Mitosis

• Mitosis is the term usually used to describe the entire process of cell division in eukaryotic cells.
• Mitosis refers to nuclear division, which is an essential stage in cell division.
• Mitosis ensures that both daughter cells produced when a parent cell divides are genetically identical, except in the rare events where mutations occur.
• Each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes.
• Mitosis is necessary when all the daughter cells have to be identical, which is the case during growth, replacement and repair of tissues in multicellular organisms such as animals, plants, and fungi.
• Mitosis is also necessary for asexual reproduction, which is the production of genetically identical offspring from one parent in multicellular organisms including plants, fungi, and some animals, and also in eukaryotic single-celled organisms such as Ameoba species.
• Prokaryotic organisms, including bacteria, do not have a nucleus and they reproduce asexually by a different process known as binary fission.
• Before mitosis can occur, all of the DNA in the nucleus is replicated during interphase.
• Each DNA molecule (chromosome) is converted into two identical DNA molecules, called chromatids.
• The two chromatids are joined together at a region called the centromere.
• It is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred and segregated equally, one each into the two new daughter cells.
• There are four stages of mitosis: prophase, metaphase, anaphase and telophase.
• These stages of mitosis flow seamlessly from one to another.
• Each of these phases of mitosis can be viewed and identified using a light microscope.
• Dividing cells can be easily obtained from growing root tips of plants.
• The root tips can be treated with a chemical to allow the cells to be separated, then they can be squashed to form a single layer of cells on a microscope slide.
• Stains that bind DNA are used to make the chromosomes clearly visible.
• Prophase

  Chromatin fibres begin to coil and condense to form chromosomes that will take up stain to become visible under the light microscope.
• Prophase

  The nucleolus, a distinct area of the nucleus responsible for RNA synthesis, disappears.
• Prophase

  The nuclear membrane begins to break down.
• Prophase

  Protein microtubules form spindle-shaped structures linking the poles of the cell.
• Prophase

  The fibres forming the spindle are necessary to move the chromosomes into the correct positions before division.
• Prophase

  In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell.
• Prophase

  The centrioles are cylindrical bundles of proteins that help in the formation of the spindle.
• Prophase

  The spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell.
• Prophase

  By the end of prophase, the nuclear envelope has disappeared.
• Metaphase

  Chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.
• Anaphase

  The centromeres holding together the pairs of chromatids in each chromosome divide.
• Anaphase

  The chromatids are separated - pulled to opposite poles of the cell by the shortening spindle fibres.
• Anaphase

  The characteristic "V' shape of the chromatids moving towards the poles is a result of them being dragged by their centromeres through the liquid cytosol.
• Telophase

  The chromatids have reached the poles and are now called chromosomes.
• Telophase

  The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them.
• Telophase

  The chromosomes start to uncoil and the nucleolus is formed.
• Telophase

  At the end of this phase, cell division, or cytokinesis, begins.
• Cytokinesis


  The actual division of the cell into two separate cells, begins during telophase.
• In animal cells, a cleavage furrow forms around the middle of the cell.
• The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells.
• In plant cells, where cell walls prevent the formation of a cleavage furrow, vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed.
• The vesicles fuse with each other and the cell surface membrane, dividing the cell into two.
• New sections of cell wall then form along the new sections of membrane if the dividing cell wall were formed before the daughter cells separated they would immediately undergo osmotic lysis from the surrounding water.