• 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 varies depending on:

  • The 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 cell cycle

S = synthesis (of DNA); G = growth; M = mitosis

Interphase

• Interphase?is the?longest?and?most?active phase?of the?cell cycle
• During interphase, the cell:
  • Increases?in?mass?and?size
  • Carries out many cellular functions in the nucleus and cytoplasm eg.?synthesising proteins?and?replicating its DNA?ready for mitosis (these only occur during interphase)
  • Increases?the number of?mitochondria
  • Increases?the number of?chloroplasts?(if they are a plant or algae cell)

  The phases of interphase

  Interphase consists of?three?phases:

  • G₁?phase
  • S phase
  • G₂?phase

  The gap between the previous cell division and the S phase is called the G₁?phase?–?G?stands for?growth

  • Cells make the?RNA, enzymes and other proteins required for growth?during the G₁?phase

  It is at some point during the G₁?phase a?signal?is received telling the cell to?divide?again (although some cells do not receive this signal and will?never divide;?they enter the?G₀?phase)

  After the G₁?phase of interphase the cell enters the next phase of the cell cycle, the?S phase?–?S?stands for?synthesis?(of DNA)

  • The S phase is relatively short
  • The?DNA in the nucleus replicates,?resulting in each chromosome consisting of two identical sister chromatids

  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

  Exam Tip

• The?cell cycle?is a sequence of stages including?interphase?(G₁, S & G),?mitosis?and?cytokinesis
• The cycle is?controlled?by?cyclins?(a group of proteins) and?kinases?(enzymes)
• There are?four different cyclins?(D, E, A & B) whose concentrations rise and fall over the cycle:
  • D –?present first, triggers cells to?move?from?G₁?to S phase
  • E –?highest?concentration at the start of?S phase,?prepares?the cell for?DNA replication?during S phase
  • A –?highest?concentration in?G₂?phase?but activates two different kinases that trigger two processes:
    • In the?S phase, it?activates DNA replication
    • In?G₂?phase, it?prepares?the cell for?mitosis

    B - ?highest?concentration at the?beginning?of?mitosis,?promotes?the?formation?of the?mitotic spindle

Cyclins control the cell cycle. The presence of certain cyclins triggers a specific stage of the cell cycle.

• When each of the different?cyclins?reach a?certain concentration?they?trigger the next stage?of the cell cycle
• This ensures?key processes?(e.g. DNA replication, organelle multiplication and protein synthesis)?occur?at the?correct time
• When a specific cyclin has reached a certain concentration it will?bind?with another group of proteins (cyclin-dependent kinases) forming a?complex?which is?activated
• This complex?phosphorylates?(attaches a phosphate) a?target protein?which?activates it,?causing it to trigger?specific functions?(e.g. DNA replication)
• Once the specific function is complete the phosphate is released, the cyclin breaks down and the cyclin-dependent kinases become inactive

The mechanism for cell cycle control by cyclins

NOS: Serendipity and scientific discoveries; the discovery of cyclins was accidental

• Some scientific discoveries occur by accident, meaning that the scientist might not necessarily have been deliberately searching for information about the particular mechanism, process, molecule or structure that they ended up discovering
• The discovery of the cyclins was?serendipitous?(occurred by chance)
• Tim Hunt and his team were researching protein synthesis in sea urchin eggs, however, whilst doing this research they noticed a protein (later named by Hunt as?cyclin) that repeatedly increased and decreased in concentration and that these coincided with the phases of the cell cycle
• This discovery led to new research which found the presence of other cyclins and their function as a key factor in the regulation of the cell cycle

Exam Tip

Cytokinesis

• Follows the nuclear division (mitosis) phase
• Once the nucleus has divided into two genetically identical nuclei, the?cell divides in two?with one nucleus moving into each cell to?create two genetically identical daughter cells

Mitosis in animal and plant cells

• The process differs slightly in animal and plant cells
• In?animal?cells:
  • A ‘cleavage furrow’ forms and separates the daughter cells
  • The cleavage furrow forms when actin and myosin proteins form a contractile ring just under the plasma membrane
  • This ring is formed at the equator (centre) of the cell
  • As the proteins contract, they pull the plasma membrane towards the centre eventually separating the cell into two daughter cells

  In plants?cells:

  • A ‘cell plate’ (the precursor to a new cell wall) forms at the equator. Once the cell plate reaches the cell walls of the parent cell, new cell walls are produced, separating the new daughter cells
  • The cell plate is formed from vesicles carrying carbohydrates, lipids and proteins fusing together to create the two plasma membranes
  • After this other vesicles, carrying pectin and cellulose, deposit these substances by exocytosis in the gap between the two new membranes leading to the formation of new cell walls

Cytokinesis in an animal cell and a plant cell