Cell Division

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  • Cell Division
    • 'A process by which a cell splits to form daughter cells'
    • Two types of cell division
      • Mitosis
        • In mitosis the cell divides once to produce two diploid daughter cells that are genetically identical to the parent cell
        • In mitosis dividing cells undergo a regular pattern of events called the cell cycle
        • Uncontrolled mitosis can lead to the formation of groups of cells called tumours
          • Benign tumours are slow growing and harmless
          • Malignant tumours are cancerous and can spread to other tissues
      • Meiosis
        • In sexual reproduction meiosis is used to form gametes containing half the number of chromosomes
        • In meiosis, the cell divides twice to produce 4 daughter cells which are genetically different to the parent with half the number of chromosomes
        • It is important that the daughter cells only have half the number of the parents chromosomes to prevent doubling of the chromosomes when the gametes fuse in sexual reproduction
    • Chromosomes
      • Made up of DNA, protein and a small amount of RNA
        • DNA occurs as a single strand in the form of a double helix running the length of the chromosome
          • Each DNA molecule is made up of many sections called genes
      • It is only at the onset of cell division that chromosomes become visible
        • Just before cell division starts each DNA molecule makes a copy of itself
          • The single thread of DNA becomes two identical threads. These are called chromatids. They are joined at the centromere
      • Replicated Chromosome
        • Replicated chromosomes are two strands of identical chromosomal material called chromatids (sister chromatids)
      • Humans always have 46 chromosomes
        • Chromosomes are found in matching pairs called homologous pair. Humans have 23 homologous pairs of chromosomes
          • The total number of chromosomes is called the diploid number
          • Gametes have half the diploid number, called the haploid number. Human gametes have 23 chromosomes
    • Cell Cycle
      • 1. Interphase
        • Makes up about 90% of the cell cycle
        • The phase between cell divisions and although called the resting phase the cell is metabolically active during this phase
        • Nucleus and Nucleolus are visible
        • The DNA in the nucleus replicates and doubles in quantity
        • New organelles are made e.g. Mitochondria
        • All the ATP needed for mitosis is made
        • No chromosomes are visible because chromatin is dispersed throughout nucleus in diffuse form
      • 2. Prophase
        • Longest phase, so in a sample of cells this will be the most commonly seen
        • The nucleus disappears and the nuclear membrane disintegrates
        • The chromatin material coils up and becomes shorter and thicker forming chromosomes
        • Centrioles divide and migrate to opposite poles of the nucleus and begin to form the spindle
          • By the end of this phase, spindle fibres have developed from the centrioles and extend from pole to pole
      • 3. Metaphase
        • A relatively short phase
        • Chromatids move to the equator and attach to the spindle fibres by their centromeres
          • (Plant cells do not contain centrioles)
      • 4. Anaphase
        • Also a short phase (the fastest phase)
        • The centromeres divide to separate the chromatid pairs
        • The spindle fibres contract pulling the chromatids to opposite poles (centromere first)
      • 5. Telophase
        • The chromatids have now reached the poles and can be regarded as distinct chromosomes
        • A nuclear membrane forms around each set of chromosomes and the nucleus reappears
        • By the end of this phase the chromosomes uncoil to form diffuse chromatin which are no longer visible
      • 6. Cytokinesis
        • At the end of telophase, the cell divides by cytokinesis to form two daughter cells
        • In animal cells this is done by 'pinching in' of the cell membrane
      • In animals the cell cycle lasts for 8-24 hours (20 minutes in bacteria) but the actual period of cell division accounts for less than 10% of the total cycle


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