mitosis

• in multicellular organisms not all cells keep their ability to divide - ones that do follow a process called the cell cycle
• 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 (cell growth) is subdivided into three separate growth stages:

  • G1
  • S
  • G2
• Growth/ gap phase 1: cell grows and new organelles and proteins are made
• synthesis = cell replicates its DNA ready to divide by mitosis
• growth/ gap phase 2 = cell keeps growing and proteins needed for cell division are made
• interphase - during interphase the cell carries out normal functions, but also prepares to divide:

  • the cell's DNA is unravelled and replicated to double its genetic content
  • the organelles also replicate so the cell has spare
  • ATP content is increased (provides the energy needed for cell division)
• mitosis: form of cell division that occurs during the cell cycle
• • parent cell divides to produce two genetically identical daughter cells (contain an exact copy of the DNA of the parent cell)
  • needed for growth in multicellular organisms
  • for repairing damaged tissues
  • really one continuous process but described as s series of division stages
• • 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 2 strands bc each chromosome has already made an identical copy of itself during interphase
  • when mitosis is over - chromatids end up as one-strand chromosomes in the daughter cells
• prophase:

  • 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
  • nuclear envelope breaks down and chromosomes lie free in the cytoplasm
• metaphase:

  • the chromosomes (each with 2 chromatids) line up along the middle o the cell
  • become attached to the spindle by their centromere
• anaphase:

  • centromeres divide
  • separating each pair of sister chromatids
  • spindles contract pulling chromatids to opposite poles of the spindle centromere first
  • makes the chromatids appear v-shaped
• telopahse:

  • chromatids reach opposite poles on the spindle
  • uncoil and become long and thin again
  • now called chromosomes again
  • nuclear envelope forms around each group of chromosomes so there are now 2 nuclei
  • cytoplasm divides (cytokinesis) and there are now 2 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
• mitosis and the cell cycle are controlled by genes - normally when cells have divided enough times to make enough new cells they stop. But it there is a mutation in a gene that controls cell division - cells can grow out of control - keep on dividing to make more and more cells - forms a tumour
• cancer is a tumour that invades surrounding tissue
• some treatments for cancer are designed to control the rate of cell division in tumour cells by disrupting the cell cycle. This kills the tumour cells. These treatments do not distinguish between normal and tumour cells though and so also kill normal body cells that are dividing. However, tumour cells divide much more frequently than normal cells, so the treatments are more likely to kill tumour cells
• some chemical drugs (chemotherapy) prevent the synthesis of enzymes needed for DNA replication. If these are not produced the cell is unable to enter the synthesis phase disrupting the cell cycle and forcing the cell to kill itself
• radiation and some drugs damage DNA. at several pints in the cell cycle, the DNA in the cell is checked for damage. if severe DNA damage is detected the cell will kill itself - preventing further tumour growth (S phase - DNA rep)

  (other one is G1)