Cell Cycle and Mitosis

• DNA Replication Recap

  1. DNA double helix unwinds and unzips with the help of an enzyme (Helicase).
  2. An enzyme (DNA Polymerase) adds new nucleotides to the leading strand continuously in the direction of the replication fork.
  3. Nucleotides are added in Okazaki fragments to the lagging strand and these fragments are joined by an enzyme (Ligase).
  4. The two identical daughter strands of DNA replication wind back into a double helix.

  
• Cell Division - Mitosis

  Mitosis is a form of cell division which produces two identical daughter cells. 

  
  

  The purpose of mitosis is for the growth of an organism and to replace old or damaged cells.
• Cell Division - Mitosis

  Mitosis takes place rapidly when new cells are forming during periods of growth (zygote, embryo, infant, young organisms and plants coming out of periods of dormancy).

  A high rate of mitosis is also needed for the repairs that take place after tissue damage.

  
• Cell Division - Mitosis

  Some areas in the body are constantly producing new cells. In these places, the mitotic rate is greater than in other areas of the body.

  
  

  For example:

  • The dermis of the human skin is constantly active, forming new cells to replace those lost daily from the epidermis.
  • Bone marrow constantly produces red blood cells, which remain in circulation for only about 100 days before breaking down.

  
• Process of Mitosis

  A) Interphase

  B) Prophase

  C) Metaphase

  D) Anaphase

  E) Telophase

  
• Summary of Mitosis Steps

  1.The chromosomes replicate to form two chromatids

  
  

  2.The chromatids are held together by centromeres. Spindle fibres also develop from the centrioles.

  
  

  3.The chromatids line up at the cell equator.
• Summary of Mitosis Steps

  4. Individual chromatids are pulled apart by spindle fibres and move towards the poles of the cell.

  
  

  5. Nuclear membranes begin to form around the two groups of chromatids.

  
  

  6. The plasma membrane divides the two cells and the cytoplasm divides between the two identical daughter cells.
• Cell Cycle

  The cell cycle can be divided into two main parts:

  • Interphase - When cells grow, accumulate nutrients, and replicate the DNA.
  • Mitosis - Includes both mitosis and cytokinesis, when the replicated chromosomes divide and then the cell divides in two.

  
• Factors Affecting Mitosis

  • Stage of Life
  • Availability of Nutrients and Energy
  • Location of Cells
  • Environmental Factors
• Factors Affecting the Rate of Mitosis - STAGE OF LIFE


  The rate of mitosis is high in organisms that are undergoing growth and repair.

  
  

  For example:

  • During the early years of development in animals (i.e. childhood, especially infancy and puberty-related growth spurts in humans) and plants (i.e. during the breaking of dormancy and germination).
  • During seasonal growth in plants (i.e. during spring).
  • Following damage to the organism when repair of tissue is necessary.
• Factors Affecting the Rate of Mitosis- AVAILABILITY OF NUTRIENTS

  To replicate a cell, certain substances are required to build new organelles and cell components.

  
  

  Replication is slowed or prevented if these nutrients are not available.
• Factors Affecting the Rate of Mitosis- LOCATION OF CELLS

  Specific areas of an organism may have a higher rate of mitosis where most growth or replacement of cells is occurring, such as root/shoot tips, hair follicles, bone marrow, skin cells, mucous membranes etc.
• Factors Affecting The Rate of Mitosis - ENVIRONMENTAL FACTORS

  The process of mitosis (and DNA replication) involves enzymes.

  
  

  Therefore, the rate of mitosis is affected by factors that affect enzymes.

  
  

  These factors include:

  • temperature - low temperature results in slower reactions, too high-temperature results in the enzymes denaturing
  • pH - enzymes have an optimum pH at which they work best
  • salt - high levels of salt ions denature enzymes and some ions behave as essential enzyme cofactors or toxins.