AQA Biology Unit 2: 11 The Cell Cycle

• 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

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

• 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

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

• 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)

Cancer

• 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