Cell division

• In multicellular organisms not all cells keep their ability to divide. The ones that do follow a process called the cell cycle. The cell cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells. The cell cycle consists of a period of cell growth and DNA replication, called interphase, and a period of cell division called mitosis. Interphase is subdivided into three separate growth phases G1, S, G2
• Interphase

  • During interphase the cells DNA is unravelled and replicated, to double its genetic content.
  • The organelles are also replicated so it has spare ones and its ATP content is increased
• • There are two types of cell division - mitosis and meiosis. Mitosis is the form of cell division that occurs during the cell cycle
  • In mitosis a parent cell divides to produce two genetically identical daughter cells
  • Mitosis is needed for the growth and repair of multicellular organisms
• • As mitosis begins, the chromosomes are made of two strands joined in the middle by a centromere.
  • The separate strands are called chromatids.
  • Two strands on the same chromosome are called sister chromatids
  • There are two strands because each chromosome has already made identical copy of itself during interphase
  • When mitosis is over the chromatids end up as one-strand chromosomes in the new daughter cells
• Prophase

  • The chromosomes condense getting shorter and fatter
  • Tiny bundles of protein called centrioles start moving to opposite ends of the cell forming a network of protein fibres across it called the spindle
  • The nuclear envelope breaks down and chromosomes lie free in the cytoplasm
• Metaphase

  • The chromosomes line up along the middle of the cell and become attached to the spindle by their centromere
• Anaphase

  • The centromeres divide separating each pair of sister chromatids
  • The spindles contract, pulling chromatids to opposite poles of the spindle, centromere first
  • This makes the chromatids appear V-shaped
• Telophase

  • The chromatids reach the opposite poles on the spindle
  • They uncoil and become long and thin again
  • A nuclear envelope forms around each group of chromosomes, so there are now two nuclei
  • Divisions of the cytoplasm finishes in telophase
  • There are now two daughter cells that are genetically identical to the original cell and to each other.
  • Mitosis is finished and each daughter cell starts the interphase part of the cell cycle to get ready for the next round of mitosis
• miotic index = number of cells with visible chromosomes/ total number of cells observed