Cell Division

• DNA replication occurs during the interphase of the cell cycle
• Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication
• Cell division must occur in multicellular organisms in order for them to grow and to replace damaged or worn out tissues
• Not all cells keep the ability to divide, many specialised cells only go through the cell cycle once, they have to be replaced by stem cells e.g red blood cells
• In eukaryotic cells, there are two types of cell division- mitosis and meiosis
• Mitosis produces 2 genetically identical diploid daughter cells
• Meiosis produces 4 genetically non-identical haploid daughter cells (gametes)
• Mitosis is a form of asexual reproduction. it is the process for growth of tissues, replacement of lost cells, repairing damages tissue, formation of clones of white blood cells in an immune response, and it is also how cancerous tumours form
• Meiosis is the method of cell division that takes place in reproductive organs which is used to produce haploid sex cells for sexual reproduction
• Specialised cells and stem cells move from G1 into a resting phase known as G0, this can be temporary
• The majority of the cell cycle is spent in interphase- this is when the cell is growing and DNA is replicated
• During interphase in the cell cycle the DNA is uncondensed and replicated so the amount of DNA doubles so each cell will have the same amount

  Organelles are also replicated so there is enough for both new cells

  Amount of ATP produced is increased as energy is required for cell division
• There are three stages to interphase:

  G1 (Gap 1)

  S (Synthesis)

  G2 (Gap 2)
• G1 (Gap 1)-phase where the cell elongates and new organelles and proteins are made
• S (Synthesis)-phase where the cell replicates its DNA which is essential before the cell can divide
• G2 (Gap 2)-phase where the cell keeps elongating and proteins needed for cell division are mad
• Mitosis stage is the formation of two new identical daughter cells form an original cell
• Mitosis stage involved nuclear division

  It is a continuous process but can be described in stages
• Cytokinesis is the final stage where the cytoplasm divides and two separate genetically identical cells are formed
• After cytokinesis each cell will start interphase to prepare for the next round of mitosis
• A chromosome consists of two strands called (sister) chromatids, they are joined in the middle by a centromere
• Chromosomes are only visible during cell division because they are formed during interphase when the DNA is replicated
• Each sister chromatid on a chromosome contains identical genetic information so that each daughter cell also contains identical information
• To remember stages of mitosis in order you can use I.P.M.A.T
• In interphase the chromosomes are not visible because the DNA is uncondensed

  The nucleolus is still visible and the nuclear envelope is still intact

  The centrioles have replicated and may be visible
• Prophase:

  The chromosomes condense (thicken and shorten as DNA is coiled tightly around proteins called histones) and become visible. Nucleolus disappears and nuclear envelope begins to break down.The centrioles (bundles of protein) begin to produce spindle fibres which start to extend towards the chromosomes
• Metaphase:

  The nuclear membrane has disappeared. Spindle fibres have attached to the centromere or the chromosomes. Each centromere has a spindle fibre from each pole of the cell. Chromosomes are pulled to the middle/equator of the cell where they line up
• Anaphase:

  The spindle fibres contract. The centromere of each chromosome splits in half so that one chromatid from each chromosome can be pulled to opposite poles of the cell. This stage makes sure that each half of the cell receives one chromatid from each chromosome
• Anaphase stage can be recognised by the V-shape of the chromatids as they are dragged across
• Telophase:

  Nuclear envelopes begin to reform around each new group of chromosomes. Spindle fibres disappear. The chromosomes begin to uncoil/unravel and become less distinct. The cytoplasm begins to divide to form two new genetically identical daughter cells. Each new cell will have half the amount of DNA compared to cell in interphase, until interphase begins again and DNA is replicated
• Calculating time taken for stages of mitosis:

  Step 1- calculate the proportion of cells that are in prophase

  Step 2-convert cell cycle time to unit required for answer

  Step 3-calculate the proportion of time spent in prophase
• Light microscopes are commonly used as they are relatively cheap, easy to use and they can magnify living cells or small organisms so their activity and behaviour can be studied
• Light has to pass through the specimens for them to be seen in light microscopes so a stain is often used to colour cell structures so it is easier to see
• Magnification is making an image larger than the object
• When we use a microscope we can increase the magnification by using a higher powered objective lens

  This allows us to see in more detail because it increases the size of the image of small details
• When we focus an image on the microscope, we make the image more clear, sharp and easy to see. We can focus the microscopes using the coarse or fine focus knobs which move the slide closer or further away from objective lens. At low power magnification we can use fine focus knob so we dont need to move stage by too much. You can bring the objective lens too close which may break the lens and slide
• Using a light microscope:

  Step 1- Take the slide with the sample and put it on the stage

  Step 2- Set to lowest magnification of objective lens (x4). This will give x40 magnification

  Step 3- Turn coarse focus knob until cell is in focus

  Step 4- Turn fine focus knob to clearly focus the cells

  Step 5- Increase magnification of the objective lens to x10 by turning objective lens

  Step 6- Refocus cells using fine focus knob
• In plants, cell division by mitosis is restricted to specific tissues, called meristems, that are found at the growing root and shoot tips
• Cells in meristems are undifferentiated
• Stem cells in plant meristems divide to produce cells that increase the height of the plant, length of the roots and girth of the stem also produce cells that develop into leaves and flowers