Cell Division

• The nucleus of a cell contains chromosomes made of DNA molecules and each chromosome carries a large number of genes
• In body cells the chromosomes are normally found in pairs
• The nucleus contains genetic material in the form of chromosomes
• Cells divide in a series of stages called the cell cycle
• During the cell cycle the genetic material is doubled and then divided into two identical cells
• Cell division by mitosis is important in the growth, repairing tissue and development of multicellular organisms
• Before a cell can divide it needs to grow and increase the number of sub-cellular structures such as ribosomes and mitochondria ; The DNA replicates to form two copies of each chromosome
• Mitosis

  • The chromosomes line up at the centre of the cell and spindle fibres pull them apart
  • Chromosomes are pulled to each end of the cell
  • Membranes form around each set of chromosomes an the nucleus divides
  • The cell membrane and cytoplasm divide
• After cell division occurs 2 daughter cells are produced which are genetically identical to the parent cell
• Cells divide when an organism grows or and organism has become damaged
• The cell cycle includes

  • Growth phase (chromosome doubling and subcellular structures increase)
  • Mitosis
  • Cytokinesis (division of cytoplasm and the cell membrane)
• Humans have 46 chromosomes in the nucleus of all their body cells, found in 23 pairs
• Chromosomes are made from highly coiled strands of relatively long DNA and each chromosome is made from one DNA molecule
• Prokaryotic cells replicate by binary fission
• Binary fission

  • Circular DNA and plasmids replicate
  • The cell gets bigger and the circular DNA stands move to opposite poles (ends) of the cell
  • The cytoplasm divides and new cell walls form
• After binary fission takes place 2 daughter cells are produced and each has one copy of the circular DNA but can have variable copies of plasmids
• Bacteria multiply by simple cell division (binary fission) as often as once every 20 minutes if they have enough nutrients and a suitable temperature