Cell Division

• In most parts of the body, [cells] need to divide, so that Organisms can grow and replace old or damaged cells
• [MEIosis] when cells divide to produce Sex Cells or Gametes, which contain only HALF the original number of chromosomes
• [Human Body Cells] 46 Chromosomes in 23 Pairs called Homologous Pairs. These are Diploid cells – two copies of each chromosome

  [Sex Cells] 23 chromosomes, so they are Haploid cells - one copy of each chromosome
• 1. Prophase

  • DNA replicates and the chromosomes form two exact copies called chromatids
  • They are joined in the Middle by a Centromere
  • The nuclear membrane breaks down

  A) Centromere

  
• 2. Metaphase

  • A structure called the Spindle forms
  • Chromosomes line up at the Metaphase Plate (middle), attached to the Spindle by their Centromeres

  A) Spindle

  
• 3. Anaphase

  • The Spindle fibres shorten and pull the Chromatids to the opposite ends (‘poles’) of the cell
  • The Chromatids separate to become the Chromosomes of the two Daughter Cells

  
• 4. Telophase

  • Two new Nuclei (Nucleus) form at the poles of the cell
  • The Cytoplasm starts to divide, to produce two Daughter Cells
  • Both Daughter Cells have a copy of each Chromosome from the Parent cell

  
• A Human begins life as a Zygote (fertilised egg), which divides by Mitosis to form two cells, then four, then eight and so on, until an Embryo is formed, containing millions of cells

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  As the developing embryo grows, the cells become Specialised to perform particular roles. This specialisation is also under the control of the genes, and is called Cell Differentiation

  
• Types of MUSCLE Cells: (1 of 3)

  [Voluntary Muscle Cell]

  FUNCTION:

  • Fast contraction to move Bones (also called skeletal muscle)
  • They are also under Voluntary control by the BRAIN

  STRUCTURE:

  • Striped (or Striated) due to the alignment of Protein Filaments in the Cell
  • Many Nuclei per cell

  
• Types of Muscle Cells: (2 of 3)

  [Involuntary Muscle Cell]

  STRUCTURE:

  • Non-striated (NOT striped) because Protein Filaments are NOT aligned in the cell (also called smooth muscle)
  • One nucleus per cell + Not branched
  • The Cell is tapered at both ends

  FUNCTION:

  • Slow, rhythmic Contraction in walls of the Gut, and blood vessels
  • NOT under Voluntary Control by the brain

  
• Types of Muscle Cells: (3 of 3)

  [Cardiac Muscle Cell]

  FUNCTION:

  • Only present in the HEART
  • Contracts rhythmically and constantly throughout life without getting tired
  • NOT under Voluntary control by the Brain

  STRUCTURE:

  • Striated (Striped)
  • Many nuclei per cell
  • Branched cells forming a strong mesh-like network