The Cell Cycle

• Stages of the cell cycle

  1. Interphase

  2. Prophase

  3. Metaphase

  4. Anaphase

  5. Telophase

  6. Cytokinesis
• Interphase


  The cell spends most of its life in this phase. The DNA in chromosomes copies itself ready for mitosis.
• Prophase


  The DNA in chromosomes and their copies condenses to become more visible. The membrane around the nucleus disappears.
• Metaphase


  Chromosomes and their copies line up in the middle of the cell.
• Anaphase


  Chromosomes and their copies are pulled to different ends of the cell.
• Telophase


  New membranes form around the chromosomes at each end of the cell.
• Cytokinesis


  The cell membrane pinches in and eventually divides into two daughter cells.
• Stages of mitosis


  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• 1.Stem cell = undifferentiated cell 

  2.Cell differentiation  = when stem cells form specialised cells 

  3.Clone = genetically identical cells or organisms 
• During mitosis, the cell starts with 46 individual chromosomes.

  23 are from the mother and 23 are from the father.

  These cells are called diploid Mitosis produces two ‘daughter’ cells that are identical to each other.

  Each daughter cell has 46 chromosomes also.

  In multi-celled creatures mitosis is used for growth & repair.  
• During meiosis, the cell starts with 46 individual chromosomes (23 pairs).

  The chromosomes in this cell are then replicated producing a cell with 92  chromosomes (46 pairs).

  This cell then divides twice . 

  Meiosis produces four haploid cells that are different to each other. These gamete cells (sex cells) have 23 chromosomes.

  These gametes then join at fertilisation to restore the usual 23  pairs of chromosomes.
• Mitosis is important because it allows multicellular organisms to grow and develop by increasing their number of cells. It also enables damaged tissues to be repaired. Meiosis is important as it creates genetic variation between offspring which helps them adapt to changing environments.
• Chromatin condenses into visible chromosomes during prophase. Centrioles move towards opposite poles of the cell. Spindle fibers attach to centromeres on sister chromatids. Chromosomes line up along equatorial plane. Sister chromatids separate and spindle fibers pull them apart. Nuclear envelope breaks down. New nuclei form around sets of chromosomes.
• What word is given to the fusion of two gamete cells?

  fertilisation

  What name is given to a fertilised cell?

  zygote 

  What form of cell division does a zygote undergo? 

  mitosis 

  What form of cell division does a diploid cell undergo to form a haploid sex cell?

  meiosis 

  What would happen if the number of chromosomes was not halved during meiosis? 

  Reproduction could not take place to produce diploid zygote 

  Where in the body does meiosis happen?

  Testes and ovaries