The Cell Cycle.

Biology AQA new AS level unit 2, the cell cycle.

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Replication of DNA.

Cell division occurs in 2 stages: nuclear division - nucleus divides (mitosis and meiosis), and cell division - follows nuclear and whole cell divides. Before nuclear DNA must replicate so all cells have genetic info.

Semi-Conservative Replication:

  • Requirements: 4 types of nucleotides, each with bases, must be present; both strands must act as template; enzyme DNA Polymerase needed to catalyse; and a source of chemical energy.
  • 1. Enzyme DNA Helicase breaks hydrogen bonds linking bases.
  • 2. Leads to double helix separating to 2 strands.
  • 3. Each polynucleotide strand acts as template that complimentary nucleotides attach to.
  • 4. Energy activates nucleotides.
  • 5. Activated nucleotides joined by DNA Polymerase to form missing strand on each strand.
  • 6. Hydrogen bonds form.
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Division of nucleus leads to daughter cells having exact copy of DNA, expect in mutations. It is always preceded by a period when the cell is not dividing where DNA replicates.

  • 1. Interphase - protein synthesis, DNA replicates, chromosomes invisible.
  • 2. Prophase - chromosomes become visible, nuclear envelope disintegrates.
  • 3. Metaphase - chromosomes line up on equator.
  • 4. Anaphase - spindles contract, chromatids pulled to poles.
  • 5. Telophase- chromatids reach poles & become distinct, nucleolus and nuclear envelope reform, spindle disintergrates.

Importance of Mitosis:

  • 1. Growth - when haploid cells form diploid and diploid divides then all cells must be identical to this.
  • 2. Differentiation - cells change to become specialised which then divide. Tissues can only function if all cells have same structure and function.
  • 3. Repair - new cells must be the same as dead ones or tissue will not function.
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The Cell Cycle.

Cells don't divide continuously but undergo a regular cycle separated by periods of cell growth (cell cycle):

  • 1. Interphase - majority of cycle (resting phase).
  • a. 1st Growth Phase (G1) - proteins that cell organelles are synthesised from are produced.
  • b. Synthesis Phase (S) - DNA replicated.
  • c. 2nd Growth Phase (G2) - organelles grow and divide and energy stores released.
  • 2. Nuclear Division - nucleus divides into 2 (mitosis) or 4 (meiosis).
  • 3. Cell Division - follows nuclear and whole cell divides into 2 or 4.
  • Cancer:
  • Growth disorder of cells - damage to genes that regulate mitosis and cell cycle.
  • Uncontrolled growth so group of abnormal cells develop and expands.
  • Treatment blocks part of cell cycle so cell division ceases.
  • Drugs prevent DNA replicating (cisplatin) or inhibit metaphase as interfere with spindle formation. They disrupt normal cells too but are more effective with rapidly dividing cells.
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