cell would not be formed
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The Cell Cycle

  • Cells do not divide continuously, they have a cycle of division separated by periods of cell growth. The Cell Cycle which has 3 stages:
    • Interphase: most of cell cycle, no division occurs, divided into 3 parts:
      • G1 (first growth): Proteins from which cell organelles are synthesised are produced
      • S (Synthesis): DNA is replicated
      • G2 (second growth): Organelles grow and divide, energy stores are increased
    • Nuclear Division: Nucleus divides by mitosis or meiosis
    • Cell Division: Whole cell divides into two (Mitosis) or four (meiosis)


  • Caused by growth disorder of cells
  • Result of damage to the genes that regulate mitosis and the cell cycle
  • Leads to uncontrolled growth of cells, forming abnormal cells known as a tumour
  • The tumour can expand
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