1.4.2 Phases of Mitosis

• Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)
• Significance of mitosis: mitosis occurs whenever the production of genetically identical nuclei are required in eukaryotic cells
  • E.g. during embryonic development, growth, tissue repair and asexual reproduction

Embryonic development and growth of multicellular organisms

• Unicellular zygotes divide by mitosis in order to grow in size

• After a certain amount of growth, they then differentiate into embryos
• Growth of multicellular organisms occurs as the number of new cells increases due to mitosis
• This growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Replacement of cells & repair of tissues

• Damaged tissues can be repaired by mitosis followed by cell division
• As cells are constantly dying they need to be continually replaced by genetically identical cells
• In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut
• Some animals can regenerate body parts, for example, zebrafish can regenerate fins and axolotls regenerate legs and their tail amongst other parts

Asexual reproduction

• Asexual reproduction is the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent
• For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring
• For multicellular organisms, new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways
• This type of reproduction can be observed in different plant, fungi and animal species
• Some examples of these are budding in Hydra and yeast and runners from strawberries

Phases of Mitosis

• Although mitosis is, in reality, one continuous process, it can be divided into four main stages or phases

• These stages are:
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

• Most organisms contain many chromosomes in the nuclei of their cells (eg. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for simplicity
• The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Prophase

• Chromosomes condense and are now visible when stained
• The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
• The two centrosomes (replicated in the G₂ phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
• Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
• The nuclear envelope (nuclear membrane) breaks down into small vesicles
• The nucleolus disappears

Prophase

Metaphase

• Centrosomes reach opposite poles
• Spindle fibres (protein microtubules) continue to extend from centrosomes
• Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
• Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
  • This attachment involves specific proteins called kinetochores

• Each sister chromatid is attached to a spindle fibre originating from opposite poles

Metaphase

Anaphase

• The sister chromatids separate at the centromere (the centromere divides in two)
• Spindle fibres (protein microtubules) begin to shorten
• The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Anaphase

Telophase

• Chromosomes arrive at opposite poles and begin to decondense
• Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
• The spindle fibres break down
• New nucleoli form within each nucleus

Telophase

Condensation of chromosomes

• DNA molecules are very long molecules (human DNA can be more than 50,000 µm) that need to fit within much smaller nuclei (human nuclei average less than 5 µm)
• Prior to mitosis, the DNA molecules are loosely coiled (around histones in eukaryotic cells) to form a complex called chromatin
• During prophase, the chromatin gets condensed by supercoiling to form chromosomes
• Condensation occurs by the repeated coiling of the DNA molecule (supercoiling)
• This supercoiling is aided in eukaryotic cells by the presence of histone proteins and enzymes

DNA is coiled around histone proteins to make chromatin