Mitosis (SQA National 5 Biology): Revision Note

Mitosis is defined as nuclear division, giving rise to genetically identical cells

Mitosis is used for:

• growth

• repair of damaged tissues

• replacement of dead or damaged cells

• asexual reproduction 

Most body cells have two copies of each chromosome

We describe these cells as diploid

Before a cell divides, each chromosome duplicates its DNA, forming two identical chromatids joined together

During mitosis, the chromatids are separated so that each new daughter cell receives a complete set of chromosomes

This ensures that both new cells remain diploid, maintaining the correct chromosome number for body cells

Before mitosis, each chromosome in the nucleus of a cell has one chromatid

The chromatids copy themselves exactly during DNA replication

• Each chromosome now has two chromatids (two identical DNA molecules joined together)

  • It’s still counted as one chromosome until the chromatids separate during mitosis

• DNA replication must occur before mitosis begins

During mitosis, different stages occur to create two identical daughter cell

The stages are as follows:

1. Chromosomes condense (becoming visible) and the nuclear membrane breaks down

2. Chromosomes then line up along the equator of the cell

3. Spindle fibres split the chromosomes in half, pulling one chromatid to each pole (end) of the cell

4. A new nuclear membrane forms around the two separated chromatids

After mitosis, the cell divides into two cells, with an identical copy of each of the chromosomes in each cell

As mitosis results in the formation of two genetically identical daughter cells, the daughter cells have the same chromosome number as the parent cell