1.6 - Cell division

• Cell division

  • Allows for the growth of the organism by producing more copies of cells - and also allows more cell differentiation to occur.
• Mitosis is most frequently happening in developing embryos
• New cells are need on a regular basis to replace dead, damaged or infected cells.
• Mitosis is required for:

  • Growth: Multicellular organisms increase their size by increasing their number of cells through mitosis
• Mitosis is required for:

  • Asexual reproduction: Certain eukaryotic organisms may reproduce asexually by mitosis (e.g. vegetative reproduction)
• Mitosis is required for:

  • Tissue Repair: Damaged tissue can recover by replacing dead or damaged cells
• Mitosis is required for:

  • Embryonic development: A fertilized egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo
• The Cell Cycle

  • A series of events through which cells pass to divide and create two identical daughter cells.
• Every eukaryote has genes on chromosomes.
• Each chromosome has a partner - one from each parent. Both copies are required for the cell to function.
• Diploid = 2n

  Haploid = n
• Gametes: (sex cells)

  They are haploid and pair up with eachother during fertilisation.
• Interphase consists of the parts of the cell cycle that don't involve cell division.
• Interphase:

  •  G1  (Gap 1)

  1. Increase the volume of cytoplasm
  2. Organelles produced
  3. Proteins synthesised

  • S (Synthesis)

  1. DNA replicated

  •  G2  (Gap 2)

  1. Increase the volume of cytoplasm
  2. Organelles produced
  3. Proteins synthesised
• Interphase:

  • Cells spend the majority of their time in interphase. It is a very active phase of the cycle
  • This when the cell carries out it’s normal functions
• Interphase functions:

  • Metabolic reactions (e.g. respiration to produce ATP) are necessary for the life of the cell
  • Protein synthesis -  proteins and enzymes are necessary to allow cell grow
  • Organelles numbers are increased to first support the enlarged cell
  • DNA is replicated to ensure a second copy is available to enable mitosis
• Centromere:

  • Is the part of a chromosome that links sister chromatids
• Sister chromatids are duplicated chromosomes attached by a centromere

  After anaphase when the sister chromatids separate they should then be referred to as chromosomes.
• Centrioles:

  • Organise spindle microtubules

  Spindle microtubules:

  • (also referred to as spindle fibres)

  In animal cells two centrioles are held by a protein mass referred to as a centrosome
• Prophase:

  • DNA supercoils chromatin condenses and becomes sister chromatids, which are visible under a light microscope
  • The centrosomes move to opposite poles of the cell and spindle fibres begin to form between them
  • The nuclear membrane is broken down and disappears
• Metaphase:

  • Spindle fibres from each of the two centrosomes attach to the centromere of each pair of sister chromatids
  • Contraction of the microtubule spindle fibres cause the sister chromatids to line up along the centre of the cell.
• Anaphase:

  • Continued contraction of the microtubule spindle fibres cause the separation of the sister chromatids
  • The chromatids are now referred to as chromosomes
  • Chromosomes move to the opposite poles of the cell
• Telophase:

  • Chromosomes arrive at the poles.
  • The chromosomes uncoil de-condense to chromatin (and are no longer visible under a light microscope).
  • Microtubule spindle fibres disappear
  • New nuclear membranes reform around each set of chromosome
• Mitosis is the nucleus division

  Cytokinesis is the division of the cytoplasm

  and hence the cell.

  The division of the cell into two daughter cells (cytokinesis) occurs concurrently with telophase.
• Cytokinesis in Animal Cells:

  • A ring of contractile protein (microfilaments) immediately inside the plasma membrane at the equator pulls the plasma membrane inward.
  • The inward pull on the plasma membrane produces the characteristic cleavage furrow.
  • When the cleavage furrow reaches the centre of the cells it is pinched apart to form two daughter cells. 
• Cytokinesis in plant cells (Pt 1):

  • Membrane-enclosed vesicles derived from the Golgi apparatus migrate to the centre of the cell.
  • Vesicles fuse to form tubular structures that merge (with the addition of more vesicles) to form two layers of plasma membrane (i.e. the cell plate)
  • Cell plate continues to develop until it connects with the existing cell’s plasma membrane.
  • This completes the division of the cytoplasm and the formation of two daughter cells.
• Cytokinesis in Plant cells (Pt 2):

  • Vesicles deposit, by exocytosis, pectins and other substances in the lumen between the daughter cells to form the middle lamella (‘gluing’ the cells together).
  • Both daughter cell secrete cellulose to form their new adjoining cell walls. 
• Why supercoil chromosomes?

  • Chromosomes need to be stored compactly to fit within the nuclei of cells.
  • This problem becomes more acute during mitosis when chromosomes need to be short and compact enough that they can be separated and moved to each end of the cell.
• How are chromosomes supercoiled?

  • Strain is placed on a DNA helix by over-winding or under-winding of the helix
  • This causes the DNA molecule to coil back on itself becoming shorter and wider
• Cyclins:

  • A family of proteins that control the progression of cells through the cell cycle
• Cyclins:

  1. Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches it threshold.
  2. Cyclins bind to enzymes called cyclin-dependent kinases
  3. These kinases then become active and attach phosphate groups to other proteins in the cell.
  4. The attachment of phosphate triggers the other proteins to become active and carry out tasks (specific to one of the phases of the cell cycle).
• Cyclin D:

  • Triggers cells to move from G0 to G1 and from G1 into S phase.
• Cyclin E:

  • Prepares the cell for DNA replication in S phase.
• Cyclin A:

  • Activates DNA replication inside the nucleus in S phase.
• Cyclin B:

  • Promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.
• Tumours:

  • Are abnormal growth of tissue that develop at any stage of life in any part of the body.
• A cancer:

  • Is a malignant  tumour and is named after the part of the body where the cancer (primary tumour) first develops
• Mutation:

  • A change in an organisms genetic code.
  • A mutation/change in the base sequence of a certain genes can result in cancer.
• Mutagens:

  • Agents that cause gene mutations.
• Not all mutations result in cancers, but anything that causes a mutation has the potential to cause a cancer.