Cell Division

• What do cells need to be copied for?

  • growth and repair of tissues (replacing cells) in multicellular organisms.
  • asexual reproduction in unicellular and multicellular organisms.
  • for producing gametes for sexual reproduction.
• Acellular organisms (like viruses) also need to copy themselves to infect more cells.

  can only reproduce when they infect host cells
• Cells with ability to divide:

  • stem cells (animals)
  • Meristem cells (plants)
• Bacteria (and other unicellular organisms) reproduce by binary fission
• Multicellular organisms (e.g animals/plants)

  • growth and repair of tissues (replacing cells) by mitosis
  • asexual reproduction by mitosis
  • gametes for sexual reproduction by meiosis. (weird rare cases when gametes are produced by mitosis).
  • Meiosis and mitosis only used when describing eukaryotic cells
• It's necessary to replicate DNA so that:

  • In binary fission / mitosis - both daughter cells have the same / identical DNA as the parent cell.
  • In meiosis - each of the four daughter cells have half the DNA as the parent cell.
• Binary fission occurs in prokaryotic/bacterial cells

  A) Cytoplasm

  B) Murein cell wall

  C) Cell membrane

  D) Circular DNA

  E) May also have plasmid

  
• Process of binary fission

  1. Cell volume increases.
  2. Protein synthesis occurs.
  3. More ribosome synthesis.
  4. DNA Replication occurs. (If there are plasmids, their DNA is also replicated.)
  5. The cell is divided into two (new cell membrane, cell wall etc are synthesised).
  6. Each new cell has one copy of the DNA from the original cell, ie, has identical DNA to the parent cell.
• Cell Cycle

  • Interphase
  • G1
  • Protein synthesis
  • ATP synthesis
  • Organelle synthesis
  • [volume of cytoplasm increases]
  • S-Phase
  • DNA replication
  • G2
  • Mitosis
  • Prophase, metaphase, anaphase, telophase, cytokinesis
• During interphase, DNA is present in form of Chromatin,

  it is decondensed. Allows DNA replication and transcription to occur easily
• During Mitosis, the DNA is present in the form of CHROMOSOMES, condensed. Allows equal amounts of DNA to be given to two daughter cells easily
• Interphase

  • Synthesis of proteins, organelles, ATP etc (G1 phase).
  • DNA Replication (S-phase).
  • DNA is present in the decondensed form, i.e., long & threadlike (chromatin).
• Prophase

  • Chromatin becomes condensed, i.e., visible into individual chromosomes.
  • Each chromosome consists of a centromere with two chromatids attached.
  • There are two chromatids with identical DNA sequence (sister chromatids) because DNA Replication has occurred.
  • The nuclear envelope starts to disappear.
  • Centrioles become prominent and move to opposite poles.
• Metaphase

  • Chromosomes line up along the equatorial plane of the cell.
  • The nuclear envelope is lost completely.
  • Spindle fibres attach from the centrioles to the centromere of each chromosome.
• Anaphase

  • Spindle fibres contract.
  • The centromere of each chromosome is split.
  • Sister chromatids are pulled towards the opposite poles of the cell.
• Telophase

  • New nuclear envelopes are formed (phospholipid synthesis).
  • Centrioles are replicated.
  • Chromosomes (which have one chromatid each) start to decondense.
• Cytokinesis

  • A cleavage furrow is formed.
  • Phospholipids are added to make new cell membrane to separate the two cells.
  • Chromosomes decondense and become thread-like (chromatin).
  • Two daughter cells, identical to each other and to the parent cell, are obtained.
• Differences in plant cell division w/ animal

  1. Plant cells do NOT have centrioles. (Spindle fibres are still made, just not connected to centrioles.)
  2. During cytokinesis, the cleavage plate moves from the centre outwards in plant cells, whereas it moves from the outside towards the centre in animal cells.
• Differences between mitosis and meiosis

  1. 2 daugher cells in mitosis, 4 daughter cells in meiosis
  2. 1 division in mitosis, 2 divisions in meiosis
  3. each daughter cell has same number of chromosomes as parent cell in mitosis, each daughter cell has half number of chromosomes as parent cell in meiosis
  4. daughter cells genetically identical to each other and to parent cell in mitosis, daughter cells genetically different from each other and from parent cell
  5. Function is growth, repair of tissues, and asexual reproduction in mitosis, function is to produce gametes for sexual reproduction in meiosis
• Mitotic index

  Mitotic index = Number of cells in mitosis / total number of cells
• Cancer

  • Result of uncontrolled mitosis
  • Usually, if DNA replication is incomplete or chromosomes have not been divided correctly, cell stays in interphase until corrected
  • IF it can't be corrected, cell is destroyed by immune system
  • If immune system hasn't destroyed such cells, cancer is the result
  • In cancer cells, there's a mutation that prevents system from staying in interphase until problem is corrected
  • Mutation causes cell to keep dividing
• Cancer treatments

  1. Surgery
  2. Radiotherapy
  3. Chemotherapy
  4. Target dividing cells. Kill cancer cells because they divide faster than normal cells, but they also kill normal dividing cells
  5. Immunotherapy