• The cel carries out normal functions, but also prepares to divide. The cell's DNA is unravelled and replicated, to double its genetic material. The organelles are also repliacted so it has spare ones and its ATP content is increased too
  • G1
    • Increase in size
    • new proteins and organelles made
  • S
    • DNA synthesis occurs in nucleus (DNA replication)
  • G2
    • Preparation to divide
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  • The chromosomes condense, getting shorter and fatter, using histone proteins. Tiny bundles of proteins called centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle. The nuclear envelope breaks down and chromosomes are free in the cytoplasm

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  • The chromosomes line up along the middle of the cell and become attached to the spindle by their centromere

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  • The centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite poles of the spindle, centromere first. This make the chromatids appear A-shaped

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  • The chromatides reach the opposite poles on the spindle. They uncoil and become long and thin again. They're now called chromosomes again. A nuclear envelope forms around each group of chromosomes, so there are now two nuclei. The cytoplasm divides (cytokinesis) and there are now two daughter cells that are genetically identical to the original cell and to each other. Mitosis is finished and each daughter cell starts the interphase part of the cell cycle to get ready for the next round of mitosis

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  • Mitosis is controlled by genes, and controls how many times they divide, but if there's a mutation in a gene that controls cell division, the cells can grow out of control. If the cells keep dividing, they can form a tumour, and cancer is a tumour that invades surrounding tissue
  • Treatment:
    • G1 - Some chemical drugs (chemotherapy) prevent the synthesis of enzymes needed for DNA replication. If these aren't produced, the cell is unable to enter the synthesis phase, disrupting the cell cycle and forcing the cell to kill itself
    • S-phase - Radiation and some drugs damage DNA. At several points in the cell cycle (including just before and during S phase) the DNA in the cell is checked for damage. If severe damage is detected, the cell will kill itself, preventing further tumour growth
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Required Practical - Root Tips

  • Cut off a 1cm length of root from a garlic bulb. Place root in a watch glass with acetic orcein for 10 mins
  • Remove the root from the acetic orcein. Transfer into a beaker of cold water to wash for 5 mins, then dry.

  • Pre-heat 1M hydrochloric acid in a waterbath set at 60˚C.Using forceps, transfer the root to the acid and leave for 5 mins.
  • Transfer the root into a beaker of cold water for 5 minutes to rinse off the excess acid, and then dry on filter paper.
  • Remove root tip from the water. Using a sharp scalpel cut off the final 2mm of the root tip.
  • Discard the rest of the root. Blot any excess water from the slide using filter paper,
  • Using the end of a mounted needle, squash the root tip.
  • Add one drop of the acetic orcein stain and leave for about 2 minutes.
  • Lower a clean glass cover slip onto the squashed cells.
  • Cover the slide with several layer thickness of paper towels. Press down firmly on the slide to further squash and spread out the root tip.
  • Most of the stain will have been absorbed by the paper. Place a further drop of stain next to the cover slip and draw it across the section using a small piece of filter paper
  • Examine the section under low power of the light microscope.
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Graticule and Micrometer

  • A bit like rulers, they are used to calculate the size of the cells you are looking at
  • The eyepiece graticule is fitted onto the eyepiece. It has numbers but no units
  • The stage micrometer is on the stage, and its used to work out the value of the divisions on the eyepiece graticule at a particular magnification
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