The Cell Cycle

Cell Continuity:

Cell continuity is where cells arise from cells of the same type OR organisms arise from organisms of the same type.

Living organisms maintain cell continuity by mitosis and/or meiosis and cell division.

Chromatin and Chromosomes

  • Chromatin is elongated DNA
  • Chromosomes are composed of DNA and protein
    • Chromosomes are arranged into homologous pairs – pairs of chromosomes that contain genes that control the same characteristics

Haploid and diploid

  • Haploid means one set of chromosomes
  • Diploid means two sets of chromosomes
    • In humans the haploid number is 23
    • The diploid number is 46 – arranged into 23 pairs
Haploid karyotypes

The Cell Cycle

  • The cell cycle occurs under three distinct stages:
    • Interphase
    • Mitosis
    • Cell division

​1. Interphase

  • Interphase is the stage where a cell is not dividing and a cell spends the vast majority of its time in this stage​
  • Chromosomes are elongated and the cell goes about its daily functions
Cell at interphase
  • Towards the end of interphase the cell organelles and all the DNA is replicated

End of interphase

  • The DNA is replicated and condenses into duplicated chromosomes which are held together by centromeres
Duplicated chromosome homologous pairs at the end of interphase

​2. Mitosis

  • Mitosis is nuclear division that leads to the formation of two identical daughter cells
  • Stage 1: Prophase
  • Stage 2: Metaphase
  • Stage 3: Anaphase
  • Stage 4: Telophase

Stage 1: Prophase

  • The nuclear membrane begins to disappear and spindle fibres begin to appear from the centrioles:

Stage 2: Metaphase

  • The replicated chromosomes line up along the equator of the cell held in place by spindle fibres:

Stage 3: Anaphase

  • The spindle fibres begin to contract pulling one chromosome from each pair to each pole:

Stage 4: Telophase

  • The chromosomes at each pole begin to unravel and lengthen; spindle fibres disappear; and the nuclear membranes reform at each pole:

3. Cell Division (Cytokinesis)

  • Cell division occurs immediately after mitosis and is different depending on whether in animals cells in plant cells.
  • In animal cells, the membrane pinches inwards forming a cleavage furrow, with two cells eventually being formed.
  • In plant cells, vesicles form down the middle of the dividing cell, forming a cell plate which eventually becomes the cell wall, forming two individual cells.
Cell division in animal cell (top); cell division in plant cells (bottom)

Cell division – Meiosis:

  • Meiosis is nuclear division whereby four new daughter cells are produced each with half the number of chromosomes as the parent cell


  • Cancer is a disease whereby a cell loses its ability to control both the rate of mitosis and cell division
  • Mitosis is normally carefully controlled but when it is out of control it can result in cancer
  • Cancer can be either benign or malignant
    • Benign cancer involves cells that divide out of control for a limited period of time and do not spread
    • Malignant cancers involve cells that divide rapidly uncontrollably and spread and invade other tissues – destroying that tissue in the process
  • Anything that has the potential to cause cancer is called a carcinogen – which damage DNA
  • Two carcinogens:
    • UV light – damages DNA of skin cells
    • Cigarette smoke – damages the DNA of cells lining the mouth, airways and lungs