the cell cycle

• Mitosis is part of a controlled sequence of events known as the cell cycle. The cell cycle occurs between one cell division and the next

  
• In eukaryotic cells the cell cycle has two main phases :

  interphase -

  • G1
  • S
  • G2

  mitotic phase -

  • mitosis
  • cytokinesis
• Interphase = G1+S+G2

  Cells spend the majority of their time in this phase. During this phase cells are not dividing but the cell is very active!

  • DNA is replicated and checked for errors
  • Protein synthesis occurs in the cytoplasm
  • Mitochondria grow and divide
• G1 - Known as the “first growth phase”- cell increases in size, protein synthesis and organelle replication.
• S - The “synthesis” phase- a short phase in which DNA is replicated.
• G2 - The “second growth phase”- cell continues to grow, new duplicated DNA is checked. Tubulin protein made.
• DNA associates with proteins called histones to form a complex called chromatin . This coils and condenses to form chromosomes . These become visible when the cell prepares to divide.
• Before mitosis can occur the DNA in the nucleus is replicated (In S phase).

  Each chromosome is converted into two identical DNA molecules called chromatids

  These two chromatids are joined together at a region called the centromere.

  
• chromatin - Uncondensed DNA in a complex with histones
• chromatids - Two identical copies of DNA held together at a centromere
• chromosome - Structures of condensed and coiled DNA
• centromere - Region at which two chromatids are held together
• prophase -

  • Chromosomes coil up (and become visible under stains)
  • Nuclear membrane breaks down
  • Centrioles move to poles and start to form spindles

  
• metaphase -

  • Chromosomes line up along equator (middle of spindle)
  • Spindle fibres attach to centromeres

  
• anaphase -

  • Chromatids split
  • They are then pulled towards opposite poles by the spindle fibres and centriole

  
• telophase -

  • Spindle fibres break down
  • Nuclear envelope reforms
  • Centrioles reform
  • Chromosomes unravel

  
• cytokinesis -

  • Division of the cell into 2 separate cells
  • A ring of cytoskeletal fibres (the contractile ring) forms at the centre of the cell allowing a cleavage furrow forms around the middle of the cell.
  • The cell surface membrane is pulled inwards by the cytoskeletal fibres until it is close enough to fuse around the middle, forming 2 cells.

  
• anaphase

  
• prophase

  
• telophase

  
• anaphase

  
• interphase

  
• prophase

  
• telophase

  
• G0 - This is the phase when the cell leaves the cycle. This is usually permanent

  • DNA is damaged
  • senescent cells
  • differentiation
• Cytokinesis is different in plant cells due to the fact that they have cell walls.

  1. Spindle fibres guide Golgi to equator
  2. Vesicles from the golgi apparatus assemble in the metaphase plate (in the centre
  3. Vesicles fuse with each other and the cell surface membrane dividing the cell in two.
  4. New sections of cell wall form along the new sections of membrane.

  
• G1 checkpoint: This is at the end part of G1 before entry to S phase.

  If no error identified and cell meets requirements it will trigger DNA replication.

  If not, the cell enters G0 resting state
• G2 checkpoint: This is at the end of G2 phase. Cell needs to check DNA is replicated without error. If it passes the cell moves into the  mitotic phase.
• Spindle assembly checkpoint: This is also known as the metaphase checkpoint. This check occurs at the point when all chromosomes should be attached to spindles and should be aligned.
• The movement through each phase of the cell cycle is controlled by cyclins

  These bind to kinases forming a cyclin-dependant kinase complex (CDK). These complexes catalyse activation of cell cycle proteins ensuring that the cell progresses through the phases of the cell cycle at the appropriate time.
• senescent cells - These are cells in permanent cell arrest as they have depleted their limited number of divisions
• Asexual reproduction is the reproduction of organisms resulting in genetically identical offspring.
• Dividing cells can be well visualised under a microscope in growing root tips of plants. Stains that bind DNA can be added to make chromosomes more visible. We are then able to observe cells as they move through different stages of mitosis. We can calculate how long cells spend in a phase