All cells arise from other cells

• Cells: A group of organelles which complete a specific function
• Tissue: A group of cells designed for a specific function working together
• Organ: A group of tissues designed to perform a specific function
• Organ system: A collection of organs to complete a process.
• Differentiation: The process which cells become more specialised
• The more differentiated a cell becomes the more specialised it also becomes.
• Stages of cell cycle in order:

  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokenises
• Binary fusion is the process where prokaryotic cells divide.
• Binary fusion process:

  • Circular DNA and plasmid DNA replicate
  • The cell swells / enlarges
  • Both circular DNA and plasmid DNA move to opposite poles
  • Cytoplasm divides (cytokinesis)
  • Cell wall reforms for 2 identical daughter cells
• Prophase is the first stage of mitosis. It is the process which separates the duplicated genetic material carried by the nucleus of a parent cell into 2 identical daughter cells.
• Prophase:

  • Occupies a small part of the cell cycle
  • Half of mitosis is prophase

  This is because most cellular processes and functions take place such as chromosomes condensing.
• In prophase:

  • Chromosomes condense and become visable
  • In animals two cylindrical organelles (centrioles) move to the poles of the cell
  • The centrioles create spindle fibres which form spindle apparatus. (Not vital)
  • Nucleolus disappears, nuclear envelope breaks down and chromosomes become free
• Under a microscope prophase looks like:

  • Thick strands of DNA
  • Early prophase an intact nucleolus may be visible
  • Late prophase centrosomes are at the poles
• Metaphase serves as a check point in the cell cycle. The cell ensures that all chromosomes are correctly aligned and attach to spindle fibres. Before processing to anaphase.
• Metaphase:

  • Chromosomes are spread out in the nucleus
  • Chromosomes form a tug of war (Chromosomes that are replicated at the centrometer are called sister chromatids)
  • Occupies 4% of the cell cycle
• During metaphase:

  • Microtubes from both copied centrosomes have completed attachment to kinectores at opposite pole ends, on condensed chromosomes
  • Convene from centrometers to a metaphase plate
  • Forms a 'tug of war' along this plate
  • Takes about 1hr (20-120 minutes)
  • Compared to interphase it's small
• Anaphase marks the moment when replicated chromosomes separate and move to opposite poles.
• During anaphase:

  • Microtubules delicately pull sister chromatids appart (allowing identical daughters + accuracy is vital to maintain genetic stability)
  • Typically occupies about 2-60 mins
  • Duration varies on cell type and physiological conditions
  • Microtubules are not attached to chromosomes elongate the cell and push apart separating poles (done through motor proteins)
• Under a microscope metaphase:

  • Chromosome alignment
  • Metaphase plate is visible as a horizontal line of chromosomes
  • Centrometer structure is visible
• Under a microscope anaphase:

  • Sister chromatids pull at opposite poles
  • Doesn't last long but is dramatic
• Telophase is the last stage of mitosis
• During telophase:

  • The cell is still elongated
  • Chromatids reach opposite poles of microtubules
  • Chromatids decondense and unfold back into chromatin
  • Nuclear envelope reforms and surrounds separated daughter cells genetic material
  • Nucleoli reforms as chromosomes decondense and transcribe into RNA
  • Microtubules disappear - except for those needed for cytokensis
  • Clevage furrow forms (cell membrane and cytoplasm divide here)
  • 1 hour in total
• Telophase:

  • The membranes reform due to the break down of old parents reforming a nuclear envelope
  • Mediated and controlled by both lamins and intergal membranes
  • Vesicles fuse + form a double membrane
  • Nuclear pore complex is reintroduced
  • Occupies 2% of the cell cycle
• Under a microscope telophase:

  • Looks like 2 joined cells
  • Chromosomes decondense
  • Nucleolus reappears
  • Nuclear envelope reforms
  • Spindles disappear
• Cytokensis is the movement of cells
• During cytokensis:

  • Clevage furrow spreads around the cell and divides the cell completely
  • In animal cells cytokenesis is complete by the contractile ring
  • Contracts and spits the cell + forms under the plasma membrane
  • New membrane is inserted into plasma membranes
• Describe binary fusion in bacteria.

  Replication of circular DNA

  Replication of plasmids

  Division of the cytoplasm
• Name the three phases of mitosis shown by C, D and E on the figure below.

  Describe the role of the spindle fibres and the behaviour of the chromosomes during each of these phases.

  C - Prophase - During this stage chromosomes condense and they attach to spindle fibres.

  D - Metaphase - The chromosomes align along the equator of the cell

  E - Anaphase - During this stage the chromosomes are pulled to opposite poles of the cell.

  
• Figure 1 (to the left) shows all the chromosomes present in one human cell during mitosis. A scientist stained and photographed the chromosomes. In Figure 2(to the right), the scientist has arranged the images of these chromosomes in homologous pairs.

  Chromosomes are only visible in this state during mitosis, because they decondense during prophase.

  Chromosomes are made up of 2 sister chromatids, this happens because DNA has been replicated.

  A) Figure 2

  
• When preparing the cells for observation the scientist placed them in a solution that had a slightly higher (less negative) water potential than the cytoplasm.

  This did not cause the cells to burst but moved the chromosomes further apart in order to reduce the overlapping of the chromosomes when observed with an optical microscope.

  Water will enter by osmosis, this makes the cytoplasm to become bigger
• The dark stain used on the chromosomes binds more to some areas of the chromosomes than others, giving the chromosomes a striped appearance.

  Suggest one way the structure of the chromosome could differ along its length to result in the stain binding more in some areas.

  Different base sequences / genes where contained
• What is a homologous pair of chromosomes?

  Two chromosomes carrying the same gene.
• Give two ways in which the arrangement of prokaryotic DNA is different from the arrangement of the human DNA

  Eukaryotic DNA is bound to histones, where as Prokaryotic DNA isn't

  Eukaryotic DNA is circular, whereas human DNA is linear