3.4 All Cells Arise From Other Cells

• Roles of Mitosis:

  • Growth of tissues
  • Repair of tissues
  • Asexual reproduction
  • Development of body plan
  • Proliferation of white blood cells (clonal expansion)
  • Production of new stem cells
• Specialised cell can't divide - only stem cells (unspecialised) can divide.
• The majority of the cell cycle is in interphase.
• The stages of the cell cycle are:

  1. G1 - Growth 1
  2. S - Synthesis
  3. G2 - Growth 2
  4. Mitosis
  5. Cytokinesis
• Growth phase 1 involves protein synthesis/transcription/formation of mRNA.
• In G1, organelles replicate and ATP is required.
• In synthesis, DNA replicates in semi-conservative replication and ATP is required.
• In G2, the cell grows.
• To prevent mistakes/mutations, the cell can only progress through the cell cycle by passing particular checkpoints. The cell cycle pauses until problems are fixed.
• Cell Growth Checkpoint:

  • occurs towards the end of G1
  • checks whether the cell is big enough and has made the proper proteins for the synthesis phase
  • if not, the cell goes through a resting period (G0) until it is ready to divide
• DNA Synthesis Checkpoint:

  • occurs during the synthesis phase
  • checks whether DNA has been replicated correctly
• Mitosis Checkpoint

  • occurs during mitosis
  • checks whether chromosomes are attached correctly to the spindle
• Cancer ----> uncontrolled cell division leading to tumours.
• Cancer is caused by mutations in genes that control mitosis/cell cycle.
• Mutant cells generally die. Those that survive clone themselves and form tumours.
• Explain how errors in the cell cycle can lead to cancer.


  DNA can be replicated incorrectly and chromosomes can be incorrectly attached to the spindle, leading to uncontrolled cell division and tumours(cancer).
• Examples of cancer drugs are Methotrexate and Vinblastine.
• Methotrexate stops the replication of DNA. It work on normal fast dividing cells too, such as hair cells, which is why people who take it lose their hair.
• Vinblastine inhibits the metaphase stage of mitosis by interfering with spindle formation. The side effects include hair loss.
• In cytokinesis, the cytoplasm splits, producing 2 new genetically identical daughter cells.
• The stages of mitosis are: prophase, metaphase, anaphase, and telophase.
• In prophase, chromosomes condense and become visible. Centrioles move to opposite poles of the cell and start to form spindle fibres. Nucleolus disintegrates. The nuclear membrane breaks down.
• In metaphase, the chromosomes line up at the metaphase plate/cell equator. Spindle fibres attach to the centromeres.
• In anaphase, centromeres split and spindle fibres contract and pull sister chromatids to opposite poles of the cell.
• In telophase, the nuclear envelope reforms, the chromosomes decondense, and spindle fibres disappear.
• 2 sister chromatids - identical strands that have been replicated.
• The centromere holds chromatin together.
• Unreplicated homologous chromosomes undergo semi-conservative replication to produce a replicated homologous chromosome.
• In interphase, the cell undergoes growth 1, synthesis, and growth 2.