3.2.2 Cell replication

• Cells divide by mitosis to produce 2 genetically identical daughter cells, allowing growth and repair in tissues
• Chromosome after DNA has been replicated is called a pair of sister chromatids

  
• Interphase - Cell carries out usual functions, DNA and organelles replicates as the cell prepares to divide

  Cell cycle:

  Prophase - Chromosomes condense, becoming a pair of sister chromatids. Nuclear membrane breaks down, spindle fibers form

  Metaphase - Chromosomes align to equator, spindle fibers attach to centromere

  Anaphase - fibers contract, pulling chromatids to opposite poles

  Telophase - Nuclear membranes reform, chromosomes uncoil, spindle fibers break down

  
• Cytokinesis

  Chromosomes not visible

  Cytoplasm and organelles divided into 2 daughter cells

  Each cell is in interphase again
• Mitotic index

  A) number of cells in mitosis

  
• Key points in root tip practical

  Use root tip - mitosis only occurs here

  Use a stain - to make the chromosomes visible

  Squash the tissue - to get a single layer of cells so light can pass through and cells are not obscuring each other
• Uncontrolled division leads to:

  • Undifferentiated cells - not specialised
  • Different antigens
  • A mass of undifferentiated cells = a tumour
  • The tumour can damage surrounding tissues / press on nerves and blood vessels
• Benign tumour


  Slow growing

  Enclosed in a capsule
• Malignant tumour

  Fast growing

  Not enclosed in a capsule

  Can metastasise - cells break away and form secondary tumours in other parts of the body
• What do cancer treatments do?

  Stops/slows cell division in healthy cells, particularly in fast dividing cells such as hair follicles and epithelial cells
• Ways to prevent cell growth

  Prevent DNA replication

  Prevent mitosis

  Prevent the spindle forming

  Prevent the centromere attaching to the spindle

  Prevent the spindle contracting

  Prevent cytokinesis
• Binary fission:

  1. Circular DNA and plasmids replicate
  2. Cells grow in size and DNA and plasmids move to each side
  3. Cytoplasm divides and the cell wall begins to form
  4. Cell separated by new cell wall
  5. Each new cell has a single copy of circular DNA and a variable number of plasmids
• Viral replication in host cells:

  1. Attachment proteins binds to specific receptor proteins on the Th cell. Capsid enters the Th cells and releases RNA and reverse transcriptase into the cell
  2. Reverse transcriptase makes a single stranded DNA copy from the viral RNA and the a double stranded DNA copy is made. Viral DNA is then inserted into host cell DNA in the nucleus
  3. Cell enzymes and organelles are used to make viral proteins, new viruses are assembled. Viruses leave the cell - killing it in the process. It picks up a layer of cell membrane and infects other Th cells