Mitosis & the Cell Cycle (AQA GCSE Biology: Combined Science)

• Mitosis is a stage in the cell cycle when the nucleus divides
• The cell cycle is a series of stages in the life cycle of a cell
• During the growth phase of the cell cycle, the genetic material of the cell (chromosomes) is doubled
• Two copies of each chromosome are produced; these initially remain attached to each other with each strand called a chromatid
• The chromatids will eventually be divided between the two genetically identical daughter cells that form from the dividing cell
• During the growth phase, the number of subcellular structures (such as ribosomes and mitochondria) also increases

• For a multicellular organism to grow, cells must divide by mitosis to produce two identical daughter cells
• During mitosis the following events occur:
  • Chromosomes replicate, resulting in X-shaped chromosomes with two 'arms' known as chromatids
  • The chromosomes line up along the equator of the cell
  • The chromatids are separated and pulled to opposite ends of the cell
  • A new nucleus forms around each new group of chromosomes
• After this, the cytoplasm and cell membrane divides in a process known as cytokinesis; this results in the formation of two genetically identical daughter cells

Diagram showing the process of cell division by mitosis

• So, if a human cell with 46 chromosomes in 23 pairs needs to divide by mitosis:
  • Its 46 chromosomes are doubled, so that there are two copies of each of the 46 chromosomes (or 92 chromatids) in total in the cell
    • The number of ribosomes and mitochondria subcellular structures also increases

  • During mitosis, the chromatids are pulled apart, with a complete set of 46 chromosomes in 23 pairs moving to each end of the cell
  • The cytoplasm and cell membrane divide, forming two new daughter cells

• Cell division by mitosis is important in the growth and development of multicellular organisms
• When a sperm cell fertilises an egg cell in human reproduction, the resulting zygote cell that forms divides by mitosis, with each subsequent cell produced dividing in the same way to form an embryo
• As the embryo continues to grow in size, with the cells dividing by mitosis (and differentiating), a fetus forms
• Therefore, for a multicellular organism to grow, cells have to divide by mitosis to produce an increase in cell number

• You may also be asked to use observations of how many cells are in each stage of the cell cycle to estimate how long each stage of the cell cycle lasts

Worked Example

Cell Cycle Length