Mitosis (Cambridge O Level Biology)

• Most body cells have two copies of each chromosome
• We describe these cells as diploid
• When cells divide their chromosomes double beforehand
• This ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)
• This type of cell division is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction and is known as mitosis
• Mitosis is defined as nuclear division giving rise to genetically identical cells

Mitosis

The process of cell division by mitosis

Process:

• Just before mitosis, each chromosome in the nucleus copies itself exactly (forms x-shaped chromosomes)
• Chromosomes line up along the centre of the cell where cell fibres pull them apart
• The cell divides into two; each new cell has a copy of each of the chromosomes

Importance:

• All cells in the body (excluding gametes) are produced by mitosis of the zygote
• Mitosis is important for replacing cells e.g, skin cells, red blood cells and for allowing growth (production of new cells e.g. when a zygote divides to form an embryo)

Occurs in:

• Growth: mitosis produces new cells
• Repair: to replace damaged or dead cells
• Asexual reproduction: mitosis produces offspring that are genetically identical to the parent

Mitosis and Stem Cells

• Many tissues in the human body contain a small number of unspecialised cells
• These are called stem cells and their function is to divide by mitosis and produce new daughter cells that can become specialised within the tissue and be used for different functions
• The ultimate stem cell is the zygote
  • A zygote divides several times by mitosis to become a ball of unspecialised cells (around 200-300 cells)
    • These are embryonic stem cells
  • These cells are all the same and start differentiating as the fetus develops with recognisable features