3.14 The Cell Cycle & Mitosis

• Mitosis is part of a precisely controlled process known as the cell cycle
• The cell cycle is the regulated sequence of events that occurs between one cell division and the next
• The cell cycle has three phases:
  • Interphase
  • Nuclear division (mitosis)
  • Cell division (cytokinesis)
• The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism
  • For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)
• The movement from one phase to another is triggered by chemical signals called cyclins

The stages of the cell cycle

Interphase

• During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)
• Interphase consists of three phases:
  • G₁ phase
  • S phase
  • G₂ phase
• It is at some point during the G₁ phase that a signal is received telling the cell to divide again
• The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)
• This phase of the interphase stage of the cell cycle is called the S phase – S stands for synthesis (of DNA)
  • The S phase is relatively short
• The gap between the previous cell division and the S phase is called the G₁ phase – G stands for gap
  • Cells make the RNA, enzymes and other proteins required for growth during the G₁ phase
• Between the S phase and next cell division event the G₂ phase occurs
  • During the G₂ phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired
  • Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
• Interphase = G₁ + S + G₂

Nuclear division (mitosis)

• Follows interphase
• Referred to as the M phase – M stands for mitosis
• Cell growth stops during the M phase
• During mitosis, the two identical sister chromatids of each chromosome separates from each other and move to opposite poles of the cell
• This ensures that each new nucleus that forms will contain the exact same genetic information as the original nucleus

Cytokinesis

• Follows M phase
• Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells
• In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

• 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)
• Although mitosis is, in reality, one continuous process, it can be divided into four main stages
• 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 the sake of 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 (which consist of two centrioles in animal cells)
• The nuclear envelope (nuclear membrane) breaks down into small vesicles

Prophase stage of mitosis where chromosomes condense into visible structures

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

Metaphase, where chromosomes line up along the equator of the cell

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, where chromosomes are pulled to the poles of the cell

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

Telophase, where the nuclei reform and the cell begins to split into two

The significance of mitosis

• The process of mitosis is of great biological significance and is fundamental to many biological processes, including:
  • The growth of multicellular organisms
  • The replacement of cells and repair of tissues
  • Asexual reproduction

Growth of multicellular organisms

• The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell
• This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms
• 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 (as seen with many plant species) new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways. Some examples of these are budding in Hydra and yeast and runners from strawberries